root/kernel/rcu/tree_plugin.h

INCLUDED FROM

DEFINITIONS

1. rcu_bootup_announce_oddness
2. rcu_bootup_announce
3. rcu_preempt_ctxt_queue
4. rcu_qs
5. rcu_note_context_switch
6. rcu_preempt_blocked_readers_cgp
7. __rcu_read_lock
8. __rcu_read_unlock
9. rcu_next_node_entry
10. rcu_preempt_has_tasks
11. rcu_preempt_deferred_qs_irqrestore
12. rcu_preempt_need_deferred_qs
13. rcu_preempt_deferred_qs
14. rcu_preempt_deferred_qs_handler
15. rcu_read_unlock_special
16. rcu_preempt_check_blocked_tasks
17. rcu_flavor_sched_clock_irq
18. exit_rcu
19. dump_blkd_tasks
20. rcu_bootup_announce
21. rcu_qs
22. rcu_all_qs
23. rcu_note_context_switch
24. rcu_preempt_blocked_readers_cgp
25. rcu_preempt_has_tasks
26. rcu_preempt_need_deferred_qs
27. rcu_preempt_deferred_qs
28. rcu_preempt_check_blocked_tasks
29. rcu_flavor_sched_clock_irq
30. exit_rcu
31. dump_blkd_tasks
32. rcu_cpu_kthread_setup
33. rcu_boost
34. rcu_boost_kthread
35. rcu_initiate_boost
36. rcu_is_callbacks_kthread
37. rcu_preempt_boost_start_gp
38. rcu_spawn_one_boost_kthread
39. rcu_boost_kthread_setaffinity
40. rcu_spawn_boost_kthreads
41. rcu_prepare_kthreads
42. rcu_initiate_boost
43. rcu_is_callbacks_kthread
44. rcu_preempt_boost_start_gp
45. rcu_boost_kthread_setaffinity
46. rcu_spawn_boost_kthreads
47. rcu_prepare_kthreads
48. rcu_needs_cpu
49. rcu_cleanup_after_idle
50. rcu_prepare_for_idle
51. rcu_try_advance_all_cbs
52. rcu_needs_cpu
53. rcu_prepare_for_idle
54. rcu_cleanup_after_idle
55. rcu_nocb_setup
56. parse_rcu_nocb_poll
57. rcu_nocb_bypass_lock
58. rcu_nocb_wait_contended
59. rcu_nocb_bypass_trylock
60. rcu_nocb_bypass_unlock
61. rcu_nocb_lock
62. rcu_nocb_unlock
63. rcu_nocb_unlock_irqrestore
64. rcu_lockdep_assert_cblist_protected
65. rcu_nocb_gp_cleanup
66. rcu_nocb_gp_get
67. rcu_init_one_nocb
68. rcu_is_nocb_cpu
69. wake_nocb_gp
70. wake_nocb_gp_defer
71. rcu_nocb_do_flush_bypass
72. rcu_nocb_flush_bypass
73. rcu_nocb_try_flush_bypass
74. rcu_nocb_try_bypass
75. __call_rcu_nocb_wake
76. do_nocb_bypass_wakeup_timer
77. nocb_gp_wait
78. rcu_nocb_gp_kthread
79. nocb_cb_wait
80. rcu_nocb_cb_kthread
81. rcu_nocb_need_deferred_wakeup
82. do_nocb_deferred_wakeup_common
83. do_nocb_deferred_wakeup_timer
84. do_nocb_deferred_wakeup
85. rcu_init_nohz
86. rcu_boot_init_nocb_percpu_data
87. rcu_spawn_one_nocb_kthread
88. rcu_spawn_cpu_nocb_kthread
89. rcu_spawn_nocb_kthreads
90. rcu_organize_nocb_kthreads
91. rcu_bind_current_to_nocb
92. show_rcu_nocb_gp_state
93. show_rcu_nocb_state
94. rcu_nocb_lock
95. rcu_nocb_unlock
96. rcu_nocb_unlock_irqrestore
97. rcu_lockdep_assert_cblist_protected
98. rcu_nocb_gp_cleanup
99. rcu_nocb_gp_get
100. rcu_init_one_nocb
101. rcu_nocb_flush_bypass
102. rcu_nocb_try_bypass
103. __call_rcu_nocb_wake
104. rcu_boot_init_nocb_percpu_data
105. rcu_nocb_need_deferred_wakeup
106. do_nocb_deferred_wakeup
107. rcu_spawn_cpu_nocb_kthread
108. rcu_spawn_nocb_kthreads
109. show_rcu_nocb_state
110. rcu_nohz_full_cpu
111. rcu_bind_gp_kthread
112. rcu_dynticks_task_enter
113. rcu_dynticks_task_exit

   1 /* SPDX-License-Identifier: GPL-2.0+ */
   2 /*
   3  * Read-Copy Update mechanism for mutual exclusion (tree-based version)
   4  * Internal non-public definitions that provide either classic
   5  * or preemptible semantics.
   6  *
   7  * Copyright Red Hat, 2009
   8  * Copyright IBM Corporation, 2009
   9  *
  10  * Author: Ingo Molnar <mingo@elte.hu>
  11  *         Paul E. McKenney <paulmck@linux.ibm.com>
  12  */
  13 
  14 #include "../locking/rtmutex_common.h"
  15 
  16 #ifdef CONFIG_RCU_NOCB_CPU
  17 static cpumask_var_t rcu_nocb_mask; /* CPUs to have callbacks offloaded. */
  18 static bool __read_mostly rcu_nocb_poll;    /* Offload kthread are to poll. */
  19 #endif /* #ifdef CONFIG_RCU_NOCB_CPU */
  20 
  21 /*
  22  * Check the RCU kernel configuration parameters and print informative
  23  * messages about anything out of the ordinary.
  24  */
  25 static void __init rcu_bootup_announce_oddness(void)
  26 {
  27         if (IS_ENABLED(CONFIG_RCU_TRACE))
  28                 pr_info("\tRCU event tracing is enabled.\n");
  29         if ((IS_ENABLED(CONFIG_64BIT) && RCU_FANOUT != 64) ||
  30             (!IS_ENABLED(CONFIG_64BIT) && RCU_FANOUT != 32))
  31                 pr_info("\tCONFIG_RCU_FANOUT set to non-default value of %d.\n",
  32                         RCU_FANOUT);
  33         if (rcu_fanout_exact)
  34                 pr_info("\tHierarchical RCU autobalancing is disabled.\n");
  35         if (IS_ENABLED(CONFIG_RCU_FAST_NO_HZ))
  36                 pr_info("\tRCU dyntick-idle grace-period acceleration is enabled.\n");
  37         if (IS_ENABLED(CONFIG_PROVE_RCU))
  38                 pr_info("\tRCU lockdep checking is enabled.\n");
  39         if (RCU_NUM_LVLS >= 4)
  40                 pr_info("\tFour(or more)-level hierarchy is enabled.\n");
  41         if (RCU_FANOUT_LEAF != 16)
  42                 pr_info("\tBuild-time adjustment of leaf fanout to %d.\n",
  43                         RCU_FANOUT_LEAF);
  44         if (rcu_fanout_leaf != RCU_FANOUT_LEAF)
  45                 pr_info("\tBoot-time adjustment of leaf fanout to %d.\n",
  46                         rcu_fanout_leaf);
  47         if (nr_cpu_ids != NR_CPUS)
  48                 pr_info("\tRCU restricting CPUs from NR_CPUS=%d to nr_cpu_ids=%u.\n", NR_CPUS, nr_cpu_ids);
  49 #ifdef CONFIG_RCU_BOOST
  50         pr_info("\tRCU priority boosting: priority %d delay %d ms.\n",
  51                 kthread_prio, CONFIG_RCU_BOOST_DELAY);
  52 #endif
  53         if (blimit != DEFAULT_RCU_BLIMIT)
  54                 pr_info("\tBoot-time adjustment of callback invocation limit to %ld.\n", blimit);
  55         if (qhimark != DEFAULT_RCU_QHIMARK)
  56                 pr_info("\tBoot-time adjustment of callback high-water mark to %ld.\n", qhimark);
  57         if (qlowmark != DEFAULT_RCU_QLOMARK)
  58                 pr_info("\tBoot-time adjustment of callback low-water mark to %ld.\n", qlowmark);
  59         if (jiffies_till_first_fqs != ULONG_MAX)
  60                 pr_info("\tBoot-time adjustment of first FQS scan delay to %ld jiffies.\n", jiffies_till_first_fqs);
  61         if (jiffies_till_next_fqs != ULONG_MAX)
  62                 pr_info("\tBoot-time adjustment of subsequent FQS scan delay to %ld jiffies.\n", jiffies_till_next_fqs);
  63         if (jiffies_till_sched_qs != ULONG_MAX)
  64                 pr_info("\tBoot-time adjustment of scheduler-enlistment delay to %ld jiffies.\n", jiffies_till_sched_qs);
  65         if (rcu_kick_kthreads)
  66                 pr_info("\tKick kthreads if too-long grace period.\n");
  67         if (IS_ENABLED(CONFIG_DEBUG_OBJECTS_RCU_HEAD))
  68                 pr_info("\tRCU callback double-/use-after-free debug enabled.\n");
  69         if (gp_preinit_delay)
  70                 pr_info("\tRCU debug GP pre-init slowdown %d jiffies.\n", gp_preinit_delay);
  71         if (gp_init_delay)
  72                 pr_info("\tRCU debug GP init slowdown %d jiffies.\n", gp_init_delay);
  73         if (gp_cleanup_delay)
  74                 pr_info("\tRCU debug GP init slowdown %d jiffies.\n", gp_cleanup_delay);
  75         if (!use_softirq)
  76                 pr_info("\tRCU_SOFTIRQ processing moved to rcuc kthreads.\n");
  77         if (IS_ENABLED(CONFIG_RCU_EQS_DEBUG))
  78                 pr_info("\tRCU debug extended QS entry/exit.\n");
  79         rcupdate_announce_bootup_oddness();
  80 }
  81 
  82 #ifdef CONFIG_PREEMPT_RCU
  83 
  84 static void rcu_report_exp_rnp(struct rcu_node *rnp, bool wake);
  85 static void rcu_read_unlock_special(struct task_struct *t);
  86 
  87 /*
  88  * Tell them what RCU they are running.
  89  */
  90 static void __init rcu_bootup_announce(void)
  91 {
  92         pr_info("Preemptible hierarchical RCU implementation.\n");
  93         rcu_bootup_announce_oddness();
  94 }
  95 
  96 /* Flags for rcu_preempt_ctxt_queue() decision table. */
  97 #define RCU_GP_TASKS    0x8
  98 #define RCU_EXP_TASKS   0x4
  99 #define RCU_GP_BLKD     0x2
 100 #define RCU_EXP_BLKD    0x1
 101 
 102 /*
 103  * Queues a task preempted within an RCU-preempt read-side critical
 104  * section into the appropriate location within the ->blkd_tasks list,
 105  * depending on the states of any ongoing normal and expedited grace
 106  * periods.  The ->gp_tasks pointer indicates which element the normal
 107  * grace period is waiting on (NULL if none), and the ->exp_tasks pointer
 108  * indicates which element the expedited grace period is waiting on (again,
 109  * NULL if none).  If a grace period is waiting on a given element in the
 110  * ->blkd_tasks list, it also waits on all subsequent elements.  Thus,
 111  * adding a task to the tail of the list blocks any grace period that is
 112  * already waiting on one of the elements.  In contrast, adding a task
 113  * to the head of the list won't block any grace period that is already
 114  * waiting on one of the elements.
 115  *
 116  * This queuing is imprecise, and can sometimes make an ongoing grace
 117  * period wait for a task that is not strictly speaking blocking it.
 118  * Given the choice, we needlessly block a normal grace period rather than
 119  * blocking an expedited grace period.
 120  *
 121  * Note that an endless sequence of expedited grace periods still cannot
 122  * indefinitely postpone a normal grace period.  Eventually, all of the
 123  * fixed number of preempted tasks blocking the normal grace period that are
 124  * not also blocking the expedited grace period will resume and complete
 125  * their RCU read-side critical sections.  At that point, the ->gp_tasks
 126  * pointer will equal the ->exp_tasks pointer, at which point the end of
 127  * the corresponding expedited grace period will also be the end of the
 128  * normal grace period.
 129  */
 130 static void rcu_preempt_ctxt_queue(struct rcu_node *rnp, struct rcu_data *rdp)
 131         __releases(rnp->lock) /* But leaves rrupts disabled. */
 132 {
 133         int blkd_state = (rnp->gp_tasks ? RCU_GP_TASKS : 0) +
 134                          (rnp->exp_tasks ? RCU_EXP_TASKS : 0) +
 135                          (rnp->qsmask & rdp->grpmask ? RCU_GP_BLKD : 0) +
 136                          (rnp->expmask & rdp->grpmask ? RCU_EXP_BLKD : 0);
 137         struct task_struct *t = current;
 138 
 139         raw_lockdep_assert_held_rcu_node(rnp);
 140         WARN_ON_ONCE(rdp->mynode != rnp);
 141         WARN_ON_ONCE(!rcu_is_leaf_node(rnp));
 142         /* RCU better not be waiting on newly onlined CPUs! */
 143         WARN_ON_ONCE(rnp->qsmaskinitnext & ~rnp->qsmaskinit & rnp->qsmask &
 144                      rdp->grpmask);
 145 
 146         /*
 147          * Decide where to queue the newly blocked task.  In theory,
 148          * this could be an if-statement.  In practice, when I tried
 149          * that, it was quite messy.
 150          */
 151         switch (blkd_state) {
 152         case 0:
 153         case                RCU_EXP_TASKS:
 154         case                RCU_EXP_TASKS + RCU_GP_BLKD:
 155         case RCU_GP_TASKS:
 156         case RCU_GP_TASKS + RCU_EXP_TASKS:
 157 
 158                 /*
 159                  * Blocking neither GP, or first task blocking the normal
 160                  * GP but not blocking the already-waiting expedited GP.
 161                  * Queue at the head of the list to avoid unnecessarily
 162                  * blocking the already-waiting GPs.
 163                  */
 164                 list_add(&t->rcu_node_entry, &rnp->blkd_tasks);
 165                 break;
 166 
 167         case                                              RCU_EXP_BLKD:
 168         case                                RCU_GP_BLKD:
 169         case                                RCU_GP_BLKD + RCU_EXP_BLKD:
 170         case RCU_GP_TASKS +                               RCU_EXP_BLKD:
 171         case RCU_GP_TASKS +                 RCU_GP_BLKD + RCU_EXP_BLKD:
 172         case RCU_GP_TASKS + RCU_EXP_TASKS + RCU_GP_BLKD + RCU_EXP_BLKD:
 173 
 174                 /*
 175                  * First task arriving that blocks either GP, or first task
 176                  * arriving that blocks the expedited GP (with the normal
 177                  * GP already waiting), or a task arriving that blocks
 178                  * both GPs with both GPs already waiting.  Queue at the
 179                  * tail of the list to avoid any GP waiting on any of the
 180                  * already queued tasks that are not blocking it.
 181                  */
 182                 list_add_tail(&t->rcu_node_entry, &rnp->blkd_tasks);
 183                 break;
 184 
 185         case                RCU_EXP_TASKS +               RCU_EXP_BLKD:
 186         case                RCU_EXP_TASKS + RCU_GP_BLKD + RCU_EXP_BLKD:
 187         case RCU_GP_TASKS + RCU_EXP_TASKS +               RCU_EXP_BLKD:
 188 
 189                 /*
 190                  * Second or subsequent task blocking the expedited GP.
 191                  * The task either does not block the normal GP, or is the
 192                  * first task blocking the normal GP.  Queue just after
 193                  * the first task blocking the expedited GP.
