root/kernel/softirq.c

DEFINITIONS

1. wakeup_softirqd
2. ksoftirqd_running
3. __local_bh_disable_ip
4. __local_bh_enable
5. _local_bh_enable
6. __local_bh_enable_ip
7. lockdep_softirq_start
8. lockdep_softirq_end
9. lockdep_softirq_start
10. lockdep_softirq_end
11. __do_softirq
12. do_softirq
13. irq_enter
14. invoke_softirq
15. tick_irq_exit
16. irq_exit
17. raise_softirq_irqoff
18. raise_softirq
19. __raise_softirq_irqoff
20. open_softirq
21. __tasklet_schedule_common
22. __tasklet_schedule
23. __tasklet_hi_schedule
24. tasklet_action_common
25. tasklet_action
26. tasklet_hi_action
27. tasklet_init
28. tasklet_kill
29. softirq_init
30. ksoftirqd_should_run
31. run_ksoftirqd
32. tasklet_kill_immediate
33. takeover_tasklets
34. spawn_ksoftirqd
35. early_irq_init
36. arch_probe_nr_irqs
37. arch_early_irq_init
38. arch_dynirq_lower_bound

   1 // SPDX-License-Identifier: GPL-2.0-only
   2 /*
   3  *      linux/kernel/softirq.c
   4  *
   5  *      Copyright (C) 1992 Linus Torvalds
   6  *
   7  *      Rewritten. Old one was good in 2.2, but in 2.3 it was immoral. --ANK (990903)
   8  */
   9 
  10 #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
  11 
  12 #include <linux/export.h>
  13 #include <linux/kernel_stat.h>
  14 #include <linux/interrupt.h>
  15 #include <linux/init.h>
  16 #include <linux/mm.h>
  17 #include <linux/notifier.h>
  18 #include <linux/percpu.h>
  19 #include <linux/cpu.h>
  20 #include <linux/freezer.h>
  21 #include <linux/kthread.h>
  22 #include <linux/rcupdate.h>
  23 #include <linux/ftrace.h>
  24 #include <linux/smp.h>
  25 #include <linux/smpboot.h>
  26 #include <linux/tick.h>
  27 #include <linux/irq.h>
  28 
  29 #define CREATE_TRACE_POINTS
  30 #include <trace/events/irq.h>
  31 
  32 /*
  33    - No shared variables, all the data are CPU local.
  34    - If a softirq needs serialization, let it serialize itself
  35      by its own spinlocks.
  36    - Even if softirq is serialized, only local cpu is marked for
  37      execution. Hence, we get something sort of weak cpu binding.
  38      Though it is still not clear, will it result in better locality
  39      or will not.
  40 
  41    Examples:
  42    - NET RX softirq. It is multithreaded and does not require
  43      any global serialization.
  44    - NET TX softirq. It kicks software netdevice queues, hence
  45      it is logically serialized per device, but this serialization
  46      is invisible to common code.
  47    - Tasklets: serialized wrt itself.
  48  */
  49 
  50 #ifndef __ARCH_IRQ_STAT
  51 DEFINE_PER_CPU_ALIGNED(irq_cpustat_t, irq_stat);
  52 EXPORT_PER_CPU_SYMBOL(irq_stat);
  53 #endif
  54 
  55 static struct softirq_action softirq_vec[NR_SOFTIRQS] __cacheline_aligned_in_smp;
  56 
  57 DEFINE_PER_CPU(struct task_struct *, ksoftirqd);
  58 
  59 const char * const softirq_to_name[NR_SOFTIRQS] = {
  60         "HI", "TIMER", "NET_TX", "NET_RX", "BLOCK", "IRQ_POLL",
  61         "TASKLET", "SCHED", "HRTIMER", "RCU"
  62 };
  63 
  64 /*
  65  * we cannot loop indefinitely here to avoid userspace starvation,
  66  * but we also don't want to introduce a worst case 1/HZ latency
  67  * to the pending events, so lets the scheduler to balance
  68  * the softirq load for us.