 194                  */
 195                 list_add(&t->rcu_node_entry, rnp->exp_tasks);
 196                 break;
 197 
 198         case RCU_GP_TASKS +                 RCU_GP_BLKD:
 199         case RCU_GP_TASKS + RCU_EXP_TASKS + RCU_GP_BLKD:
 200 
 201                 /*
 202                  * Second or subsequent task blocking the normal GP.
 203                  * The task does not block the expedited GP. Queue just
 204                  * after the first task blocking the normal GP.
 205                  */
 206                 list_add(&t->rcu_node_entry, rnp->gp_tasks);
 207                 break;
 208 
 209         default:
 210 
 211                 /* Yet another exercise in excessive paranoia. */
 212                 WARN_ON_ONCE(1);
 213                 break;
 214         }
 215 
 216         /*
 217          * We have now queued the task.  If it was the first one to
 218          * block either grace period, update the ->gp_tasks and/or
 219          * ->exp_tasks pointers, respectively, to reference the newly
 220          * blocked tasks.
 221          */
 222         if (!rnp->gp_tasks && (blkd_state & RCU_GP_BLKD)) {
 223                 WRITE_ONCE(rnp->gp_tasks, &t->rcu_node_entry);
 224                 WARN_ON_ONCE(rnp->completedqs == rnp->gp_seq);
 225         }
 226         if (!rnp->exp_tasks && (blkd_state & RCU_EXP_BLKD))
 227                 rnp->exp_tasks = &t->rcu_node_entry;
 228         WARN_ON_ONCE(!(blkd_state & RCU_GP_BLKD) !=
 229                      !(rnp->qsmask & rdp->grpmask));
 230         WARN_ON_ONCE(!(blkd_state & RCU_EXP_BLKD) !=
 231                      !(rnp->expmask & rdp->grpmask));
 232         raw_spin_unlock_rcu_node(rnp); /* interrupts remain disabled. */
 233 
 234         /*
 235          * Report the quiescent state for the expedited GP.  This expedited
 236          * GP should not be able to end until we report, so there should be
 237          * no need to check for a subsequent expedited GP.  (Though we are
 238          * still in a quiescent state in any case.)
 239          */
 240         if (blkd_state & RCU_EXP_BLKD && rdp->exp_deferred_qs)
 241                 rcu_report_exp_rdp(rdp);
 242         else
 243                 WARN_ON_ONCE(rdp->exp_deferred_qs);
 244 }
 245 
 246 /*
 247  * Record a preemptible-RCU quiescent state for the specified CPU.
 248  * Note that this does not necessarily mean that the task currently running
 249  * on the CPU is in a quiescent state:  Instead, it means that the current
 250  * grace period need not wait on any RCU read-side critical section that
 251  * starts later on this CPU.  It also means that if the current task is
 252  * in an RCU read-side critical section, it has already added itself to
 253  * some leaf rcu_node structure's ->blkd_tasks list.  In addition to the
 254  * current task, there might be any number of other tasks blocked while
 255  * in an RCU read-side critical section.
 256  *
 257  * Callers to this function must disable preemption.
 258  */
 259 static void rcu_qs(void)
 260 {
 261         RCU_LOCKDEP_WARN(preemptible(), "rcu_qs() invoked with preemption enabled!!!\n");
 262         if (__this_cpu_read(rcu_data.cpu_no_qs.s)) {
 263                 trace_rcu_grace_period(TPS("rcu_preempt"),
 264                                        __this_cpu_read(rcu_data.gp_seq),
 265                                        TPS("cpuqs"));
 266                 __this_cpu_write(rcu_data.cpu_no_qs.b.norm, false);
 267                 barrier(); /* Coordinate with rcu_flavor_sched_clock_irq(). */
 268                 WRITE_ONCE(current->rcu_read_unlock_special.b.need_qs, false);
 269         }
 270 }
 271 
 272 /*
 273  * We have entered the scheduler, and the current task might soon be
 274  * context-switched away from.  If this task is in an RCU read-side
 275  * critical section, we will no longer be able to rely on the CPU to
 276  * record that fact, so we enqueue the task on the blkd_tasks list.
 277  * The task will dequeue itself when it exits the outermost enclosing
 278  * RCU read-side critical section.  Therefore, the current grace period
 279  * cannot be permitted to complete until the blkd_tasks list entries
 280  * predating the current grace period drain, in other words, until
 281  * rnp->gp_tasks becomes NULL.
 282  *
 283  * Caller must disable interrupts.
 284  */
 285 void rcu_note_context_switch(bool preempt)
 286 {
 287         struct task_struct *t = current;
 288         struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
 289         struct rcu_node *rnp;
 290 
 291         trace_rcu_utilization(TPS("Start context switch"));
 292         lockdep_assert_irqs_disabled();
 293         WARN_ON_ONCE(!preempt && t->rcu_read_lock_nesting > 0);
 294         if (t->rcu_read_lock_nesting > 0 &&
 295             !t->rcu_read_unlock_special.b.blocked) {
 296 
 297                 /* Possibly blocking in an RCU read-side critical section. */
 298                 rnp = rdp->mynode;
 299                 raw_spin_lock_rcu_node(rnp);
 300                 t->rcu_read_unlock_special.b.blocked = true;
 301                 t->rcu_blocked_node = rnp;
 302 
 303                 /*
 304                  * Verify the CPU's sanity, trace the preemption, and
 305                  * then queue the task as required based on the states
 306                  * of any ongoing and expedited grace periods.
 307                  */
 308                 WARN_ON_ONCE((rdp->grpmask & rcu_rnp_online_cpus(rnp)) == 0);
 309                 WARN_ON_ONCE(!list_empty(&t->rcu_node_entry));
 310                 trace_rcu_preempt_task(rcu_state.name,
 311                                        t->pid,
 312                                        (rnp->qsmask & rdp->grpmask)
 313                                        ? rnp->gp_seq
 314                                        : rcu_seq_snap(&rnp->gp_seq));
 315                 rcu_preempt_ctxt_queue(rnp, rdp);
 316         } else {
 317                 rcu_preempt_deferred_qs(t);
 318         }
 319 
 320         /*
 321          * Either we were not in an RCU read-side critical section to
 322          * begin with, or we have now recorded that critical section
 323          * globally.  Either way, we can now note a quiescent state
 324          * for this CPU.  Again, if we were in an RCU read-side critical
 325          * section, and if that critical section was blocking the current
 326          * grace period, then the fact that the task has been enqueued
 327          * means that we continue to block the current grace period.
 328          */
 329         rcu_qs();
 330         if (rdp->exp_deferred_qs)
 331                 rcu_report_exp_rdp(rdp);
 332         trace_rcu_utilization(TPS("End context switch"));
 333 }
 334 EXPORT_SYMBOL_GPL(rcu_note_context_switch);
 335 
 336 /*
 337  * Check for preempted RCU readers blocking the current grace period
 338  * for the specified rcu_node structure.  If the caller needs a reliable
 339  * answer, it must hold the rcu_node's ->lock.
 340  */
 341 static int rcu_preempt_blocked_readers_cgp(struct rcu_node *rnp)
 342 {
 343         return READ_ONCE(rnp->gp_tasks) != NULL;
 344 }
 345 
 346 /* Bias and limit values for ->rcu_read_lock_nesting. */
 347 #define RCU_NEST_BIAS INT_MAX
 348 #define RCU_NEST_NMAX (-INT_MAX / 2)
 349 #define RCU_NEST_PMAX (INT_MAX / 2)
 350 
 351 /*
 352  * Preemptible RCU implementation for rcu_read_lock().
 353  * Just increment ->rcu_read_lock_nesting, shared state will be updated
 354  * if we block.
 355  */
 356 void __rcu_read_lock(void)
 357 {
 358         current->rcu_read_lock_nesting++;
 359         if (IS_ENABLED(CONFIG_PROVE_LOCKING))
 360                 WARN_ON_ONCE(current->rcu_read_lock_nesting > RCU_NEST_PMAX);
 361         barrier();  /* critical section after entry code. */
 362 }
 363 EXPORT_SYMBOL_GPL(__rcu_read_lock);
 364 
 365 /*
 366  * Preemptible RCU implementation for rcu_read_unlock().
 367  * Decrement ->rcu_read_lock_nesting.  If the result is zero (outermost
 368  * rcu_read_unlock()) and ->rcu_read_unlock_special is non-zero, then
 369  * invoke rcu_read_unlock_special() to clean up after a context switch
 370  * in an RCU read-side critical section and other special cases.
 371  */
 372 void __rcu_read_unlock(void)
 373 {
 374         struct task_struct *t = current;
 375 
 376         if (t->rcu_read_lock_nesting != 1) {
 377                 --t->rcu_read_lock_nesting;
 378         } else {
 379                 barrier();  /* critical section before exit code. */
 380                 t->rcu_read_lock_nesting = -RCU_NEST_BIAS;
 381                 barrier();  /* assign before ->rcu_read_unlock_special load */
 382                 if (unlikely(READ_ONCE(t->rcu_read_unlock_special.s)))
 383                         rcu_read_unlock_special(t);
 384                 barrier();  /* ->rcu_read_unlock_special load before assign */
 385                 t->rcu_read_lock_nesting = 0;
 386         }
 387         if (IS_ENABLED(CONFIG_PROVE_LOCKING)) {
 388                 int rrln = t->rcu_read_lock_nesting;
 389 
 390                 WARN_ON_ONCE(rrln < 0 && rrln > RCU_NEST_NMAX);
 391         }
 392 }
 393 EXPORT_SYMBOL_GPL(__rcu_read_unlock);
 394 
 395 /*
 396  * Advance a ->blkd_tasks-list pointer to the next entry, instead
 397  * returning NULL if at the end of the list.
 398  */
 399 static struct list_head *rcu_next_node_entry(struct task_struct *t,
 400                                              struct rcu_node *rnp)
 401 {
 402         struct list_head *np;
 403 
 404         np = t->rcu_node_entry.next;
 405         if (np == &rnp->blkd_tasks)
 406                 np = NULL;
 407         return np;
 408 }
 409 
 410 /*
 411  * Return true if the specified rcu_node structure has tasks that were
 412  * preempted within an RCU read-side critical section.
 413  */
 414 static bool rcu_preempt_has_tasks(struct rcu_node *rnp)
 415 {
 416         return !list_empty(&rnp->blkd_tasks);
 417 }
 418 
 419 /*
 420  * Report deferred quiescent states.  The deferral time can
 421  * be quite short, for example, in the case of the call from
 422  * rcu_read_unlock_special().
 423  */
 424 static void
 425 rcu_preempt_deferred_qs_irqrestore(struct task_struct *t, unsigned long flags)
 426 {
 427         bool empty_exp;
 428         bool empty_norm;
 429         bool empty_exp_now;
 430         struct list_head *np;
 431         bool drop_boost_mutex = false;
 432         struct rcu_data *rdp;
 433         struct rcu_node *rnp;
 434         union rcu_special special;
 435 
 436         /*
 437          * If RCU core is waiting for this CPU to exit its critical section,
 438          * report the fact that it has exited.  Because irqs are disabled,
 439          * t->rcu_read_unlock_special cannot change.
 440          */
 441         special = t->rcu_read_unlock_special;
 442         rdp = this_cpu_ptr(&rcu_data);
 443         if (!special.s && !rdp->exp_deferred_qs) {
 444                 local_irq_restore(flags);
 445                 return;
 446         }
 447         t->rcu_read_unlock_special.b.deferred_qs = false;
 448         if (special.b.need_qs) {
 449                 rcu_qs();
 450                 t->rcu_read_unlock_special.b.need_qs = false;
 451                 if (!t->rcu_read_unlock_special.s && !rdp->exp_deferred_qs) {
 452                         local_irq_restore(flags);
 453                         return;
 454                 }
 455         }
 456 
 457         /*
 458          * Respond to a request by an expedited grace period for a
 459          * quiescent state from this CPU.  Note that requests from
 460          * tasks are handled when removing the task from the
 461          * blocked-tasks list below.
 462          */
 463         if (rdp->exp_deferred_qs) {
 464                 rcu_report_exp_rdp(rdp);
 465                 if (!t->rcu_read_unlock_special.s) {
 466                         local_irq_restore(flags);
 467                         return;
 468                 }
 469         }
 470 
 471         /* Clean up if blocked during RCU read-side critical section. */
 472         if (special.b.blocked) {
 473                 t->rcu_read_unlock_special.b.blocked = false;
 474 
 475                 /*
 476                  * Remove this task from the list it blocked on.  The task
 477                  * now remains queued on the rcu_node corresponding to the
 478                  * CPU it first blocked on, so there is no longer any need
 479                  * to loop.  Retain a WARN_ON_ONCE() out of sheer paranoia.
 480                  */
 481                 rnp = t->rcu_blocked_node;
 482                 raw_spin_lock_rcu_node(rnp); /* irqs already disabled. */
 483                 WARN_ON_ONCE(rnp != t->rcu_blocked_node);
 484                 WARN_ON_ONCE(!rcu_is_leaf_node(rnp));
 485                 empty_norm = !rcu_preempt_blocked_readers_cgp(rnp);
 486                 WARN_ON_ONCE(rnp->completedqs == rnp->gp_seq &&
 487                              (!empty_norm || rnp->qsmask));
 488                 empty_exp = sync_rcu_preempt_exp_done(rnp);
 489                 smp_mb(); /* ensure expedited fastpath sees end of RCU c-s. */
 490                 np = rcu_next_node_entry(t, rnp);
 491                 list_del_init(&t->rcu_node_entry);
 492                 t->rcu_blocked_node = NULL;
 493                 trace_rcu_unlock_preempted_task(TPS("rcu_preempt"),
 494                                                 rnp->gp_seq, t->pid);
 495                 if (&t->rcu_node_entry == rnp->gp_tasks)
 496                         WRITE_ONCE(rnp->gp_tasks, np);
 497                 if (&t->rcu_node_entry == rnp->exp_tasks)
 498                         rnp->exp_tasks = np;
 499                 if (IS_ENABLED(CONFIG_RCU_BOOST)) {
 500                         /* Snapshot ->boost_mtx ownership w/rnp->lock held. */
 501                         drop_boost_mutex = rt_mutex_owner(&rnp->boost_mtx) == t;
 502                         if (&t->rcu_node_entry == rnp->boost_tasks)
 503                                 rnp->boost_tasks = np;
 504                 }
 505 
 506                 /*
 507                  * If this was the last task on the current list, and if
 508                  * we aren't waiting on any CPUs, report the quiescent state.
 509                  * Note that rcu_report_unblock_qs_rnp() releases rnp->lock,
 510                  * so we must take a snapshot of the expedited state.
 511                  */
 512                 empty_exp_now = sync_rcu_preempt_exp_done(rnp);
 513                 if (!empty_norm && !rcu_preempt_blocked_readers_cgp(rnp)) {
 514                         trace_rcu_quiescent_state_report(TPS("preempt_rcu"),
 515                                                          rnp->gp_seq,
 516                                                          0, rnp->qsmask,
 517                                                          rnp->level,
 518                                                          rnp->grplo,
 519                                                          rnp->grphi,
 520                                                          !!rnp->gp_tasks);
 521                         rcu_report_unblock_qs_rnp(rnp, flags);
 522                 } else {
 523                         raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 524                 }
 525 
 526                 /* Unboost if we were boosted. */
 527                 if (IS_ENABLED(CONFIG_RCU_BOOST) && drop_boost_mutex)
 528                         rt_mutex_futex_unlock(&rnp->boost_mtx);
 529 
 530                 /*
 531                  * If this was the last task on the expedited lists,
 532                  * then we need to report up the rcu_node hierarchy.
 533                  */
 534                 if (!empty_exp && empty_exp_now)
 535                         rcu_report_exp_rnp(rnp, true);
 536         } else {
 537                 local_irq_restore(flags);
 538         }
 539 }
 540 
 541 /*
 542  * Is a deferred quiescent-state pending, and are we also not in
 543  * an RCU read-side critical section?  It is the caller's responsibility
 544  * to ensure it is otherwise safe to report any deferred quiescent
 545  * states.  The reason for this is that it is safe to report a
 546  * quiescent state during context switch even though preemption
 547  * is disabled.  This function cannot be expected to understand these
 548  * nuances, so the caller must handle them.
 549  */
 550 static bool rcu_preempt_need_deferred_qs(struct task_struct *t)
 551 {
 552         return (__this_cpu_read(rcu_data.exp_deferred_qs) ||
 553                 READ_ONCE(t->rcu_read_unlock_special.s)) &&
 554                t->rcu_read_lock_nesting <= 0;
 555 }
 556 
 557 /*
 558  * Report a deferred quiescent state if needed and safe to do so.
 559  * As with rcu_preempt_need_deferred_qs(), "safe" involves only
 560  * not being in an RCU read-side critical section.  The caller must
 561  * evaluate safety in terms of interrupt, softirq, and preemption
 562  * disabling.
 563  */
 564 static void rcu_preempt_deferred_qs(struct task_struct *t)
 565 {
 566         unsigned long flags;
 567         bool couldrecurse = t->rcu_read_lock_nesting >= 0;
 568 
 569         if (!rcu_preempt_need_deferred_qs(t))
 570                 return;
 571         if (couldrecurse)
 572                 t->rcu_read_lock_nesting -= RCU_NEST_BIAS;
 573         local_irq_save(flags);
 574         rcu_preempt_deferred_qs_irqrestore(t, flags);
 575         if (couldrecurse)
 576                 t->rcu_read_lock_nesting += RCU_NEST_BIAS;
 577 }
 578 
 579 /*
 580  * Minimal handler to give the scheduler a chance to re-evaluate.
 581  */
 582 static void rcu_preempt_deferred_qs_handler(struct irq_work *iwp)
 583 {
 584         struct rcu_data *rdp;
 585 
 586         rdp = container_of(iwp, struct rcu_data, defer_qs_iw);
 587         rdp->defer_qs_iw_pending = false;
 588 }
 589 
 590 /*
 591  * Handle special cases during rcu_read_unlock(), such as needing to
 592  * notify RCU core processing or task having blocked during the RCU
 593  * read-side critical section.
 594  */
 595 static void rcu_read_unlock_special(struct task_struct *t)
 596 {
 597         unsigned long flags;
 598         bool preempt_bh_were_disabled =
 599                         !!(preempt_count() & (PREEMPT_MASK | SOFTIRQ_MASK));
 600         bool irqs_were_disabled;
 601 
 602         /* NMI handlers cannot block and cannot safely manipulate state. */
 603         if (in_nmi())
 604                 return;
 605 
 606         local_irq_save(flags);
 607         irqs_were_disabled = irqs_disabled_flags(flags);
 608         if (preempt_bh_were_disabled || irqs_were_disabled) {
 609                 bool exp;
 610                 struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
 611                 struct rcu_node *rnp = rdp->mynode;
 612 
 613                 t->rcu_read_unlock_special.b.exp_hint = false;
 614                 exp = (t->rcu_blocked_node && t->rcu_blocked_node->exp_tasks) ||
 615                       (rdp->grpmask & READ_ONCE(rnp->expmask)) ||
 616                       tick_nohz_full_cpu(rdp->cpu);
 617                 // Need to defer quiescent state until everything is enabled.
 618                 if (irqs_were_disabled && use_softirq &&
 619                     (in_interrupt() ||
 620                      (exp && !t->rcu_read_unlock_special.b.deferred_qs))) {
 621                         // Using softirq, safe to awaken, and we get
 622                         // no help from enabling irqs, unlike bh/preempt.
 623                         raise_softirq_irqoff(RCU_SOFTIRQ);
 624                 } else {
 625                         // Enabling BH or preempt does reschedule, so...
 626                         // Also if no expediting or NO_HZ_FULL, slow is OK.
 627                         set_tsk_need_resched(current);
 628                         set_preempt_need_resched();
 629                         if (IS_ENABLED(CONFIG_IRQ_WORK) && irqs_were_disabled &&
 630                             !rdp->defer_qs_iw_pending && exp) {
 631                                 // Get scheduler to re-evaluate and call hooks.
 632                                 // If !IRQ_WORK, FQS scan will eventually IPI.
 633                                 init_irq_work(&rdp->defer_qs_iw,
 634                                               rcu_preempt_deferred_qs_handler);
 635                                 rdp->defer_qs_iw_pending = true;
 636                                 irq_work_queue_on(&rdp->defer_qs_iw, rdp->cpu);
 637                         }
 638                 }
 639                 t->rcu_read_unlock_special.b.deferred_qs = true;
 640                 local_irq_restore(flags);
 641                 return;
 642         }
 643         WRITE_ONCE(t->rcu_read_unlock_special.b.exp_hint, false);
 644         rcu_preempt_deferred_qs_irqrestore(t, flags);
 645 }
 646 
 647 /*
 648  * Check that the list of blocked tasks for the newly completed grace
 649  * period is in fact empty.  It is a serious bug to complete a grace
 650  * period that still has RCU readers blocked!  This function must be
 651  * invoked -before- updating this rnp's ->gp_seq, and the rnp's ->lock
 652  * must be held by the caller.
 653  *
 654  * Also, if there are blocked tasks on the list, they automatically
 655  * block the newly created grace period, so set up ->gp_tasks accordingly.
 656  */
 657 static void rcu_preempt_check_blocked_tasks(struct rcu_node *rnp)
 658 {
 659         struct task_struct *t;
 660 
 661         RCU_LOCKDEP_WARN(preemptible(), "rcu_preempt_check_blocked_tasks() invoked with preemption enabled!!!\n");
 662         if (WARN_ON_ONCE(rcu_preempt_blocked_readers_cgp(rnp)))
 663                 dump_blkd_tasks(rnp, 10);
 664         if (rcu_preempt_has_tasks(rnp) &&
 665             (rnp->qsmaskinit || rnp->wait_blkd_tasks)) {
 666                 WRITE_ONCE(rnp->gp_tasks, rnp->blkd_tasks.next);
 667                 t = container_of(rnp->gp_tasks, struct task_struct,
 668                                  rcu_node_entry);
 669                 trace_rcu_unlock_preempted_task(TPS("rcu_preempt-GPS"),
 670                                                 rnp->gp_seq, t->pid);
 671         }
 672         WARN_ON_ONCE(rnp->qsmask);
 673 }
 674 
 675 /*
 676  * Check for a quiescent state from the current CPU, including voluntary
 677  * context switches for Tasks RCU.  When a task blocks, the task is
 678  * recorded in the corresponding CPU's rcu_node structure, which is checked
 679  * elsewhere, hence this function need only check for quiescent states
 680  * related to the current CPU, not to those related to tasks.
 681  */
 682 static void rcu_flavor_sched_clock_irq(int user)
 683 {
 684         struct task_struct *t = current;
 685 
 686         if (user || rcu_is_cpu_rrupt_from_idle()) {
 687                 rcu_note_voluntary_context_switch(current);
 688         }
 689         if (t->rcu_read_lock_nesting > 0 ||
 690             (preempt_count() & (PREEMPT_MASK | SOFTIRQ_MASK))) {
 691                 /* No QS, force context switch if deferred. */
 692                 if (rcu_preempt_need_deferred_qs(t)) {
 693                         set_tsk_need_resched(t);
 694                         set_preempt_need_resched();
 695                 }
 696         } else if (rcu_preempt_need_deferred_qs(t)) {
 697                 rcu_preempt_deferred_qs(t); /* Report deferred QS. */
 698                 return;
 699         } else if (!t->rcu_read_lock_nesting) {
 700                 rcu_qs(); /* Report immediate QS. */
 701                 return;
 702         }
 703 
 704         /* If GP is oldish, ask for help from rcu_read_unlock_special(). */
 705         if (t->rcu_read_lock_nesting > 0 &&
 706             __this_cpu_read(rcu_data.core_needs_qs) &&
 707             __this_cpu_read(rcu_data.cpu_no_qs.b.norm) &&
 708             !t->rcu_read_unlock_special.b.need_qs &&
 709             time_after(jiffies, rcu_state.gp_start + HZ))
 710                 t->rcu_read_unlock_special.b.need_qs = true;
 711 }
 712 
 713 /*
 714  * Check for a task exiting while in a preemptible-RCU read-side
 715  * critical section, clean up if so.  No need to issue warnings, as
 716  * debug_check_no_locks_held() already does this if lockdep is enabled.
 717  * Besides, if this function does anything other than just immediately
 718  * return, there was a bug of some sort.  Spewing warnings from this
 719  * function is like as not to simply obscure important prior warnings.
 720  */
 721 void exit_rcu(void)
 722 {
 723         struct task_struct *t = current;
 724 
 725         if (unlikely(!list_empty(&current->rcu_node_entry))) {
 726                 t->rcu_read_lock_nesting = 1;
 727                 barrier();
 728                 WRITE_ONCE(t->rcu_read_unlock_special.b.blocked, true);
 729         } else if (unlikely(t->rcu_read_lock_nesting)) {
 730                 t->rcu_read_lock_nesting = 1;
 731         } else {
 732                 return;
 733         }
 734         __rcu_read_unlock();
 735         rcu_preempt_deferred_qs(current);
 736 }
 737 
 738 /*
 739  * Dump the blocked-tasks state, but limit the list dump to the
 740  * specified number of elements.
 741  */
 742 static void
 743 dump_blkd_tasks(struct rcu_node *rnp, int ncheck)
 744 {
 745         int cpu;
 746         int i;
 747         struct list_head *lhp;
 748         bool onl;
 749         struct rcu_data *rdp;
 750         struct rcu_node *rnp1;
 751 
 752         raw_lockdep_assert_held_rcu_node(rnp);
 753         pr_info("%s: grp: %d-%d level: %d ->gp_seq %ld ->completedqs %ld\n",
 754                 __func__, rnp->grplo, rnp->grphi, rnp->level,
 755                 (long)rnp->gp_seq, (long)rnp->completedqs);
 756         for (rnp1 = rnp; rnp1; rnp1 = rnp1->parent)
 757                 pr_info("%s: %d:%d ->qsmask %#lx ->qsmaskinit %#lx ->qsmaskinitnext %#lx\n",
 758                         __func__, rnp1->grplo, rnp1->grphi, rnp1->qsmask, rnp1->qsmaskinit, rnp1->qsmaskinitnext);
 759         pr_info("%s: ->gp_tasks %p ->boost_tasks %p ->exp_tasks %p\n",
 760                 __func__, READ_ONCE(rnp->gp_tasks), rnp->boost_tasks,
 761                 rnp->exp_tasks);
 762         pr_info("%s: ->blkd_tasks", __func__);
 763         i = 0;
 764         list_for_each(lhp, &rnp->blkd_tasks) {
 765                 pr_cont(" %p", lhp);
 766                 if (++i >= ncheck)
 767                         break;
 768         }
 769         pr_cont("\n");
 770         for (cpu = rnp->grplo; cpu <= rnp->grphi; cpu++) {
 771                 rdp = per_cpu_ptr(&rcu_data, cpu);
 772                 onl = !!(rdp->grpmask & rcu_rnp_online_cpus(rnp));
 773                 pr_info("\t%d: %c online: %ld(%d) offline: %ld(%d)\n",
 774                         cpu, ".o"[onl],
 775                         (long)rdp->rcu_onl_gp_seq, rdp->rcu_onl_gp_flags,
 776                         (long)rdp->rcu_ofl_gp_seq, rdp->rcu_ofl_gp_flags);
 777         }
 778 }
 779 
 780 #else /* #ifdef CONFIG_PREEMPT_RCU */
 781 
 782 /*
 783  * Tell them what RCU they are running.
 784  */
 785 static void __init rcu_bootup_announce(void)
 786 {
 787         pr_info("Hierarchical RCU implementation.\n");
 788         rcu_bootup_announce_oddness();
 789 }
 790 
 791 /*
 792  * Note a quiescent state for PREEMPT=n.  Because we do not need to know
 793  * how many quiescent states passed, just if there was at least one since
 794  * the start of the grace period, this just sets a flag.  The caller must
 795  * have disabled preemption.
 796  */
 797 static void rcu_qs(void)
 798 {
 799         RCU_LOCKDEP_WARN(preemptible(), "rcu_qs() invoked with preemption enabled!!!");
 800         if (!__this_cpu_read(rcu_data.cpu_no_qs.s))
 801                 return;
 802         trace_rcu_grace_period(TPS("rcu_sched"),
 803                                __this_cpu_read(rcu_data.gp_seq), TPS("cpuqs"));
 804         __this_cpu_write(rcu_data.cpu_no_qs.b.norm, false);
 805         if (!__this_cpu_read(rcu_data.cpu_no_qs.b.exp))
 806                 return;
 807         __this_cpu_write(rcu_data.cpu_no_qs.b.exp, false);
 808         rcu_report_exp_rdp(this_cpu_ptr(&rcu_data));
 809 }
 810 
 811 /*
 812  * Register an urgently needed quiescent state.  If there is an
 813  * emergency, invoke rcu_momentary_dyntick_idle() to do a heavy-weight
 814  * dyntick-idle quiescent state visible to other CPUs, which will in
 815  * some cases serve for expedited as well as normal grace periods.
 816  * Either way, register a lightweight quiescent state.
 817  */
 818 void rcu_all_qs(void)
 819 {
 820         unsigned long flags;
 821 
 822         if (!raw_cpu_read(rcu_data.rcu_urgent_qs))
 823                 return;
 824         preempt_disable();
 825         /* Load rcu_urgent_qs before other flags. */
 826         if (!smp_load_acquire(this_cpu_ptr(&rcu_data.rcu_urgent_qs))) {
 827                 preempt_enable();
 828                 return;
 829         }
 830         this_cpu_write(rcu_data.rcu_urgent_qs, false);
 831         if (unlikely(raw_cpu_read(rcu_data.rcu_need_heavy_qs))) {
 832                 local_irq_save(flags);
 833                 rcu_momentary_dyntick_idle();
 834                 local_irq_restore(flags);
 835         }
 836         rcu_qs();
 837         preempt_enable();
 838 }
 839 EXPORT_SYMBOL_GPL(rcu_all_qs);
 840 
 841 /*
 842  * Note a PREEMPT=n context switch.  The caller must have disabled interrupts.
 843  */
 844 void rcu_note_context_switch(bool preempt)
 845 {
 846         trace_rcu_utilization(TPS("Start context switch"));
 847         rcu_qs();
 848         /* Load rcu_urgent_qs before other flags. */
 849         if (!smp_load_acquire(this_cpu_ptr(&rcu_data.rcu_urgent_qs)))
 850                 goto out;
 851         this_cpu_write(rcu_data.rcu_urgent_qs, false);
 852         if (unlikely(raw_cpu_read(rcu_data.rcu_need_heavy_qs)))
 853                 rcu_momentary_dyntick_idle();
 854         if (!preempt)
 855                 rcu_tasks_qs(current);
 856 out:
 857         trace_rcu_utilization(TPS("End context switch"));
 858 }
 859 EXPORT_SYMBOL_GPL(rcu_note_context_switch);
 860 
 861 /*
 862  * Because preemptible RCU does not exist, there are never any preempted
 863  * RCU readers.
 864  */
 865 static int rcu_preempt_blocked_readers_cgp(struct rcu_node *rnp)
 866 {
 867         return 0;
 868 }
 869 
 870 /*
 871  * Because there is no preemptible RCU, there can be no readers blocked.