  69  */
  70 static void wakeup_softirqd(void)
  71 {
  72         /* Interrupts are disabled: no need to stop preemption */
  73         struct task_struct *tsk = __this_cpu_read(ksoftirqd);
  74 
  75         if (tsk && tsk->state != TASK_RUNNING)
  76                 wake_up_process(tsk);
  77 }
  78 
  79 /*
  80  * If ksoftirqd is scheduled, we do not want to process pending softirqs
  81  * right now. Let ksoftirqd handle this at its own rate, to get fairness,
  82  * unless we're doing some of the synchronous softirqs.
  83  */
  84 #define SOFTIRQ_NOW_MASK ((1 << HI_SOFTIRQ) | (1 << TASKLET_SOFTIRQ))
  85 static bool ksoftirqd_running(unsigned long pending)
  86 {
  87         struct task_struct *tsk = __this_cpu_read(ksoftirqd);
  88 
  89         if (pending & SOFTIRQ_NOW_MASK)
  90                 return false;
  91         return tsk && (tsk->state == TASK_RUNNING) &&
  92                 !__kthread_should_park(tsk);
  93 }
  94 
  95 /*
  96  * preempt_count and SOFTIRQ_OFFSET usage:
  97  * - preempt_count is changed by SOFTIRQ_OFFSET on entering or leaving
  98  *   softirq processing.
  99  * - preempt_count is changed by SOFTIRQ_DISABLE_OFFSET (= 2 * SOFTIRQ_OFFSET)
 100  *   on local_bh_disable or local_bh_enable.
 101  * This lets us distinguish between whether we are currently processing
 102  * softirq and whether we just have bh disabled.
 103  */
 104 
 105 /*
 106  * This one is for softirq.c-internal use,
 107  * where hardirqs are disabled legitimately:
 108  */
 109 #ifdef CONFIG_TRACE_IRQFLAGS
 110 void __local_bh_disable_ip(unsigned long ip, unsigned int cnt)
 111 {
 112         unsigned long flags;
 113 
 114         WARN_ON_ONCE(in_irq());
 115 
 116         raw_local_irq_save(flags);
 117         /*
 118          * The preempt tracer hooks into preempt_count_add and will break
 119          * lockdep because it calls back into lockdep after SOFTIRQ_OFFSET
 120          * is set and before current->softirq_enabled is cleared.
 121          * We must manually increment preempt_count here and manually
 122          * call the trace_preempt_off later.
 123          */
 124         __preempt_count_add(cnt);
 125         /*
 126          * Were softirqs turned off above:
 127          */
 128         if (softirq_count() == (cnt & SOFTIRQ_MASK))
 129                 trace_softirqs_off(ip);
 130         raw_local_irq_restore(flags);
 131 
 132         if (preempt_count() == cnt) {
 133 #ifdef CONFIG_DEBUG_PREEMPT
 134                 current->preempt_disable_ip = get_lock_parent_ip();
 135 #endif
 136                 trace_preempt_off(CALLER_ADDR0, get_lock_parent_ip());
 137         }
 138 }
 139 EXPORT_SYMBOL(__local_bh_disable_ip);
 140 #endif /* CONFIG_TRACE_IRQFLAGS */
 141 
 142 static void __local_bh_enable(unsigned int cnt)
 143 {
 144         lockdep_assert_irqs_disabled();
 145 
 146         if (preempt_count() == cnt)
 147                 trace_preempt_on(CALLER_ADDR0, get_lock_parent_ip());
 148 
 149         if (softirq_count() == (cnt & SOFTIRQ_MASK))
 150                 trace_softirqs_on(_RET_IP_);
 151 
 152         __preempt_count_sub(cnt);
 153 }
 154 
 155 /*
 156  * Special-case - softirqs can safely be enabled by __do_softirq(),
 157  * without processing still-pending softirqs:
 158  */
 159 void _local_bh_enable(void)
 160 {
 161         WARN_ON_ONCE(in_irq());
 162         __local_bh_enable(SOFTIRQ_DISABLE_OFFSET);
 163 }
 164 EXPORT_SYMBOL(_local_bh_enable);
 165 
 166 void __local_bh_enable_ip(unsigned long ip, unsigned int cnt)
 167 {
 168         WARN_ON_ONCE(in_irq());
 169         lockdep_assert_irqs_enabled();
 170 #ifdef CONFIG_TRACE_IRQFLAGS
 171         local_irq_disable();
 172 #endif
 173         /*
 174          * Are softirqs going to be turned on now:
 175          */
 176         if (softirq_count() == SOFTIRQ_DISABLE_OFFSET)
 177                 trace_softirqs_on(ip);
 178         /*
 179          * Keep preemption disabled until we are done with
 180          * softirq processing:
 181          */
 182         preempt_count_sub(cnt - 1);
 183 
 184         if (unlikely(!in_interrupt() && local_softirq_pending())) {
 185                 /*
 186                  * Run softirq if any pending. And do it in its own stack
 187                  * as we may be calling this deep in a task call stack already.