 872  */
 873 static bool rcu_preempt_has_tasks(struct rcu_node *rnp)
 874 {
 875         return false;
 876 }
 877 
 878 /*
 879  * Because there is no preemptible RCU, there can be no deferred quiescent
 880  * states.
 881  */
 882 static bool rcu_preempt_need_deferred_qs(struct task_struct *t)
 883 {
 884         return false;
 885 }
 886 static void rcu_preempt_deferred_qs(struct task_struct *t) { }
 887 
 888 /*
 889  * Because there is no preemptible RCU, there can be no readers blocked,
 890  * so there is no need to check for blocked tasks.  So check only for
 891  * bogus qsmask values.
 892  */
 893 static void rcu_preempt_check_blocked_tasks(struct rcu_node *rnp)
 894 {
 895         WARN_ON_ONCE(rnp->qsmask);
 896 }
 897 
 898 /*
 899  * Check to see if this CPU is in a non-context-switch quiescent state,
 900  * namely user mode and idle loop.
 901  */
 902 static void rcu_flavor_sched_clock_irq(int user)
 903 {
 904         if (user || rcu_is_cpu_rrupt_from_idle()) {
 905 
 906                 /*
 907                  * Get here if this CPU took its interrupt from user
 908                  * mode or from the idle loop, and if this is not a
 909                  * nested interrupt.  In this case, the CPU is in
 910                  * a quiescent state, so note it.
 911                  *
 912                  * No memory barrier is required here because rcu_qs()
 913                  * references only CPU-local variables that other CPUs
 914                  * neither access nor modify, at least not while the
 915                  * corresponding CPU is online.
 916                  */
 917 
 918                 rcu_qs();
 919         }
 920 }
 921 
 922 /*
 923  * Because preemptible RCU does not exist, tasks cannot possibly exit
 924  * while in preemptible RCU read-side critical sections.
 925  */
 926 void exit_rcu(void)
 927 {
 928 }
 929 
 930 /*
 931  * Dump the guaranteed-empty blocked-tasks state.  Trust but verify.
 932  */
 933 static void
 934 dump_blkd_tasks(struct rcu_node *rnp, int ncheck)
 935 {
 936         WARN_ON_ONCE(!list_empty(&rnp->blkd_tasks));
 937 }
 938 
 939 #endif /* #else #ifdef CONFIG_PREEMPT_RCU */
 940 
 941 /*
 942  * If boosting, set rcuc kthreads to realtime priority.
 943  */
 944 static void rcu_cpu_kthread_setup(unsigned int cpu)
 945 {
 946 #ifdef CONFIG_RCU_BOOST
 947         struct sched_param sp;
 948 
 949         sp.sched_priority = kthread_prio;
 950         sched_setscheduler_nocheck(current, SCHED_FIFO, &sp);
 951 #endif /* #ifdef CONFIG_RCU_BOOST */
 952 }
 953 
 954 #ifdef CONFIG_RCU_BOOST
 955 
 956 /*
 957  * Carry out RCU priority boosting on the task indicated by ->exp_tasks
 958  * or ->boost_tasks, advancing the pointer to the next task in the
 959  * ->blkd_tasks list.
 960  *
 961  * Note that irqs must be enabled: boosting the task can block.
 962  * Returns 1 if there are more tasks needing to be boosted.
 963  */
 964 static int rcu_boost(struct rcu_node *rnp)
 965 {
 966         unsigned long flags;
 967         struct task_struct *t;
 968         struct list_head *tb;
 969 
 970         if (READ_ONCE(rnp->exp_tasks) == NULL &&
 971             READ_ONCE(rnp->boost_tasks) == NULL)
 972                 return 0;  /* Nothing left to boost. */
 973 
 974         raw_spin_lock_irqsave_rcu_node(rnp, flags);
 975 
 976         /*
 977          * Recheck under the lock: all tasks in need of boosting
 978          * might exit their RCU read-side critical sections on their own.
 979          */
 980         if (rnp->exp_tasks == NULL && rnp->boost_tasks == NULL) {
 981                 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 982                 return 0;
 983         }
 984 
 985         /*
 986          * Preferentially boost tasks blocking expedited grace periods.
 987          * This cannot starve the normal grace periods because a second
 988          * expedited grace period must boost all blocked tasks, including
 989          * those blocking the pre-existing normal grace period.
 990          */
 991         if (rnp->exp_tasks != NULL)
 992                 tb = rnp->exp_tasks;
 993         else
 994                 tb = rnp->boost_tasks;
 995 
 996         /*
 997          * We boost task t by manufacturing an rt_mutex that appears to
 998          * be held by task t.  We leave a pointer to that rt_mutex where
 999          * task t can find it, and task t will release the mutex when it
1000          * exits its outermost RCU read-side critical section.  Then
1001          * simply acquiring this artificial rt_mutex will boost task
1002          * t's priority.  (Thanks to tglx for suggesting this approach!)
1003          *
1004          * Note that task t must acquire rnp->lock to remove itself from
1005          * the ->blkd_tasks list, which it will do from exit() if from
1006          * nowhere else.  We therefore are guaranteed that task t will
1007          * stay around at least until we drop rnp->lock.  Note that
1008          * rnp->lock also resolves races between our priority boosting
1009          * and task t's exiting its outermost RCU read-side critical
1010          * section.
1011          */
1012         t = container_of(tb, struct task_struct, rcu_node_entry);
1013         rt_mutex_init_proxy_locked(&rnp->boost_mtx, t);
1014         raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
1015         /* Lock only for side effect: boosts task t's priority. */
1016         rt_mutex_lock(&rnp->boost_mtx);
1017         rt_mutex_unlock(&rnp->boost_mtx);  /* Then keep lockdep happy. */
1018 
1019         return READ_ONCE(rnp->exp_tasks) != NULL ||
1020                READ_ONCE(rnp->boost_tasks) != NULL;
1021 }
1022 
1023 /*
1024  * Priority-boosting kthread, one per leaf rcu_node.
1025  */
1026 static int rcu_boost_kthread(void *arg)
1027 {
1028         struct rcu_node *rnp = (struct rcu_node *)arg;
1029         int spincnt = 0;
1030         int more2boost;
1031 
1032         trace_rcu_utilization(TPS("Start boost kthread@init"));
1033         for (;;) {
1034                 rnp->boost_kthread_status = RCU_KTHREAD_WAITING;
1035                 trace_rcu_utilization(TPS("End boost kthread@rcu_wait"));
1036                 rcu_wait(rnp->boost_tasks || rnp->exp_tasks);
1037                 trace_rcu_utilization(TPS("Start boost kthread@rcu_wait"));
1038                 rnp->boost_kthread_status = RCU_KTHREAD_RUNNING;
1039                 more2boost = rcu_boost(rnp);
1040                 if (more2boost)
1041                         spincnt++;
1042                 else
1043                         spincnt = 0;
1044                 if (spincnt > 10) {
1045                         rnp->boost_kthread_status = RCU_KTHREAD_YIELDING;
1046                         trace_rcu_utilization(TPS("End boost kthread@rcu_yield"));
1047                         schedule_timeout_interruptible(2);
1048                         trace_rcu_utilization(TPS("Start boost kthread@rcu_yield"));
1049                         spincnt = 0;
1050                 }
1051         }
1052         /* NOTREACHED */
1053         trace_rcu_utilization(TPS("End boost kthread@notreached"));
1054         return 0;
1055 }
1056 
1057 /*
1058  * Check to see if it is time to start boosting RCU readers that are
1059  * blocking the current grace period, and, if so, tell the per-rcu_node
1060  * kthread to start boosting them.  If there is an expedited grace
1061  * period in progress, it is always time to boost.
1062  *
1063  * The caller must hold rnp->lock, which this function releases.
1064  * The ->boost_kthread_task is immortal, so we don't need to worry
1065  * about it going away.
1066  */
1067 static void rcu_initiate_boost(struct rcu_node *rnp, unsigned long flags)
1068         __releases(rnp->lock)
1069 {
1070         raw_lockdep_assert_held_rcu_node(rnp);
1071         if (!rcu_preempt_blocked_readers_cgp(rnp) && rnp->exp_tasks == NULL) {
1072                 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
1073                 return;
1074         }
1075         if (rnp->exp_tasks != NULL ||
1076             (rnp->gp_tasks != NULL &&
1077              rnp->boost_tasks == NULL &&
1078              rnp->qsmask == 0 &&
1079              ULONG_CMP_GE(jiffies, rnp->boost_time))) {
1080                 if (rnp->exp_tasks == NULL)
1081                         rnp->boost_tasks = rnp->gp_tasks;
1082                 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
1083                 rcu_wake_cond(rnp->boost_kthread_task,
1084                               rnp->boost_kthread_status);
1085         } else {
1086                 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
1087         }
1088 }
1089 
1090 /*
1091  * Is the current CPU running the RCU-callbacks kthread?
1092  * Caller must have preemption disabled.
1093  */
1094 static bool rcu_is_callbacks_kthread(void)
1095 {
1096         return __this_cpu_read(rcu_data.rcu_cpu_kthread_task) == current;
1097 }
1098 
1099 #define RCU_BOOST_DELAY_JIFFIES DIV_ROUND_UP(CONFIG_RCU_BOOST_DELAY * HZ, 1000)
1100 
1101 /*
1102  * Do priority-boost accounting for the start of a new grace period.
1103  */
1104 static void rcu_preempt_boost_start_gp(struct rcu_node *rnp)
1105 {
1106         rnp->boost_time = jiffies + RCU_BOOST_DELAY_JIFFIES;
1107 }
1108 
1109 /*
1110  * Create an RCU-boost kthread for the specified node if one does not
1111  * already exist.  We only create this kthread for preemptible RCU.
1112  * Returns zero if all is well, a negated errno otherwise.
1113  */
1114 static void rcu_spawn_one_boost_kthread(struct rcu_node *rnp)
1115 {
1116         int rnp_index = rnp - rcu_get_root();
1117         unsigned long flags;
1118         struct sched_param sp;
1119         struct task_struct *t;
1120 
1121         if (!IS_ENABLED(CONFIG_PREEMPT_RCU))
1122                 return;
1123 
1124         if (!rcu_scheduler_fully_active || rcu_rnp_online_cpus(rnp) == 0)
1125                 return;
1126 
1127         rcu_state.boost = 1;
1128 
1129         if (rnp->boost_kthread_task != NULL)
1130                 return;
1131 
1132         t = kthread_create(rcu_boost_kthread, (void *)rnp,
1133                            "rcub/%d", rnp_index);
1134         if (WARN_ON_ONCE(IS_ERR(t)))
1135                 return;
1136 
1137         raw_spin_lock_irqsave_rcu_node(rnp, flags);
1138         rnp->boost_kthread_task = t;
1139         raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
1140         sp.sched_priority = kthread_prio;
1141         sched_setscheduler_nocheck(t, SCHED_FIFO, &sp);
1142         wake_up_process(t); /* get to TASK_INTERRUPTIBLE quickly. */
1143 }
1144 
1145 /*
1146  * Set the per-rcu_node kthread's affinity to cover all CPUs that are
1147  * served by the rcu_node in question.  The CPU hotplug lock is still
1148  * held, so the value of rnp->qsmaskinit will be stable.
1149  *
1150  * We don't include outgoingcpu in the affinity set, use -1 if there is
1151  * no outgoing CPU.  If there are no CPUs left in the affinity set,
1152  * this function allows the kthread to execute on any CPU.
1153  */
1154 static void rcu_boost_kthread_setaffinity(struct rcu_node *rnp, int outgoingcpu)
1155 {
1156         struct task_struct *t = rnp->boost_kthread_task;
1157         unsigned long mask = rcu_rnp_online_cpus(rnp);
1158         cpumask_var_t cm;
1159         int cpu;
1160 
1161         if (!t)
1162                 return;
1163         if (!zalloc_cpumask_var(&cm, GFP_KERNEL))
1164                 return;
1165         for_each_leaf_node_possible_cpu(rnp, cpu)
1166                 if ((mask & leaf_node_cpu_bit(rnp, cpu)) &&
1167                     cpu != outgoingcpu)
1168                         cpumask_set_cpu(cpu, cm);
1169         if (cpumask_weight(cm) == 0)
1170                 cpumask_setall(cm);
1171         set_cpus_allowed_ptr(t, cm);
1172         free_cpumask_var(cm);
1173 }
1174 
1175 /*
1176  * Spawn boost kthreads -- called as soon as the scheduler is running.
1177  */
1178 static void __init rcu_spawn_boost_kthreads(void)
1179 {
1180         struct rcu_node *rnp;
1181 
1182         rcu_for_each_leaf_node(rnp)
1183                 rcu_spawn_one_boost_kthread(rnp);
1184 }
1185 
1186 static void rcu_prepare_kthreads(int cpu)
1187 {
1188         struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
1189         struct rcu_node *rnp = rdp->mynode;
1190 
1191         /* Fire up the incoming CPU's kthread and leaf rcu_node kthread. */
1192         if (rcu_scheduler_fully_active)
1193                 rcu_spawn_one_boost_kthread(rnp);
1194 }
1195 
1196 #else /* #ifdef CONFIG_RCU_BOOST */
1197 
1198 static void rcu_initiate_boost(struct rcu_node *rnp, unsigned long flags)
1199         __releases(rnp->lock)
1200 {
1201         raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
1202 }
1203 
1204 static bool rcu_is_callbacks_kthread(void)
1205 {
1206         return false;
1207 }
1208 
1209 static void rcu_preempt_boost_start_gp(struct rcu_node *rnp)
1210 {
1211 }
1212 
1213 static void rcu_boost_kthread_setaffinity(struct rcu_node *rnp, int outgoingcpu)
1214 {
1215 }
1216 
1217 static void __init rcu_spawn_boost_kthreads(void)
1218 {
1219 }
1220 
1221 static void rcu_prepare_kthreads(int cpu)
1222 {
1223 }
1224 
1225 #endif /* #else #ifdef CONFIG_RCU_BOOST */
1226 
1227 #if !defined(CONFIG_RCU_FAST_NO_HZ)
1228 
1229 /*
1230  * Check to see if any future non-offloaded RCU-related work will need
1231  * to be done by the current CPU, even if none need be done immediately,
1232  * returning 1 if so.  This function is part of the RCU implementation;
1233  * it is -not- an exported member of the RCU API.
1234  *
1235  * Because we not have RCU_FAST_NO_HZ, just check whether or not this
1236  * CPU has RCU callbacks queued.
1237  */
1238 int rcu_needs_cpu(u64 basemono, u64 *nextevt)
1239 {
1240         *nextevt = KTIME_MAX;
1241         return !rcu_segcblist_empty(&this_cpu_ptr(&rcu_data)->cblist) &&
1242                !rcu_segcblist_is_offloaded(&this_cpu_ptr(&rcu_data)->cblist);
1243 }
1244 
1245 /*
1246  * Because we do not have RCU_FAST_NO_HZ, don't bother cleaning up
1247  * after it.
1248  */
1249 static void rcu_cleanup_after_idle(void)
1250 {
1251 }
1252 
1253 /*
1254  * Do the idle-entry grace-period work, which, because CONFIG_RCU_FAST_NO_HZ=n,
1255  * is nothing.
1256  */
1257 static void rcu_prepare_for_idle(void)
1258 {
1259 }
1260 
1261 #else /* #if !defined(CONFIG_RCU_FAST_NO_HZ) */
1262 
1263 /*
1264  * This code is invoked when a CPU goes idle, at which point we want
1265  * to have the CPU do everything required for RCU so that it can enter
1266  * the energy-efficient dyntick-idle mode.  This is handled by a
1267  * state machine implemented by rcu_prepare_for_idle() below.
1268  *
1269  * The following three proprocessor symbols control this state machine:
1270  *
1271  * RCU_IDLE_GP_DELAY gives the number of jiffies that a CPU is permitted
1272  *      to sleep in dyntick-idle mode with RCU callbacks pending.  This
1273  *      is sized to be roughly one RCU grace period.  Those energy-efficiency
1274  *      benchmarkers who might otherwise be tempted to set this to a large
1275  *      number, be warned: Setting RCU_IDLE_GP_DELAY too high can hang your
1276  *      system.  And if you are -that- concerned about energy efficiency,
1277  *      just power the system down and be done with it!
1278  * RCU_IDLE_LAZY_GP_DELAY gives the number of jiffies that a CPU is
1279  *      permitted to sleep in dyntick-idle mode with only lazy RCU
1280  *      callbacks pending.  Setting this too high can OOM your system.
1281  *
1282  * The values below work well in practice.  If future workloads require
1283  * adjustment, they can be converted into kernel config parameters, though
1284  * making the state machine smarter might be a better option.
1285  */
1286 #define RCU_IDLE_GP_DELAY 4             /* Roughly one grace period. */
1287 #define RCU_IDLE_LAZY_GP_DELAY (6 * HZ) /* Roughly six seconds. */
1288 
1289 static int rcu_idle_gp_delay = RCU_IDLE_GP_DELAY;
1290 module_param(rcu_idle_gp_delay, int, 0644);
1291 static int rcu_idle_lazy_gp_delay = RCU_IDLE_LAZY_GP_DELAY;
1292 module_param(rcu_idle_lazy_gp_delay, int, 0644);
1293 
1294 /*
1295  * Try to advance callbacks on the current CPU, but only if it has been
1296  * awhile since the last time we did so.  Afterwards, if there are any
1297  * callbacks ready for immediate invocation, return true.
1298  */
1299 static bool __maybe_unused rcu_try_advance_all_cbs(void)
1300 {
1301         bool cbs_ready = false;
1302         struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
1303         struct rcu_node *rnp;
1304 
1305         /* Exit early if we advanced recently. */
1306         if (jiffies == rdp->last_advance_all)
1307                 return false;
1308         rdp->last_advance_all = jiffies;
1309 
1310         rnp = rdp->mynode;
1311 
1312         /*
1313          * Don't bother checking unless a grace period has
1314          * completed since we last checked and there are
1315          * callbacks not yet ready to invoke.
1316          */
1317         if ((rcu_seq_completed_gp(rdp->gp_seq,
1318                                   rcu_seq_current(&rnp->gp_seq)) ||
1319              unlikely(READ_ONCE(rdp->gpwrap))) &&
1320             rcu_segcblist_pend_cbs(&rdp->cblist))
1321                 note_gp_changes(rdp);
1322 
1323         if (rcu_segcblist_ready_cbs(&rdp->cblist))
1324                 cbs_ready = true;
1325         return cbs_ready;
1326 }
1327 
1328 /*
1329  * Allow the CPU to enter dyntick-idle mode unless it has callbacks ready
1330  * to invoke.  If the CPU has callbacks, try to advance them.  Tell the
1331  * caller to set the timeout based on whether or not there are non-lazy
1332  * callbacks.
1333  *
1334  * The caller must have disabled interrupts.
1335  */
1336 int rcu_needs_cpu(u64 basemono, u64 *nextevt)
1337 {
1338         struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
1339         unsigned long dj;
1340 
1341         lockdep_assert_irqs_disabled();
1342 
1343         /* If no non-offloaded callbacks, RCU doesn't need the CPU. */
1344         if (rcu_segcblist_empty(&rdp->cblist) ||
1345             rcu_segcblist_is_offloaded(&this_cpu_ptr(&rcu_data)->cblist)) {
1346                 *nextevt = KTIME_MAX;
1347                 return 0;
1348         }
1349 
1350         /* Attempt to advance callbacks. */
1351         if (rcu_try_advance_all_cbs()) {
1352                 /* Some ready to invoke, so initiate later invocation. */
1353                 invoke_rcu_core();
1354                 return 1;
1355         }
1356         rdp->last_accelerate = jiffies;
1357 
1358         /* Request timer delay depending on laziness, and round. */
1359         rdp->all_lazy = !rcu_segcblist_n_nonlazy_cbs(&rdp->cblist);
1360         if (rdp->all_lazy) {
1361                 dj = round_jiffies(rcu_idle_lazy_gp_delay + jiffies) - jiffies;
1362         } else {
1363                 dj = round_up(rcu_idle_gp_delay + jiffies,
1364                                rcu_idle_gp_delay) - jiffies;
1365         }
1366         *nextevt = basemono + dj * TICK_NSEC;
1367         return 0;
1368 }
1369 
1370 /*
1371  * Prepare a CPU for idle from an RCU perspective.  The first major task
1372  * is to sense whether nohz mode has been enabled or disabled via sysfs.
1373  * The second major task is to check to see if a non-lazy callback has
1374  * arrived at a CPU that previously had only lazy callbacks.  The third
1375  * major task is to accelerate (that is, assign grace-period numbers to)
1376  * any recently arrived callbacks.
1377  *
1378  * The caller must have disabled interrupts.
1379  */
1380 static void rcu_prepare_for_idle(void)
1381 {
1382         bool needwake;
1383         struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
1384         struct rcu_node *rnp;
1385         int tne;
1386 
1387         lockdep_assert_irqs_disabled();
1388         if (rcu_segcblist_is_offloaded(&rdp->cblist))
1389                 return;
1390 
1391         /* Handle nohz enablement switches conservatively. */
1392         tne = READ_ONCE(tick_nohz_active);
1393         if (tne != rdp->tick_nohz_enabled_snap) {
1394                 if (!rcu_segcblist_empty(&rdp->cblist))
1395                         invoke_rcu_core(); /* force nohz to see update. */
1396                 rdp->tick_nohz_enabled_snap = tne;
1397                 return;
1398         }
1399         if (!tne)
1400                 return;
1401 
1402         /*
1403          * If a non-lazy callback arrived at a CPU having only lazy
1404          * callbacks, invoke RCU core for the side-effect of recalculating
1405          * idle duration on re-entry to idle.
1406          */
1407         if (rdp->all_lazy && rcu_segcblist_n_nonlazy_cbs(&rdp->cblist)) {
1408                 rdp->all_lazy = false;
1409                 invoke_rcu_core();
1410                 return;
1411         }
1412 
1413         /*
1414          * If we have not yet accelerated this jiffy, accelerate all
1415          * callbacks on this CPU.
1416          */
1417         if (rdp->last_accelerate == jiffies)
1418                 return;
1419         rdp->last_accelerate = jiffies;
1420         if (rcu_segcblist_pend_cbs(&rdp->cblist)) {
1421                 rnp = rdp->mynode;
1422                 raw_spin_lock_rcu_node(rnp); /* irqs already disabled. */
1423                 needwake = rcu_accelerate_cbs(rnp, rdp);
1424                 raw_spin_unlock_rcu_node(rnp); /* irqs remain disabled. */
1425                 if (needwake)
1426                         rcu_gp_kthread_wake();
1427         }
1428 }
1429 
1430 /*
1431  * Clean up for exit from idle.  Attempt to advance callbacks based on
1432  * any grace periods that elapsed while the CPU was idle, and if any
1433  * callbacks are now ready to invoke, initiate invocation.
1434  */
1435 static void rcu_cleanup_after_idle(void)
1436 {
1437         struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
1438 
1439         lockdep_assert_irqs_disabled();
1440         if (rcu_segcblist_is_offloaded(&rdp->cblist))
1441                 return;
1442         if (rcu_try_advance_all_cbs())
1443                 invoke_rcu_core();
1444 }
1445 
1446 #endif /* #else #if !defined(CONFIG_RCU_FAST_NO_HZ) */
1447 
1448 #ifdef CONFIG_RCU_NOCB_CPU
1449 
1450 /*
1451  * Offload callback processing from the boot-time-specified set of CPUs
1452  * specified by rcu_nocb_mask.  For the CPUs in the set, there are kthreads
1453  * created that pull the callbacks from the corresponding CPU, wait for
1454  * a grace period to elapse, and invoke the callbacks.  These kthreads
1455  * are organized into GP kthreads, which manage incoming callbacks, wait for
1456  * grace periods, and awaken CB kthreads, and the CB kthreads, which only
1457  * invoke callbacks.  Each GP kthread invokes its own CBs.  The no-CBs CPUs
1458  * do a wake_up() on their GP kthread when they insert a callback into any
1459  * empty list, unless the rcu_nocb_poll boot parameter has been specified,
1460  * in which case each kthread actively polls its CPU.  (Which isn't so great
1461  * for energy efficiency, but which does reduce RCU's overhead on that CPU.)
1462  *
1463  * This is intended to be used in conjunction with Frederic Weisbecker's
1464  * adaptive-idle work, which would seriously reduce OS jitter on CPUs
1465  * running CPU-bound user-mode computations.
1466  *
1467  * Offloading of callbacks can also be used as an energy-efficiency
1468  * measure because CPUs with no RCU callbacks queued are more aggressive
1469  * about entering dyntick-idle mode.
1470  */
1471 
1472 
1473 /*
1474  * Parse the boot-time rcu_nocb_mask CPU list from the kernel parameters.
1475  * The string after the "rcu_nocbs=" is either "all" for all CPUs, or a
1476  * comma-separated list of CPUs and/or CPU ranges.  If an invalid list is
1477  * given, a warning is emitted and all CPUs are offloaded.
1478  */
1479 static int __init rcu_nocb_setup(char *str)
1480 {
1481         alloc_bootmem_cpumask_var(&rcu_nocb_mask);
1482         if (!strcasecmp(str, "all"))
1483                 cpumask_setall(rcu_nocb_mask);
1484         else
1485                 if (cpulist_parse(str, rcu_nocb_mask)) {
1486                         pr_warn("rcu_nocbs= bad CPU range, all CPUs set\n");
1487                         cpumask_setall(rcu_nocb_mask);
1488                 }
1489         return 1;
1490 }
1491 __setup("rcu_nocbs=", rcu_nocb_setup);
1492 
1493 static int __init parse_rcu_nocb_poll(char *arg)
1494 {
1495         rcu_nocb_poll = true;
1496         return 0;
1497 }
1498 early_param("rcu_nocb_poll", parse_rcu_nocb_poll);
1499 
1500 /*
1501  * Don't bother bypassing ->cblist if the call_rcu() rate is low.
1502  * After all, the main point of bypassing is to avoid lock contention
1503  * on ->nocb_lock, which only can happen at high call_rcu() rates.
1504  */
1505 int nocb_nobypass_lim_per_jiffy = 16 * 1000 / HZ;
1506 module_param(nocb_nobypass_lim_per_jiffy, int, 0);
1507 
1508 /*
1509  * Acquire the specified rcu_data structure's ->nocb_bypass_lock.  If the
1510  * lock isn't immediately available, increment ->nocb_lock_contended to
1511  * flag the contention.
1512  */
1513 static void rcu_nocb_bypass_lock(struct rcu_data *rdp)
1514 {
1515         lockdep_assert_irqs_disabled();
1516         if (raw_spin_trylock(&rdp->nocb_bypass_lock))
1517                 return;
1518         atomic_inc(&rdp->nocb_lock_contended);
1519         WARN_ON_ONCE(smp_processor_id() != rdp->cpu);
1520         smp_mb__after_atomic(); /* atomic_inc() before lock. */
1521         raw_spin_lock(&rdp->nocb_bypass_lock);
1522         smp_mb__before_atomic(); /* atomic_dec() after lock. */
1523         atomic_dec(&rdp->nocb_lock_contended);
1524 }
1525 
1526 /*
1527  * Spinwait until the specified rcu_data structure's ->nocb_lock is
1528  * not contended.  Please note that this is extremely special-purpose,
1529  * relying on the fact that at most two kthreads and one CPU contend for
1530  * this lock, and also that the two kthreads are guaranteed to have frequent
1531  * grace-period-duration time intervals between successive acquisitions
1532  * of the lock.  This allows us to use an extremely simple throttling
1533  * mechanism, and further to apply it only to the CPU doing floods of
1534  * call_rcu() invocations.  Don't try this at home!
1535  */
1536 static void rcu_nocb_wait_contended(struct rcu_data *rdp)
1537 {
1538         WARN_ON_ONCE(smp_processor_id() != rdp->cpu);
1539         while (WARN_ON_ONCE(atomic_read(&rdp->nocb_lock_contended)))
1540                 cpu_relax();
1541 }
1542 
1543 /*
1544  * Conditionally acquire the specified rcu_data structure's
1545  * ->nocb_bypass_lock.
1546  */
1547 static bool rcu_nocb_bypass_trylock(struct rcu_data *rdp)
1548 {
1549         lockdep_assert_irqs_disabled();
1550         return raw_spin_trylock(&rdp->nocb_bypass_lock);
1551 }
1552 
1553 /*
1554  * Release the specified rcu_data structure's ->nocb_bypass_lock.
1555  */
1556 static void rcu_nocb_bypass_unlock(struct rcu_data *rdp)
1557 {
1558         lockdep_assert_irqs_disabled();
1559         raw_spin_unlock(&rdp->nocb_bypass_lock);
1560 }
1561 
1562 /*
1563  * Acquire the specified rcu_data structure's ->nocb_lock, but only
1564  * if it corresponds to a no-CBs CPU.
1565  */
1566 static void rcu_nocb_lock(struct rcu_data *rdp)
1567 {
1568         lockdep_assert_irqs_disabled();
1569         if (!rcu_segcblist_is_offloaded(&rdp->cblist))
1570                 return;
1571         raw_spin_lock(&rdp->nocb_lock);
1572 }
1573 
1574 /*
1575  * Release the specified rcu_data structure's ->nocb_lock, but only
1576  * if it corresponds to a no-CBs CPU.
1577  */
1578 static void rcu_nocb_unlock(struct rcu_data *rdp)
1579 {
1580         if (rcu_segcblist_is_offloaded(&rdp->cblist)) {
1581                 lockdep_assert_irqs_disabled();
1582                 raw_spin_unlock(&rdp->nocb_lock);
1583         }
1584 }
1585 
1586 /*
1587  * Release the specified rcu_data structure's ->nocb_lock and restore
1588  * interrupts, but only if it corresponds to a no-CBs CPU.
1589  */
1590 static void rcu_nocb_unlock_irqrestore(struct rcu_data *rdp,
1591                                        unsigned long flags)
1592 {
1593         if (rcu_segcblist_is_offloaded(&rdp->cblist)) {
1594                 lockdep_assert_irqs_disabled();
1595                 raw_spin_unlock_irqrestore(&rdp->nocb_lock, flags);
1596         } else {
1597                 local_irq_restore(flags);
1598         }
1599 }
1600 
1601 /* Lockdep check that ->cblist may be safely accessed. */
1602 static void rcu_lockdep_assert_cblist_protected(struct rcu_data *rdp)
1603 {
1604         lockdep_assert_irqs_disabled();
1605         if (rcu_segcblist_is_offloaded(&rdp->cblist) &&
1606             cpu_online(rdp->cpu))
1607                 lockdep_assert_held(&rdp->nocb_lock);
1608 }
1609 
1610 /*
1611  * Wake up any no-CBs CPUs' kthreads that were waiting on the just-ended
1612  * grace period.
1613  */
1614 static void rcu_nocb_gp_cleanup(struct swait_queue_head *sq)
1615 {
1616         swake_up_all(sq);
1617 }
1618 
1619 static struct swait_queue_head *rcu_nocb_gp_get(struct rcu_node *rnp)
1620 {
1621         return &rnp->nocb_gp_wq[rcu_seq_ctr(rnp->gp_seq) & 0x1];
1622 }
1623 
1624 static void rcu_init_one_nocb(struct rcu_node *rnp)
1625 {
1626         init_swait_queue_head(&rnp->nocb_gp_wq[0]);
1627         init_swait_queue_head(&rnp->nocb_gp_wq[1]);
1628 }
1629 
1630 /* Is the specified CPU a no-CBs CPU? */
1631 bool rcu_is_nocb_cpu(int cpu)
1632 {
1633         if (cpumask_available(rcu_nocb_mask))
1634                 return cpumask_test_cpu(cpu, rcu_nocb_mask);
1635         return false;
1636 }
1637 
1638 /*
1639  * Kick the GP kthread for this NOCB group.  Caller holds ->nocb_lock
1640  * and this function releases it.