 188                  */
 189                 do_softirq();
 190         }
 191 
 192         preempt_count_dec();
 193 #ifdef CONFIG_TRACE_IRQFLAGS
 194         local_irq_enable();
 195 #endif
 196         preempt_check_resched();
 197 }
 198 EXPORT_SYMBOL(__local_bh_enable_ip);
 199 
 200 /*
 201  * We restart softirq processing for at most MAX_SOFTIRQ_RESTART times,
 202  * but break the loop if need_resched() is set or after 2 ms.
 203  * The MAX_SOFTIRQ_TIME provides a nice upper bound in most cases, but in
 204  * certain cases, such as stop_machine(), jiffies may cease to
 205  * increment and so we need the MAX_SOFTIRQ_RESTART limit as
 206  * well to make sure we eventually return from this method.
 207  *
 208  * These limits have been established via experimentation.
 209  * The two things to balance is latency against fairness -
 210  * we want to handle softirqs as soon as possible, but they
 211  * should not be able to lock up the box.
 212  */
 213 #define MAX_SOFTIRQ_TIME  msecs_to_jiffies(2)
 214 #define MAX_SOFTIRQ_RESTART 10
 215 
 216 #ifdef CONFIG_TRACE_IRQFLAGS
 217 /*
 218  * When we run softirqs from irq_exit() and thus on the hardirq stack we need
 219  * to keep the lockdep irq context tracking as tight as possible in order to
 220  * not miss-qualify lock contexts and miss possible deadlocks.
 221  */
 222 
 223 static inline bool lockdep_softirq_start(void)
 224 {
 225         bool in_hardirq = false;
 226 
 227         if (trace_hardirq_context(current)) {
 228                 in_hardirq = true;
 229                 trace_hardirq_exit();
 230         }
 231 
 232         lockdep_softirq_enter();
 233 
 234         return in_hardirq;
 235 }
 236 
 237 static inline void lockdep_softirq_end(bool in_hardirq)
 238 {
 239         lockdep_softirq_exit();
 240 
 241         if (in_hardirq)
 242                 trace_hardirq_enter();
 243 }
 244 #else
 245 static inline bool lockdep_softirq_start(void) { return false; }
 246 static inline void lockdep_softirq_end(bool in_hardirq) { }
 247 #endif
 248 
 249 asmlinkage __visible void __softirq_entry __do_softirq(void)
 250 {
 251         unsigned long end = jiffies + MAX_SOFTIRQ_TIME;
 252         unsigned long old_flags = current->flags;
 253         int max_restart = MAX_SOFTIRQ_RESTART;
 254         struct softirq_action *h;
 255         bool in_hardirq;
 256         __u32 pending;
 257         int softirq_bit;
 258 
 259         /*
 260          * Mask out PF_MEMALLOC as the current task context is borrowed for the
 261          * softirq. A softirq handled, such as network RX, might set PF_MEMALLOC
 262          * again if the socket is related to swapping.