1641  */
1642 static void wake_nocb_gp(struct rcu_data *rdp, bool force,
1643                            unsigned long flags)
1644         __releases(rdp->nocb_lock)
1645 {
1646         bool needwake = false;
1647         struct rcu_data *rdp_gp = rdp->nocb_gp_rdp;
1648 
1649         lockdep_assert_held(&rdp->nocb_lock);
1650         if (!READ_ONCE(rdp_gp->nocb_gp_kthread)) {
1651                 trace_rcu_nocb_wake(rcu_state.name, rdp->cpu,
1652                                     TPS("AlreadyAwake"));
1653                 rcu_nocb_unlock_irqrestore(rdp, flags);
1654                 return;
1655         }
1656         del_timer(&rdp->nocb_timer);
1657         rcu_nocb_unlock_irqrestore(rdp, flags);
1658         raw_spin_lock_irqsave(&rdp_gp->nocb_gp_lock, flags);
1659         if (force || READ_ONCE(rdp_gp->nocb_gp_sleep)) {
1660                 WRITE_ONCE(rdp_gp->nocb_gp_sleep, false);
1661                 needwake = true;
1662                 trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("DoWake"));
1663         }
1664         raw_spin_unlock_irqrestore(&rdp_gp->nocb_gp_lock, flags);
1665         if (needwake)
1666                 wake_up_process(rdp_gp->nocb_gp_kthread);
1667 }
1668 
1669 /*
1670  * Arrange to wake the GP kthread for this NOCB group at some future
1671  * time when it is safe to do so.
1672  */
1673 static void wake_nocb_gp_defer(struct rcu_data *rdp, int waketype,
1674                                const char *reason)
1675 {
1676         if (rdp->nocb_defer_wakeup == RCU_NOCB_WAKE_NOT)
1677                 mod_timer(&rdp->nocb_timer, jiffies + 1);
1678         if (rdp->nocb_defer_wakeup < waketype)
1679                 WRITE_ONCE(rdp->nocb_defer_wakeup, waketype);
1680         trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, reason);
1681 }
1682 
1683 /*
1684  * Flush the ->nocb_bypass queue into ->cblist, enqueuing rhp if non-NULL.
1685  * However, if there is a callback to be enqueued and if ->nocb_bypass
1686  * proves to be initially empty, just return false because the no-CB GP
1687  * kthread may need to be awakened in this case.
1688  *
1689  * Note that this function always returns true if rhp is NULL.
1690  */
1691 static bool rcu_nocb_do_flush_bypass(struct rcu_data *rdp, struct rcu_head *rhp,
1692                                      unsigned long j)
1693 {
1694         struct rcu_cblist rcl;
1695 
1696         WARN_ON_ONCE(!rcu_segcblist_is_offloaded(&rdp->cblist));
1697         rcu_lockdep_assert_cblist_protected(rdp);
1698         lockdep_assert_held(&rdp->nocb_bypass_lock);
1699         if (rhp && !rcu_cblist_n_cbs(&rdp->nocb_bypass)) {
1700                 raw_spin_unlock(&rdp->nocb_bypass_lock);
1701                 return false;
1702         }
1703         /* Note: ->cblist.len already accounts for ->nocb_bypass contents. */
1704         if (rhp)
1705                 rcu_segcblist_inc_len(&rdp->cblist); /* Must precede enqueue. */
1706         rcu_cblist_flush_enqueue(&rcl, &rdp->nocb_bypass, rhp);
1707         rcu_segcblist_insert_pend_cbs(&rdp->cblist, &rcl);
1708         WRITE_ONCE(rdp->nocb_bypass_first, j);
1709         rcu_nocb_bypass_unlock(rdp);
1710         return true;
1711 }
1712 
1713 /*
1714  * Flush the ->nocb_bypass queue into ->cblist, enqueuing rhp if non-NULL.
1715  * However, if there is a callback to be enqueued and if ->nocb_bypass
1716  * proves to be initially empty, just return false because the no-CB GP
1717  * kthread may need to be awakened in this case.
1718  *
1719  * Note that this function always returns true if rhp is NULL.
1720  */
1721 static bool rcu_nocb_flush_bypass(struct rcu_data *rdp, struct rcu_head *rhp,
1722                                   unsigned long j)
1723 {
1724         if (!rcu_segcblist_is_offloaded(&rdp->cblist))
1725                 return true;
1726         rcu_lockdep_assert_cblist_protected(rdp);
1727         rcu_nocb_bypass_lock(rdp);
1728         return rcu_nocb_do_flush_bypass(rdp, rhp, j);
1729 }
1730 
1731 /*
1732  * If the ->nocb_bypass_lock is immediately available, flush the
1733  * ->nocb_bypass queue into ->cblist.
1734  */
1735 static void rcu_nocb_try_flush_bypass(struct rcu_data *rdp, unsigned long j)
1736 {
1737         rcu_lockdep_assert_cblist_protected(rdp);
1738         if (!rcu_segcblist_is_offloaded(&rdp->cblist) ||
1739             !rcu_nocb_bypass_trylock(rdp))
1740                 return;
1741         WARN_ON_ONCE(!rcu_nocb_do_flush_bypass(rdp, NULL, j));
1742 }
1743 
1744 /*
1745  * See whether it is appropriate to use the ->nocb_bypass list in order
1746  * to control contention on ->nocb_lock.  A limited number of direct
1747  * enqueues are permitted into ->cblist per jiffy.  If ->nocb_bypass
1748  * is non-empty, further callbacks must be placed into ->nocb_bypass,
1749  * otherwise rcu_barrier() breaks.  Use rcu_nocb_flush_bypass() to switch
1750  * back to direct use of ->cblist.  However, ->nocb_bypass should not be
1751  * used if ->cblist is empty, because otherwise callbacks can be stranded
1752  * on ->nocb_bypass because we cannot count on the current CPU ever again
1753  * invoking call_rcu().  The general rule is that if ->nocb_bypass is
1754  * non-empty, the corresponding no-CBs grace-period kthread must not be
1755  * in an indefinite sleep state.
1756  *
1757  * Finally, it is not permitted to use the bypass during early boot,
1758  * as doing so would confuse the auto-initialization code.  Besides
1759  * which, there is no point in worrying about lock contention while
1760  * there is only one CPU in operation.
1761  */
1762 static bool rcu_nocb_try_bypass(struct rcu_data *rdp, struct rcu_head *rhp,
1763                                 bool *was_alldone, unsigned long flags)
1764 {
1765         unsigned long c;
1766         unsigned long cur_gp_seq;
1767         unsigned long j = jiffies;
1768         long ncbs = rcu_cblist_n_cbs(&rdp->nocb_bypass);
1769 
1770         if (!rcu_segcblist_is_offloaded(&rdp->cblist)) {
1771                 *was_alldone = !rcu_segcblist_pend_cbs(&rdp->cblist);
1772                 return false; /* Not offloaded, no bypassing. */
1773         }
1774         lockdep_assert_irqs_disabled();
1775 
1776         // Don't use ->nocb_bypass during early boot.
1777         if (rcu_scheduler_active != RCU_SCHEDULER_RUNNING) {
1778                 rcu_nocb_lock(rdp);
1779                 WARN_ON_ONCE(rcu_cblist_n_cbs(&rdp->nocb_bypass));
1780                 *was_alldone = !rcu_segcblist_pend_cbs(&rdp->cblist);
1781                 return false;
1782         }
1783 
1784         // If we have advanced to a new jiffy, reset counts to allow
1785         // moving back from ->nocb_bypass to ->cblist.
1786         if (j == rdp->nocb_nobypass_last) {
1787                 c = rdp->nocb_nobypass_count + 1;
1788         } else {
1789                 WRITE_ONCE(rdp->nocb_nobypass_last, j);
1790                 c = rdp->nocb_nobypass_count - nocb_nobypass_lim_per_jiffy;
1791                 if (ULONG_CMP_LT(rdp->nocb_nobypass_count,
1792                                  nocb_nobypass_lim_per_jiffy))
1793                         c = 0;
1794                 else if (c > nocb_nobypass_lim_per_jiffy)
1795                         c = nocb_nobypass_lim_per_jiffy;
1796         }
1797         WRITE_ONCE(rdp->nocb_nobypass_count, c);
1798 
1799         // If there hasn't yet been all that many ->cblist enqueues
1800         // this jiffy, tell the caller to enqueue onto ->cblist.  But flush
1801         // ->nocb_bypass first.
1802         if (rdp->nocb_nobypass_count < nocb_nobypass_lim_per_jiffy) {
1803                 rcu_nocb_lock(rdp);
1804                 *was_alldone = !rcu_segcblist_pend_cbs(&rdp->cblist);
1805                 if (*was_alldone)
1806                         trace_rcu_nocb_wake(rcu_state.name, rdp->cpu,
1807                                             TPS("FirstQ"));
1808                 WARN_ON_ONCE(!rcu_nocb_flush_bypass(rdp, NULL, j));
1809                 WARN_ON_ONCE(rcu_cblist_n_cbs(&rdp->nocb_bypass));
1810                 return false; // Caller must enqueue the callback.
1811         }
1812 
1813         // If ->nocb_bypass has been used too long or is too full,
1814         // flush ->nocb_bypass to ->cblist.
1815         if ((ncbs && j != READ_ONCE(rdp->nocb_bypass_first)) ||
1816             ncbs >= qhimark) {
1817                 rcu_nocb_lock(rdp);
1818                 if (!rcu_nocb_flush_bypass(rdp, rhp, j)) {
1819                         *was_alldone = !rcu_segcblist_pend_cbs(&rdp->cblist);
1820                         if (*was_alldone)
1821                                 trace_rcu_nocb_wake(rcu_state.name, rdp->cpu,
1822                                                     TPS("FirstQ"));
1823                         WARN_ON_ONCE(rcu_cblist_n_cbs(&rdp->nocb_bypass));
1824                         return false; // Caller must enqueue the callback.
1825                 }
1826                 if (j != rdp->nocb_gp_adv_time &&
1827                     rcu_segcblist_nextgp(&rdp->cblist, &cur_gp_seq) &&
1828                     rcu_seq_done(&rdp->mynode->gp_seq, cur_gp_seq)) {
1829                         rcu_advance_cbs_nowake(rdp->mynode, rdp);
1830                         rdp->nocb_gp_adv_time = j;
1831                 }
1832                 rcu_nocb_unlock_irqrestore(rdp, flags);
1833                 return true; // Callback already enqueued.
1834         }
1835 
1836         // We need to use the bypass.
1837         rcu_nocb_wait_contended(rdp);
1838         rcu_nocb_bypass_lock(rdp);
1839         ncbs = rcu_cblist_n_cbs(&rdp->nocb_bypass);
1840         rcu_segcblist_inc_len(&rdp->cblist); /* Must precede enqueue. */
1841         rcu_cblist_enqueue(&rdp->nocb_bypass, rhp);
1842         if (!ncbs) {
1843                 WRITE_ONCE(rdp->nocb_bypass_first, j);
1844                 trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("FirstBQ"));
1845         }
1846         rcu_nocb_bypass_unlock(rdp);
1847         smp_mb(); /* Order enqueue before wake. */
1848         if (ncbs) {
1849                 local_irq_restore(flags);
1850         } else {
1851                 // No-CBs GP kthread might be indefinitely asleep, if so, wake.
1852                 rcu_nocb_lock(rdp); // Rare during call_rcu() flood.
1853                 if (!rcu_segcblist_pend_cbs(&rdp->cblist)) {
1854                         trace_rcu_nocb_wake(rcu_state.name, rdp->cpu,
1855                                             TPS("FirstBQwake"));
1856                         __call_rcu_nocb_wake(rdp, true, flags);
1857                 } else {
1858                         trace_rcu_nocb_wake(rcu_state.name, rdp->cpu,
1859                                             TPS("FirstBQnoWake"));
1860                         rcu_nocb_unlock_irqrestore(rdp, flags);
1861                 }
1862         }
1863         return true; // Callback already enqueued.
1864 }
1865 
1866 /*
1867  * Awaken the no-CBs grace-period kthead if needed, either due to it
1868  * legitimately being asleep or due to overload conditions.
1869  *
1870  * If warranted, also wake up the kthread servicing this CPUs queues.
1871  */
1872 static void __call_rcu_nocb_wake(struct rcu_data *rdp, bool was_alldone,
1873                                  unsigned long flags)
1874                                  __releases(rdp->nocb_lock)
1875 {
1876         unsigned long cur_gp_seq;
1877         unsigned long j;
1878         long len;
1879         struct task_struct *t;
1880 
1881         // If we are being polled or there is no kthread, just leave.
1882         t = READ_ONCE(rdp->nocb_gp_kthread);
1883         if (rcu_nocb_poll || !t) {
1884                 trace_rcu_nocb_wake(rcu_state.name, rdp->cpu,
1885                                     TPS("WakeNotPoll"));
1886                 rcu_nocb_unlock_irqrestore(rdp, flags);
1887                 return;
1888         }
1889         // Need to actually to a wakeup.
1890         len = rcu_segcblist_n_cbs(&rdp->cblist);
1891         if (was_alldone) {
1892                 rdp->qlen_last_fqs_check = len;
1893                 if (!irqs_disabled_flags(flags)) {
1894                         /* ... if queue was empty ... */
1895                         wake_nocb_gp(rdp, false, flags);
1896                         trace_rcu_nocb_wake(rcu_state.name, rdp->cpu,
1897                                             TPS("WakeEmpty"));
1898                 } else {
1899                         wake_nocb_gp_defer(rdp, RCU_NOCB_WAKE,
1900                                            TPS("WakeEmptyIsDeferred"));
1901                         rcu_nocb_unlock_irqrestore(rdp, flags);
1902                 }
1903         } else if (len > rdp->qlen_last_fqs_check + qhimark) {
1904                 /* ... or if many callbacks queued. */
1905                 rdp->qlen_last_fqs_check = len;
1906                 j = jiffies;
1907                 if (j != rdp->nocb_gp_adv_time &&
1908                     rcu_segcblist_nextgp(&rdp->cblist, &cur_gp_seq) &&
1909                     rcu_seq_done(&rdp->mynode->gp_seq, cur_gp_seq)) {
1910                         rcu_advance_cbs_nowake(rdp->mynode, rdp);
1911                         rdp->nocb_gp_adv_time = j;
1912                 }
1913                 smp_mb(); /* Enqueue before timer_pending(). */
1914                 if ((rdp->nocb_cb_sleep ||
1915                      !rcu_segcblist_ready_cbs(&rdp->cblist)) &&
1916                     !timer_pending(&rdp->nocb_bypass_timer))
1917                         wake_nocb_gp_defer(rdp, RCU_NOCB_WAKE_FORCE,
1918                                            TPS("WakeOvfIsDeferred"));
1919                 rcu_nocb_unlock_irqrestore(rdp, flags);
1920         } else {
1921                 trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("WakeNot"));
1922                 rcu_nocb_unlock_irqrestore(rdp, flags);
1923         }
1924         return;
1925 }
1926 
1927 /* Wake up the no-CBs GP kthread to flush ->nocb_bypass. */
1928 static void do_nocb_bypass_wakeup_timer(struct timer_list *t)
1929 {
1930         unsigned long flags;
1931         struct rcu_data *rdp = from_timer(rdp, t, nocb_bypass_timer);
1932 
1933         trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("Timer"));
1934         rcu_nocb_lock_irqsave(rdp, flags);
1935         smp_mb__after_spinlock(); /* Timer expire before wakeup. */
1936         __call_rcu_nocb_wake(rdp, true, flags);
1937 }
1938 
1939 /*
1940  * No-CBs GP kthreads come here to wait for additional callbacks to show up
1941  * or for grace periods to end.
1942  */
1943 static void nocb_gp_wait(struct rcu_data *my_rdp)
1944 {
1945         bool bypass = false;
1946         long bypass_ncbs;
1947         int __maybe_unused cpu = my_rdp->cpu;
1948         unsigned long cur_gp_seq;
1949         unsigned long flags;
1950         bool gotcbs = false;
1951         unsigned long j = jiffies;
1952         bool needwait_gp = false; // This prevents actual uninitialized use.
1953         bool needwake;
1954         bool needwake_gp;
1955         struct rcu_data *rdp;
1956         struct rcu_node *rnp;
1957         unsigned long wait_gp_seq = 0; // Suppress "use uninitialized" warning.
1958 
1959         /*
1960          * Each pass through the following loop checks for CBs and for the
1961          * nearest grace period (if any) to wait for next.  The CB kthreads
1962          * and the global grace-period kthread are awakened if needed.
1963          */
1964         for (rdp = my_rdp; rdp; rdp = rdp->nocb_next_cb_rdp) {
1965                 trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("Check"));
1966                 rcu_nocb_lock_irqsave(rdp, flags);
1967                 bypass_ncbs = rcu_cblist_n_cbs(&rdp->nocb_bypass);
1968                 if (bypass_ncbs &&
1969                     (time_after(j, READ_ONCE(rdp->nocb_bypass_first) + 1) ||
1970                      bypass_ncbs > 2 * qhimark)) {
1971                         // Bypass full or old, so flush it.
1972                         (void)rcu_nocb_try_flush_bypass(rdp, j);
1973                         bypass_ncbs = rcu_cblist_n_cbs(&rdp->nocb_bypass);
1974                 } else if (!bypass_ncbs && rcu_segcblist_empty(&rdp->cblist)) {
1975                         rcu_nocb_unlock_irqrestore(rdp, flags);
1976                         continue; /* No callbacks here, try next. */
1977                 }
1978                 if (bypass_ncbs) {
1979                         trace_rcu_nocb_wake(rcu_state.name, rdp->cpu,
1980                                             TPS("Bypass"));
1981                         bypass = true;
1982                 }
1983                 rnp = rdp->mynode;
1984                 if (bypass) {  // Avoid race with first bypass CB.
1985                         WRITE_ONCE(my_rdp->nocb_defer_wakeup,
1986                                    RCU_NOCB_WAKE_NOT);
1987                         del_timer(&my_rdp->nocb_timer);
1988                 }
1989                 // Advance callbacks if helpful and low contention.
1990                 needwake_gp = false;
1991                 if (!rcu_segcblist_restempty(&rdp->cblist,
1992                                              RCU_NEXT_READY_TAIL) ||
1993                     (rcu_segcblist_nextgp(&rdp->cblist, &cur_gp_seq) &&
1994                      rcu_seq_done(&rnp->gp_seq, cur_gp_seq))) {
1995                         raw_spin_lock_rcu_node(rnp); /* irqs disabled. */
1996                         needwake_gp = rcu_advance_cbs(rnp, rdp);
1997                         raw_spin_unlock_rcu_node(rnp); /* irqs disabled. */
1998                 }
1999                 // Need to wait on some grace period?
2000                 WARN_ON_ONCE(!rcu_segcblist_restempty(&rdp->cblist,
2001                                                       RCU_NEXT_READY_TAIL));
2002                 if (rcu_segcblist_nextgp(&rdp->cblist, &cur_gp_seq)) {
2003                         if (!needwait_gp ||
2004                             ULONG_CMP_LT(cur_gp_seq, wait_gp_seq))
2005                                 wait_gp_seq = cur_gp_seq;
2006                         needwait_gp = true;
2007                         trace_rcu_nocb_wake(rcu_state.name, rdp->cpu,
2008                                             TPS("NeedWaitGP"));
2009                 }
2010                 if (rcu_segcblist_ready_cbs(&rdp->cblist)) {
2011                         needwake = rdp->nocb_cb_sleep;
2012                         WRITE_ONCE(rdp->nocb_cb_sleep, false);
2013                         smp_mb(); /* CB invocation -after- GP end. */
2014                 } else {
2015                         needwake = false;
2016                 }
2017                 rcu_nocb_unlock_irqrestore(rdp, flags);
2018                 if (needwake) {
2019                         swake_up_one(&rdp->nocb_cb_wq);
2020                         gotcbs = true;
2021                 }
2022                 if (needwake_gp)
2023                         rcu_gp_kthread_wake();
2024         }
2025 
2026         my_rdp->nocb_gp_bypass = bypass;
2027         my_rdp->nocb_gp_gp = needwait_gp;
2028         my_rdp->nocb_gp_seq = needwait_gp ? wait_gp_seq : 0;
2029         if (bypass && !rcu_nocb_poll) {
2030                 // At least one child with non-empty ->nocb_bypass, so set
2031                 // timer in order to avoid stranding its callbacks.
2032                 raw_spin_lock_irqsave(&my_rdp->nocb_gp_lock, flags);
2033                 mod_timer(&my_rdp->nocb_bypass_timer, j + 2);
2034                 raw_spin_unlock_irqrestore(&my_rdp->nocb_gp_lock, flags);
2035         }
2036         if (rcu_nocb_poll) {
2037                 /* Polling, so trace if first poll in the series. */
2038                 if (gotcbs)
2039                         trace_rcu_nocb_wake(rcu_state.name, cpu, TPS("Poll"));
2040                 schedule_timeout_interruptible(1);
2041         } else if (!needwait_gp) {
2042                 /* Wait for callbacks to appear. */
2043                 trace_rcu_nocb_wake(rcu_state.name, cpu, TPS("Sleep"));
2044                 swait_event_interruptible_exclusive(my_rdp->nocb_gp_wq,
2045                                 !READ_ONCE(my_rdp->nocb_gp_sleep));
2046                 trace_rcu_nocb_wake(rcu_state.name, cpu, TPS("EndSleep"));
2047         } else {
2048                 rnp = my_rdp->mynode;
2049                 trace_rcu_this_gp(rnp, my_rdp, wait_gp_seq, TPS("StartWait"));
2050                 swait_event_interruptible_exclusive(
2051                         rnp->nocb_gp_wq[rcu_seq_ctr(wait_gp_seq) & 0x1],
2052                         rcu_seq_done(&rnp->gp_seq, wait_gp_seq) ||
2053                         !READ_ONCE(my_rdp->nocb_gp_sleep));
2054                 trace_rcu_this_gp(rnp, my_rdp, wait_gp_seq, TPS("EndWait"));
2055         }
2056         if (!rcu_nocb_poll) {
2057                 raw_spin_lock_irqsave(&my_rdp->nocb_gp_lock, flags);
2058                 if (bypass)
2059                         del_timer(&my_rdp->nocb_bypass_timer);
2060                 WRITE_ONCE(my_rdp->nocb_gp_sleep, true);
2061                 raw_spin_unlock_irqrestore(&my_rdp->nocb_gp_lock, flags);
2062         }
2063         my_rdp->nocb_gp_seq = -1;
2064         WARN_ON(signal_pending(current));
2065 }
2066 
2067 /*
2068  * No-CBs grace-period-wait kthread.  There is one of these per group
2069  * of CPUs, but only once at least one CPU in that group has come online
2070  * at least once since boot.  This kthread checks for newly posted
2071  * callbacks from any of the CPUs it is responsible for, waits for a
2072  * grace period, then awakens all of the rcu_nocb_cb_kthread() instances
2073  * that then have callback-invocation work to do.
2074  */
2075 static int rcu_nocb_gp_kthread(void *arg)
2076 {
2077         struct rcu_data *rdp = arg;
2078 
2079         for (;;) {
2080                 WRITE_ONCE(rdp->nocb_gp_loops, rdp->nocb_gp_loops + 1);
2081                 nocb_gp_wait(rdp);
2082                 cond_resched_tasks_rcu_qs();
2083         }
2084         return 0;
2085 }
2086 
2087 /*
2088  * Invoke any ready callbacks from the corresponding no-CBs CPU,
2089  * then, if there are no more, wait for more to appear.
2090  */
2091 static void nocb_cb_wait(struct rcu_data *rdp)
2092 {
2093         unsigned long cur_gp_seq;
2094         unsigned long flags;
2095         bool needwake_gp = false;
2096         struct rcu_node *rnp = rdp->mynode;
2097 
2098         local_irq_save(flags);
2099         rcu_momentary_dyntick_idle();
2100         local_irq_restore(flags);
2101         local_bh_disable();
2102         rcu_do_batch(rdp);
2103         local_bh_enable();
2104         lockdep_assert_irqs_enabled();
2105         rcu_nocb_lock_irqsave(rdp, flags);
2106         if (rcu_segcblist_nextgp(&rdp->cblist, &cur_gp_seq) &&
2107             rcu_seq_done(&rnp->gp_seq, cur_gp_seq) &&
2108             raw_spin_trylock_rcu_node(rnp)) { /* irqs already disabled. */
2109                 needwake_gp = rcu_advance_cbs(rdp->mynode, rdp);
2110                 raw_spin_unlock_rcu_node(rnp); /* irqs remain disabled. */
2111         }
2112         if (rcu_segcblist_ready_cbs(&rdp->cblist)) {
2113                 rcu_nocb_unlock_irqrestore(rdp, flags);
2114                 if (needwake_gp)
2115                         rcu_gp_kthread_wake();
2116                 return;
2117         }
2118 
2119         trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("CBSleep"));
2120         WRITE_ONCE(rdp->nocb_cb_sleep, true);
2121         rcu_nocb_unlock_irqrestore(rdp, flags);
2122         if (needwake_gp)
2123                 rcu_gp_kthread_wake();
2124         swait_event_interruptible_exclusive(rdp->nocb_cb_wq,
2125                                  !READ_ONCE(rdp->nocb_cb_sleep));
2126         if (!smp_load_acquire(&rdp->nocb_cb_sleep)) { /* VVV */
2127                 /* ^^^ Ensure CB invocation follows _sleep test. */
2128                 return;
2129         }
2130         WARN_ON(signal_pending(current));
2131         trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("WokeEmpty"));
2132 }
2133 
2134 /*
2135  * Per-rcu_data kthread, but only for no-CBs CPUs.  Repeatedly invoke
2136  * nocb_cb_wait() to do the dirty work.
2137  */
2138 static int rcu_nocb_cb_kthread(void *arg)
2139 {
2140         struct rcu_data *rdp = arg;
2141 
2142         // Each pass through this loop does one callback batch, and,
2143         // if there are no more ready callbacks, waits for them.
2144         for (;;) {
2145                 nocb_cb_wait(rdp);
2146                 cond_resched_tasks_rcu_qs();
2147         }
2148         return 0;
2149 }
2150 
2151 /* Is a deferred wakeup of rcu_nocb_kthread() required? */
2152 static int rcu_nocb_need_deferred_wakeup(struct rcu_data *rdp)
2153 {
2154         return READ_ONCE(rdp->nocb_defer_wakeup);
2155 }
2156 
2157 /* Do a deferred wakeup of rcu_nocb_kthread(). */
2158 static void do_nocb_deferred_wakeup_common(struct rcu_data *rdp)
2159 {
2160         unsigned long flags;
2161         int ndw;
2162 
2163         rcu_nocb_lock_irqsave(rdp, flags);
2164         if (!rcu_nocb_need_deferred_wakeup(rdp)) {
2165                 rcu_nocb_unlock_irqrestore(rdp, flags);
2166                 return;
2167         }
2168         ndw = READ_ONCE(rdp->nocb_defer_wakeup);
2169         WRITE_ONCE(rdp->nocb_defer_wakeup, RCU_NOCB_WAKE_NOT);
2170         wake_nocb_gp(rdp, ndw == RCU_NOCB_WAKE_FORCE, flags);
2171         trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("DeferredWake"));
2172 }
2173 
2174 /* Do a deferred wakeup of rcu_nocb_kthread() from a timer handler. */
2175 static void do_nocb_deferred_wakeup_timer(struct timer_list *t)
2176 {
2177         struct rcu_data *rdp = from_timer(rdp, t, nocb_timer);
2178 
2179         do_nocb_deferred_wakeup_common(rdp);
2180 }
2181 
2182 /*
2183  * Do a deferred wakeup of rcu_nocb_kthread() from fastpath.
2184  * This means we do an inexact common-case check.  Note that if
2185  * we miss, ->nocb_timer will eventually clean things up.
2186  */
2187 static void do_nocb_deferred_wakeup(struct rcu_data *rdp)
2188 {
2189         if (rcu_nocb_need_deferred_wakeup(rdp))
2190                 do_nocb_deferred_wakeup_common(rdp);
2191 }
2192 
2193 void __init rcu_init_nohz(void)
2194 {
2195         int cpu;
2196         bool need_rcu_nocb_mask = false;
2197         struct rcu_data *rdp;
2198 
2199 #if defined(CONFIG_NO_HZ_FULL)
2200         if (tick_nohz_full_running && cpumask_weight(tick_nohz_full_mask))
2201                 need_rcu_nocb_mask = true;
2202 #endif /* #if defined(CONFIG_NO_HZ_FULL) */
2203 
2204         if (!cpumask_available(rcu_nocb_mask) && need_rcu_nocb_mask) {
2205                 if (!zalloc_cpumask_var(&rcu_nocb_mask, GFP_KERNEL)) {
2206                         pr_info("rcu_nocb_mask allocation failed, callback offloading disabled.\n");
2207                         return;
2208                 }
2209         }
2210         if (!cpumask_available(rcu_nocb_mask))
2211                 return;
2212 
2213 #if defined(CONFIG_NO_HZ_FULL)
2214         if (tick_nohz_full_running)
2215                 cpumask_or(rcu_nocb_mask, rcu_nocb_mask, tick_nohz_full_mask);
2216 #endif /* #if defined(CONFIG_NO_HZ_FULL) */
2217 
2218         if (!cpumask_subset(rcu_nocb_mask, cpu_possible_mask)) {
2219                 pr_info("\tNote: kernel parameter 'rcu_nocbs=', 'nohz_full', or 'isolcpus=' contains nonexistent CPUs.\n");
2220                 cpumask_and(rcu_nocb_mask, cpu_possible_mask,
2221                             rcu_nocb_mask);
2222         }
2223         if (cpumask_empty(rcu_nocb_mask))
2224                 pr_info("\tOffload RCU callbacks from CPUs: (none).\n");
2225         else
2226                 pr_info("\tOffload RCU callbacks from CPUs: %*pbl.\n",
2227                         cpumask_pr_args(rcu_nocb_mask));
2228         if (rcu_nocb_poll)
2229                 pr_info("\tPoll for callbacks from no-CBs CPUs.\n");
2230 
2231         for_each_cpu(cpu, rcu_nocb_mask) {
2232                 rdp = per_cpu_ptr(&rcu_data, cpu);
2233                 if (rcu_segcblist_empty(&rdp->cblist))
2234                         rcu_segcblist_init(&rdp->cblist);
2235                 rcu_segcblist_offload(&rdp->cblist);
2236         }
2237         rcu_organize_nocb_kthreads();
2238 }
2239 
2240 /* Initialize per-rcu_data variables for no-CBs CPUs. */
2241 static void __init rcu_boot_init_nocb_percpu_data(struct rcu_data *rdp)
2242 {
2243         init_swait_queue_head(&rdp->nocb_cb_wq);
2244         init_swait_queue_head(&rdp->nocb_gp_wq);
2245         raw_spin_lock_init(&rdp->nocb_lock);
2246         raw_spin_lock_init(&rdp->nocb_bypass_lock);
2247         raw_spin_lock_init(&rdp->nocb_gp_lock);
2248         timer_setup(&rdp->nocb_timer, do_nocb_deferred_wakeup_timer, 0);
2249         timer_setup(&rdp->nocb_bypass_timer, do_nocb_bypass_wakeup_timer, 0);
2250         rcu_cblist_init(&rdp->nocb_bypass);
2251 }
2252 
2253 /*
2254  * If the specified CPU is a no-CBs CPU that does not already have its
2255  * rcuo CB kthread, spawn it.  Additionally, if the rcuo GP kthread
2256  * for this CPU's group has not yet been created, spawn it as well.