 263          */
 264         current->flags &= ~PF_MEMALLOC;
 265 
 266         pending = local_softirq_pending();
 267         account_irq_enter_time(current);
 268 
 269         __local_bh_disable_ip(_RET_IP_, SOFTIRQ_OFFSET);
 270         in_hardirq = lockdep_softirq_start();
 271 
 272 restart:
 273         /* Reset the pending bitmask before enabling irqs */
 274         set_softirq_pending(0);
 275 
 276         local_irq_enable();
 277 
 278         h = softirq_vec;
 279 
 280         while ((softirq_bit = ffs(pending))) {
 281                 unsigned int vec_nr;
 282                 int prev_count;
 283 
 284                 h += softirq_bit - 1;
 285 
 286                 vec_nr = h - softirq_vec;
 287                 prev_count = preempt_count();
 288 
 289                 kstat_incr_softirqs_this_cpu(vec_nr);
 290 
 291                 trace_softirq_entry(vec_nr);
 292                 h->action(h);
 293                 trace_softirq_exit(vec_nr);
 294                 if (unlikely(prev_count != preempt_count())) {
 295                         pr_err("huh, entered softirq %u %s %p with preempt_count %08x, exited with %08x?\n",
 296                                vec_nr, softirq_to_name[vec_nr], h->action,
 297                                prev_count, preempt_count());
 298                         preempt_count_set(prev_count);
 299                 }
 300                 h++;
 301                 pending >>= softirq_bit;
 302         }
 303 
 304         if (__this_cpu_read(ksoftirqd) == current)
 305                 rcu_softirq_qs();
 306         local_irq_disable();
 307 
 308         pending = local_softirq_pending();
 309         if (pending) {
 310                 if (time_before(jiffies, end) && !need_resched() &&
 311                     --max_restart)
 312                         goto restart;
 313 
 314                 wakeup_softirqd();
 315         }
 316 
 317         lockdep_softirq_end(in_hardirq);
 318         account_irq_exit_time(current);
 319         __local_bh_enable(SOFTIRQ_OFFSET);
 320         WARN_ON_ONCE(in_interrupt());
 321         current_restore_flags(old_flags, PF_MEMALLOC);
 322 }
 323 
 324 asmlinkage __visible void do_softirq(void)
 325 {
 326         __u32 pending;
 327         unsigned long flags;
 328 
 329         if (in_interrupt())
 330                 return;
 331 
 332         local_irq_save(flags);
 333 
 334         pending = local_softirq_pending();
 335 
 336         if (pending && !ksoftirqd_running(pending))
 337                 do_softirq_own_stack();
 338 
 339         local_irq_restore(flags);
 340 }
 341 
 342 /*
 343  * Enter an interrupt context.
 344  */
 345 void irq_enter(void)
 346 {
 347         rcu_irq_enter();
 348         if (is_idle_task(current) && !in_interrupt()) {
 349                 /*
 350                  * Prevent raise_softirq from needlessly waking up ksoftirqd
 351                  * here, as softirq will be serviced on return from interrupt.
 352                  */
 353                 local_bh_disable();
 354                 tick_irq_enter();
 355                 _local_bh_enable();
 356         }
 357 
 358         __irq_enter();
 359 }
 360 
 361 static inline void invoke_softirq(void)
 362 {
 363         if (ksoftirqd_running(local_softirq_pending()))
 364                 return;
 365 
 366         if (!force_irqthreads) {
 367 #ifdef CONFIG_HAVE_IRQ_EXIT_ON_IRQ_STACK
 368                 /*
 369                  * We can safely execute softirq on the current stack if
 370                  * it is the irq stack, because it should be near empty
 371                  * at this stage.
 372                  */
 373                 __do_softirq();
 374 #else
 375                 /*
 376                  * Otherwise, irq_exit() is called on the task stack that can
 377                  * be potentially deep already. So call softirq in its own stack
 378                  * to prevent from any overrun.