2257  */
2258 static void rcu_spawn_one_nocb_kthread(int cpu)
2259 {
2260         struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
2261         struct rcu_data *rdp_gp;
2262         struct task_struct *t;
2263 
2264         /*
2265          * If this isn't a no-CBs CPU or if it already has an rcuo kthread,
2266          * then nothing to do.
2267          */
2268         if (!rcu_is_nocb_cpu(cpu) || rdp->nocb_cb_kthread)
2269                 return;
2270 
2271         /* If we didn't spawn the GP kthread first, reorganize! */
2272         rdp_gp = rdp->nocb_gp_rdp;
2273         if (!rdp_gp->nocb_gp_kthread) {
2274                 t = kthread_run(rcu_nocb_gp_kthread, rdp_gp,
2275                                 "rcuog/%d", rdp_gp->cpu);
2276                 if (WARN_ONCE(IS_ERR(t), "%s: Could not start rcuo GP kthread, OOM is now expected behavior\n", __func__))
2277                         return;
2278                 WRITE_ONCE(rdp_gp->nocb_gp_kthread, t);
2279         }
2280 
2281         /* Spawn the kthread for this CPU. */
2282         t = kthread_run(rcu_nocb_cb_kthread, rdp,
2283                         "rcuo%c/%d", rcu_state.abbr, cpu);
2284         if (WARN_ONCE(IS_ERR(t), "%s: Could not start rcuo CB kthread, OOM is now expected behavior\n", __func__))
2285                 return;
2286         WRITE_ONCE(rdp->nocb_cb_kthread, t);
2287         WRITE_ONCE(rdp->nocb_gp_kthread, rdp_gp->nocb_gp_kthread);
2288 }
2289 
2290 /*
2291  * If the specified CPU is a no-CBs CPU that does not already have its
2292  * rcuo kthread, spawn it.
2293  */
2294 static void rcu_spawn_cpu_nocb_kthread(int cpu)
2295 {
2296         if (rcu_scheduler_fully_active)
2297                 rcu_spawn_one_nocb_kthread(cpu);
2298 }
2299 
2300 /*
2301  * Once the scheduler is running, spawn rcuo kthreads for all online
2302  * no-CBs CPUs.  This assumes that the early_initcall()s happen before
2303  * non-boot CPUs come online -- if this changes, we will need to add
2304  * some mutual exclusion.
2305  */
2306 static void __init rcu_spawn_nocb_kthreads(void)
2307 {
2308         int cpu;
2309 
2310         for_each_online_cpu(cpu)
2311                 rcu_spawn_cpu_nocb_kthread(cpu);
2312 }
2313 
2314 /* How many CB CPU IDs per GP kthread?  Default of -1 for sqrt(nr_cpu_ids). */
2315 static int rcu_nocb_gp_stride = -1;
2316 module_param(rcu_nocb_gp_stride, int, 0444);
2317 
2318 /*
2319  * Initialize GP-CB relationships for all no-CBs CPU.
2320  */
2321 static void __init rcu_organize_nocb_kthreads(void)
2322 {
2323         int cpu;
2324         bool firsttime = true;
2325         bool gotnocbs = false;
2326         bool gotnocbscbs = true;
2327         int ls = rcu_nocb_gp_stride;
2328         int nl = 0;  /* Next GP kthread. */
2329         struct rcu_data *rdp;
2330         struct rcu_data *rdp_gp = NULL;  /* Suppress misguided gcc warn. */
2331         struct rcu_data *rdp_prev = NULL;
2332 
2333         if (!cpumask_available(rcu_nocb_mask))
2334                 return;
2335         if (ls == -1) {
2336                 ls = nr_cpu_ids / int_sqrt(nr_cpu_ids);
2337                 rcu_nocb_gp_stride = ls;
2338         }
2339 
2340         /*
2341          * Each pass through this loop sets up one rcu_data structure.
2342          * Should the corresponding CPU come online in the future, then
2343          * we will spawn the needed set of rcu_nocb_kthread() kthreads.
2344          */
2345         for_each_cpu(cpu, rcu_nocb_mask) {
2346                 rdp = per_cpu_ptr(&rcu_data, cpu);
2347                 if (rdp->cpu >= nl) {
2348                         /* New GP kthread, set up for CBs & next GP. */
2349                         gotnocbs = true;
2350                         nl = DIV_ROUND_UP(rdp->cpu + 1, ls) * ls;
2351                         rdp->nocb_gp_rdp = rdp;
2352                         rdp_gp = rdp;
2353                         if (dump_tree) {
2354                                 if (!firsttime)
2355                                         pr_cont("%s\n", gotnocbscbs
2356                                                         ? "" : " (self only)");
2357                                 gotnocbscbs = false;
2358                                 firsttime = false;
2359                                 pr_alert("%s: No-CB GP kthread CPU %d:",
2360                                          __func__, cpu);
2361                         }
2362                 } else {
2363                         /* Another CB kthread, link to previous GP kthread. */
2364                         gotnocbscbs = true;
2365                         rdp->nocb_gp_rdp = rdp_gp;
2366                         rdp_prev->nocb_next_cb_rdp = rdp;
2367                         if (dump_tree)
2368                                 pr_cont(" %d", cpu);
2369                 }
2370                 rdp_prev = rdp;
2371         }
2372         if (gotnocbs && dump_tree)
2373                 pr_cont("%s\n", gotnocbscbs ? "" : " (self only)");
2374 }
2375 
2376 /*
2377  * Bind the current task to the offloaded CPUs.  If there are no offloaded
2378  * CPUs, leave the task unbound.  Splat if the bind attempt fails.
2379  */
2380 void rcu_bind_current_to_nocb(void)
2381 {
2382         if (cpumask_available(rcu_nocb_mask) && cpumask_weight(rcu_nocb_mask))
2383                 WARN_ON(sched_setaffinity(current->pid, rcu_nocb_mask));
2384 }
2385 EXPORT_SYMBOL_GPL(rcu_bind_current_to_nocb);
2386 
2387 /*
2388  * Dump out nocb grace-period kthread state for the specified rcu_data
2389  * structure.
2390  */
2391 static void show_rcu_nocb_gp_state(struct rcu_data *rdp)
2392 {
2393         struct rcu_node *rnp = rdp->mynode;
2394 
2395         pr_info("nocb GP %d %c%c%c%c%c%c %c[%c%c] %c%c:%ld rnp %d:%d %lu\n",
2396                 rdp->cpu,
2397                 "kK"[!!rdp->nocb_gp_kthread],
2398                 "lL"[raw_spin_is_locked(&rdp->nocb_gp_lock)],
2399                 "dD"[!!rdp->nocb_defer_wakeup],
2400                 "tT"[timer_pending(&rdp->nocb_timer)],
2401                 "bB"[timer_pending(&rdp->nocb_bypass_timer)],
2402                 "sS"[!!rdp->nocb_gp_sleep],
2403                 ".W"[swait_active(&rdp->nocb_gp_wq)],
2404                 ".W"[swait_active(&rnp->nocb_gp_wq[0])],
2405                 ".W"[swait_active(&rnp->nocb_gp_wq[1])],
2406                 ".B"[!!rdp->nocb_gp_bypass],
2407                 ".G"[!!rdp->nocb_gp_gp],
2408                 (long)rdp->nocb_gp_seq,
2409                 rnp->grplo, rnp->grphi, READ_ONCE(rdp->nocb_gp_loops));
2410 }
2411 
2412 /* Dump out nocb kthread state for the specified rcu_data structure. */
2413 static void show_rcu_nocb_state(struct rcu_data *rdp)
2414 {
2415         struct rcu_segcblist *rsclp = &rdp->cblist;
2416         bool waslocked;
2417         bool wastimer;
2418         bool wassleep;
2419 
2420         if (rdp->nocb_gp_rdp == rdp)
2421                 show_rcu_nocb_gp_state(rdp);
2422 
2423         pr_info("   CB %d->%d %c%c%c%c%c%c F%ld L%ld C%d %c%c%c%c%c q%ld\n",
2424                 rdp->cpu, rdp->nocb_gp_rdp->cpu,
2425                 "kK"[!!rdp->nocb_cb_kthread],
2426                 "bB"[raw_spin_is_locked(&rdp->nocb_bypass_lock)],
2427                 "cC"[!!atomic_read(&rdp->nocb_lock_contended)],
2428                 "lL"[raw_spin_is_locked(&rdp->nocb_lock)],
2429                 "sS"[!!rdp->nocb_cb_sleep],
2430                 ".W"[swait_active(&rdp->nocb_cb_wq)],
2431                 jiffies - rdp->nocb_bypass_first,
2432                 jiffies - rdp->nocb_nobypass_last,
2433                 rdp->nocb_nobypass_count,
2434                 ".D"[rcu_segcblist_ready_cbs(rsclp)],
2435                 ".W"[!rcu_segcblist_restempty(rsclp, RCU_DONE_TAIL)],
2436                 ".R"[!rcu_segcblist_restempty(rsclp, RCU_WAIT_TAIL)],
2437                 ".N"[!rcu_segcblist_restempty(rsclp, RCU_NEXT_READY_TAIL)],
2438                 ".B"[!!rcu_cblist_n_cbs(&rdp->nocb_bypass)],
2439                 rcu_segcblist_n_cbs(&rdp->cblist));
2440 
2441         /* It is OK for GP kthreads to have GP state. */
2442         if (rdp->nocb_gp_rdp == rdp)
2443                 return;
2444 
2445         waslocked = raw_spin_is_locked(&rdp->nocb_gp_lock);
2446         wastimer = timer_pending(&rdp->nocb_timer);
2447         wassleep = swait_active(&rdp->nocb_gp_wq);
2448         if (!rdp->nocb_defer_wakeup && !rdp->nocb_gp_sleep &&
2449             !waslocked && !wastimer && !wassleep)
2450                 return;  /* Nothing untowards. */
2451 
2452         pr_info("   !!! %c%c%c%c %c\n",
2453                 "lL"[waslocked],
2454                 "dD"[!!rdp->nocb_defer_wakeup],
2455                 "tT"[wastimer],
2456                 "sS"[!!rdp->nocb_gp_sleep],
2457                 ".W"[wassleep]);
2458 }
2459 
2460 #else /* #ifdef CONFIG_RCU_NOCB_CPU */
2461 
2462 /* No ->nocb_lock to acquire.  */
2463 static void rcu_nocb_lock(struct rcu_data *rdp)
2464 {
2465 }
2466 
2467 /* No ->nocb_lock to release.  */
2468 static void rcu_nocb_unlock(struct rcu_data *rdp)
2469 {
2470 }
2471 
2472 /* No ->nocb_lock to release.  */
2473 static void rcu_nocb_unlock_irqrestore(struct rcu_data *rdp,
2474                                        unsigned long flags)
2475 {
2476         local_irq_restore(flags);
2477 }
2478 
2479 /* Lockdep check that ->cblist may be safely accessed. */
2480 static void rcu_lockdep_assert_cblist_protected(struct rcu_data *rdp)
2481 {
2482         lockdep_assert_irqs_disabled();
2483 }
2484 
2485 static void rcu_nocb_gp_cleanup(struct swait_queue_head *sq)
2486 {
2487 }
2488 
2489 static struct swait_queue_head *rcu_nocb_gp_get(struct rcu_node *rnp)
2490 {
2491         return NULL;
2492 }
2493 
2494 static void rcu_init_one_nocb(struct rcu_node *rnp)
2495 {
2496 }
2497 
2498 static bool rcu_nocb_flush_bypass(struct rcu_data *rdp, struct rcu_head *rhp,
2499                                   unsigned long j)
2500 {
2501         return true;
2502 }
2503 
2504 static bool rcu_nocb_try_bypass(struct rcu_data *rdp, struct rcu_head *rhp,
2505                                 bool *was_alldone, unsigned long flags)
2506 {
2507         return false;
2508 }
2509 
2510 static void __call_rcu_nocb_wake(struct rcu_data *rdp, bool was_empty,
2511                                  unsigned long flags)
2512 {
2513         WARN_ON_ONCE(1);  /* Should be dead code! */
2514 }
2515 
2516 static void __init rcu_boot_init_nocb_percpu_data(struct rcu_data *rdp)
2517 {
2518 }
2519 
2520 static int rcu_nocb_need_deferred_wakeup(struct rcu_data *rdp)
2521 {
2522         return false;
2523 }
2524 
2525 static void do_nocb_deferred_wakeup(struct rcu_data *rdp)
2526 {
2527 }
2528 
2529 static void rcu_spawn_cpu_nocb_kthread(int cpu)
2530 {
2531 }
2532 
2533 static void __init rcu_spawn_nocb_kthreads(void)
2534 {
2535 }
2536 
2537 static void show_rcu_nocb_state(struct rcu_data *rdp)
2538 {
2539 }
2540 
2541 #endif /* #else #ifdef CONFIG_RCU_NOCB_CPU */
2542 
2543 /*
2544  * Is this CPU a NO_HZ_FULL CPU that should ignore RCU so that the
2545  * grace-period kthread will do force_quiescent_state() processing?
2546  * The idea is to avoid waking up RCU core processing on such a
2547  * CPU unless the grace period has extended for too long.
2548  *
2549  * This code relies on the fact that all NO_HZ_FULL CPUs are also
2550  * CONFIG_RCU_NOCB_CPU CPUs.
2551  */
2552 static bool rcu_nohz_full_cpu(void)
2553 {
2554 #ifdef CONFIG_NO_HZ_FULL
2555         if (tick_nohz_full_cpu(smp_processor_id()) &&
2556             (!rcu_gp_in_progress() ||
2557              ULONG_CMP_LT(jiffies, READ_ONCE(rcu_state.gp_start) + HZ)))
2558                 return true;
2559 #endif /* #ifdef CONFIG_NO_HZ_FULL */
2560         return false;
2561 }
2562 
2563 /*
2564  * Bind the RCU grace-period kthreads to the housekeeping CPU.
2565  */
2566 static void rcu_bind_gp_kthread(void)
2567 {
2568         if (!tick_nohz_full_enabled())
2569                 return;
2570         housekeeping_affine(current, HK_FLAG_RCU);
2571 }
2572 
2573 /* Record the current task on dyntick-idle entry. */
2574 static void rcu_dynticks_task_enter(void)
2575 {
2576 #if defined(CONFIG_TASKS_RCU) && defined(CONFIG_NO_HZ_FULL)
2577         WRITE_ONCE(current->rcu_tasks_idle_cpu, smp_processor_id());
2578 #endif /* #if defined(CONFIG_TASKS_RCU) && defined(CONFIG_NO_HZ_FULL) */
2579 }
2580 
2581 /* Record no current task on dyntick-idle exit. */
2582 static void rcu_dynticks_task_exit(void)
2583 {
2584 #if defined(CONFIG_TASKS_RCU) && defined(CONFIG_NO_HZ_FULL)
2585         WRITE_ONCE(current->rcu_tasks_idle_cpu, -1);
2586 #endif /* #if defined(CONFIG_TASKS_RCU) && defined(CONFIG_NO_HZ_FULL) */
2587 }