 379                  */
 380                 do_softirq_own_stack();
 381 #endif
 382         } else {
 383                 wakeup_softirqd();
 384         }
 385 }
 386 
 387 static inline void tick_irq_exit(void)
 388 {
 389 #ifdef CONFIG_NO_HZ_COMMON
 390         int cpu = smp_processor_id();
 391 
 392         /* Make sure that timer wheel updates are propagated */
 393         if ((idle_cpu(cpu) && !need_resched()) || tick_nohz_full_cpu(cpu)) {
 394                 if (!in_irq())
 395                         tick_nohz_irq_exit();
 396         }
 397 #endif
 398 }
 399 
 400 /*
 401  * Exit an interrupt context. Process softirqs if needed and possible:
 402  */
 403 void irq_exit(void)
 404 {
 405 #ifndef __ARCH_IRQ_EXIT_IRQS_DISABLED
 406         local_irq_disable();
 407 #else
 408         lockdep_assert_irqs_disabled();
 409 #endif
 410         account_irq_exit_time(current);
 411         preempt_count_sub(HARDIRQ_OFFSET);
 412         if (!in_interrupt() && local_softirq_pending())
 413                 invoke_softirq();
 414 
 415         tick_irq_exit();
 416         rcu_irq_exit();
 417         trace_hardirq_exit(); /* must be last! */
 418 }
 419 
 420 /*
 421  * This function must run with irqs disabled!
 422  */
 423 inline void raise_softirq_irqoff(unsigned int nr)
 424 {
 425         __raise_softirq_irqoff(nr);
 426 
 427         /*
 428          * If we're in an interrupt or softirq, we're done
 429          * (this also catches softirq-disabled code). We will
 430          * actually run the softirq once we return from
 431          * the irq or softirq.
 432          *
 433          * Otherwise we wake up ksoftirqd to make sure we
 434          * schedule the softirq soon.
 435          */
 436         if (!in_interrupt())
 437                 wakeup_softirqd();
 438 }
 439 
 440 void raise_softirq(unsigned int nr)
 441 {
 442         unsigned long flags;
 443 
 444         local_irq_save(flags);
 445         raise_softirq_irqoff(nr);
 446         local_irq_restore(flags);
 447 }
 448 
 449 void __raise_softirq_irqoff(unsigned int nr)
 450 {
 451         trace_softirq_raise(nr);
 452         or_softirq_pending(1UL << nr);
 453 }
 454 
 455 void open_softirq(int nr, void (*action)(struct softirq_action *))
 456 {
 457         softirq_vec[nr].action = action;
 458 }
 459 
 460 /*
 461  * Tasklets
 462  */
 463 struct tasklet_head {
 464         struct tasklet_struct *head;
 465         struct tasklet_struct **tail;
 466 };
 467 
 468 static DEFINE_PER_CPU(struct tasklet_head, tasklet_vec);
 469 static DEFINE_PER_CPU(struct tasklet_head, tasklet_hi_vec);
 470 
 471 static void __tasklet_schedule_common(struct tasklet_struct *t,
 472                                       struct tasklet_head __percpu *headp,
 473                                       unsigned int softirq_nr)
 474 {
 475         struct tasklet_head *head;
 476         unsigned long flags;
 477 
 478         local_irq_save(flags);
 479         head = this_cpu_ptr(headp);
 480         t->next = NULL;
 481         *head->tail = t;
 482         head->tail = &(t->next);
 483         raise_softirq_irqoff(softirq_nr);
 484         local_irq_restore(flags);
 485 }
 486 
 487 void __tasklet_schedule(struct tasklet_struct *t)
 488 {
 489         __tasklet_schedule_common(t, &tasklet_vec,
 490                                   TASKLET_SOFTIRQ);
 491 }
 492 EXPORT_SYMBOL(__tasklet_schedule);
 493 
 494 void __tasklet_hi_schedule(struct tasklet_struct *t)
 495 {
 496         __tasklet_schedule_common(t, &tasklet_hi_vec,
 497                                   HI_SOFTIRQ);
 498 }
 499 EXPORT_SYMBOL(__tasklet_hi_schedule);
 500 
 501 static void tasklet_action_common(struct softirq_action *a,
 502                                   struct tasklet_head *tl_head,
 503                                   unsigned int softirq_nr)
 504 {
 505         struct tasklet_struct *list;
 506 
 507         local_irq_disable();
 508         list = tl_head->head;
 509         tl_head->head = NULL;
 510         tl_head->tail = &tl_head->head;
 511         local_irq_enable();
 512 
 513         while (list) {
 514                 struct tasklet_struct *t = list;
 515 
 516                 list = list->next;
 517 
 518                 if (tasklet_trylock(t)) {
 519                         if (!atomic_read(&t->count)) {
 520                                 if (!test_and_clear_bit(TASKLET_STATE_SCHED,
 521                                                         &t->state))
 522                                         BUG();
 523                                 t->func(t->data);
 524                                 tasklet_unlock(t);
 525                                 continue;
 526                         }
 527                         tasklet_unlock(t);
 528                 }
 529 
 530                 local_irq_disable();
 531                 t->next = NULL;
 532                 *tl_head->tail = t;
 533                 tl_head->tail = &t->next;
 534                 __raise_softirq_irqoff(softirq_nr);
 535                 local_irq_enable();
 536         }
 537 }
 538 
 539 static __latent_entropy void tasklet_action(struct softirq_action *a)
 540 {
 541         tasklet_action_common(a, this_cpu_ptr(&tasklet_vec), TASKLET_SOFTIRQ);
 542 }
 543 
 544 static __latent_entropy void tasklet_hi_action(struct softirq_action *a)
 545 {
 546         tasklet_action_common(a, this_cpu_ptr(&tasklet_hi_vec), HI_SOFTIRQ);
 547 }
 548 
 549 void tasklet_init(struct tasklet_struct *t,
 550                   void (*func)(unsigned long), unsigned long data)
 551 {
 552         t->next = NULL;
 553         t->state = 0;
 554         atomic_set(&t->count, 0);
 555         t->func = func;
 556         t->data = data;
 557 }
 558 EXPORT_SYMBOL(tasklet_init);
 559 
 560 void tasklet_kill(struct tasklet_struct *t)
 561 {
 562         if (in_interrupt())
 563                 pr_notice("Attempt to kill tasklet from interrupt\n");
 564 
 565         while (test_and_set_bit(TASKLET_STATE_SCHED, &t->state)) {
 566                 do {
 567                         yield();
 568                 } while (test_bit(TASKLET_STATE_SCHED, &t->state));
 569         }
 570         tasklet_unlock_wait(t);
 571         clear_bit(TASKLET_STATE_SCHED, &t->state);
 572 }
 573 EXPORT_SYMBOL(tasklet_kill);
 574 
 575 void __init softirq_init(void)
 576 {
 577         int cpu;
 578 
 579         for_each_possible_cpu(cpu) {
 580                 per_cpu(tasklet_vec, cpu).tail =
 581                         &per_cpu(tasklet_vec, cpu).head;
 582                 per_cpu(tasklet_hi_vec, cpu).tail =
 583                         &per_cpu(tasklet_hi_vec, cpu).head;
 584         }
 585 
 586         open_softirq(TASKLET_SOFTIRQ, tasklet_action);
 587         open_softirq(HI_SOFTIRQ, tasklet_hi_action);
 588 }
 589 
 590 static int ksoftirqd_should_run(unsigned int cpu)
 591 {
 592         return local_softirq_pending();
 593 }
 594 
 595 static void run_ksoftirqd(unsigned int cpu)
 596 {
 597         local_irq_disable();
 598         if (local_softirq_pending()) {
 599                 /*
 600                  * We can safely run softirq on inline stack, as we are not deep
 601                  * in the task stack here.
 602                  */
 603                 __do_softirq();
 604                 local_irq_enable();
 605                 cond_resched();
 606                 return;
 607         }
 608         local_irq_enable();
 609 }
 610 
 611 #ifdef CONFIG_HOTPLUG_CPU
 612 /*
 613  * tasklet_kill_immediate is called to remove a tasklet which can already be
 614  * scheduled for execution on @cpu.
 615  *
 616  * Unlike tasklet_kill, this function removes the tasklet
 617  * _immediately_, even if the tasklet is in TASKLET_STATE_SCHED state.
 618  *
 619  * When this function is called, @cpu must be in the CPU_DEAD state.
 620  */
 621 void tasklet_kill_immediate(struct tasklet_struct *t, unsigned int cpu)
 622 {
 623         struct tasklet_struct **i;
 624 
 625         BUG_ON(cpu_online(cpu));
 626         BUG_ON(test_bit(TASKLET_STATE_RUN, &t->state));
 627 
 628         if (!test_bit(TASKLET_STATE_SCHED, &t->state))
 629                 return;
 630 
 631         /* CPU is dead, so no lock needed. */
 632         for (i = &per_cpu(tasklet_vec, cpu).head; *i; i = &(*i)->next) {
 633                 if (*i == t) {
 634                         *i = t->next;
 635                         /* If this was the tail element, move the tail ptr */
 636                         if (*i == NULL)
 637                                 per_cpu(tasklet_vec, cpu).tail = i;
 638                         return;
 639                 }
 640         }
 641         BUG();
 642 }
 643 
 644 static int takeover_tasklets(unsigned int cpu)
 645 {
 646         /* CPU is dead, so no lock needed. */
 647         local_irq_disable();
 648 
 649         /* Find end, append list for that CPU. */
 650         if (&per_cpu(tasklet_vec, cpu).head != per_cpu(tasklet_vec, cpu).tail) {
 651                 *__this_cpu_read(tasklet_vec.tail) = per_cpu(tasklet_vec, cpu).head;
 652                 __this_cpu_write(tasklet_vec.tail, per_cpu(tasklet_vec, cpu).tail);
 653                 per_cpu(tasklet_vec, cpu).head = NULL;
 654                 per_cpu(tasklet_vec, cpu).tail = &per_cpu(tasklet_vec, cpu).head;
 655         }
 656         raise_softirq_irqoff(TASKLET_SOFTIRQ);
 657 
 658         if (&per_cpu(tasklet_hi_vec, cpu).head != per_cpu(tasklet_hi_vec, cpu).tail) {
 659                 *__this_cpu_read(tasklet_hi_vec.tail) = per_cpu(tasklet_hi_vec, cpu).head;
 660                 __this_cpu_write(tasklet_hi_vec.tail, per_cpu(tasklet_hi_vec, cpu).tail);
 661                 per_cpu(tasklet_hi_vec, cpu).head = NULL;
 662                 per_cpu(tasklet_hi_vec, cpu).tail = &per_cpu(tasklet_hi_vec, cpu).head;
 663         }
 664         raise_softirq_irqoff(HI_SOFTIRQ);
 665 
 666         local_irq_enable();
 667         return 0;
 668 }
 669 #else
 670 #define takeover_tasklets       NULL
 671 #endif /* CONFIG_HOTPLUG_CPU */
 672 
 673 static struct smp_hotplug_thread softirq_threads = {
 674         .store                  = &ksoftirqd,
 675         .thread_should_run      = ksoftirqd_should_run,
 676         .thread_fn              = run_ksoftirqd,
 677         .thread_comm            = "ksoftirqd/%u",
 678 };
 679 
 680 static __init int spawn_ksoftirqd(void)
 681 {
 682         cpuhp_setup_state_nocalls(CPUHP_SOFTIRQ_DEAD, "softirq:dead", NULL,
 683                                   takeover_tasklets);
 684         BUG_ON(smpboot_register_percpu_thread(&softirq_threads));
 685 
 686         return 0;
 687 }
 688 early_initcall(spawn_ksoftirqd);
 689 
 690 /*
 691  * [ These __weak aliases are kept in a separate compilation unit, so that
 692  *   GCC does not inline them incorrectly. ]
 693  */
 694 
 695 int __init __weak early_irq_init(void)
 696 {
 697         return 0;
 698 }
 699 
 700 int __init __weak arch_probe_nr_irqs(void)
 701 {
 702         return NR_IRQS_LEGACY;
 703 }
 704 
 705 int __init __weak arch_early_irq_init(void)
 706 {
 707         return 0;
 708 }
 709 
 710 unsigned int __weak arch_dynirq_lower_bound(unsigned int from)
 711 {
 712         return from;
 713 }