root/kernel/stop_machine.c

DEFINITIONS

1. cpu_stop_init_done
2. cpu_stop_signal_done
3. __cpu_stop_queue_work
4. cpu_stop_queue_work
5. stop_one_cpu
6. set_state
7. ack_state
8. stop_machine_yield
9. multi_cpu_stop
10. cpu_stop_queue_two_works
11. stop_two_cpus
12. stop_one_cpu_nowait
13. queue_stop_cpus_work
14. __stop_cpus
15. stop_cpus
16. try_stop_cpus
17. cpu_stop_should_run
18. cpu_stopper_thread
19. stop_machine_park
20. cpu_stop_create
21. cpu_stop_park
22. stop_machine_unpark
23. cpu_stop_init
24. stop_machine_cpuslocked
25. stop_machine
26. stop_machine_from_inactive_cpu

   1 // SPDX-License-Identifier: GPL-2.0-or-later
   2 /*
   3  * kernel/stop_machine.c
   4  *
   5  * Copyright (C) 2008, 2005     IBM Corporation.
   6  * Copyright (C) 2008, 2005     Rusty Russell rusty@rustcorp.com.au
   7  * Copyright (C) 2010           SUSE Linux Products GmbH
   8  * Copyright (C) 2010           Tejun Heo <tj@kernel.org>
   9  */
  10 #include <linux/compiler.h>
  11 #include <linux/completion.h>
  12 #include <linux/cpu.h>
  13 #include <linux/init.h>
  14 #include <linux/kthread.h>
  15 #include <linux/export.h>
  16 #include <linux/percpu.h>
  17 #include <linux/sched.h>
  18 #include <linux/stop_machine.h>
  19 #include <linux/interrupt.h>
  20 #include <linux/kallsyms.h>
  21 #include <linux/smpboot.h>
  22 #include <linux/atomic.h>
  23 #include <linux/nmi.h>
  24 #include <linux/sched/wake_q.h>
  25 
  26 /*
  27  * Structure to determine completion condition and record errors.  May
  28  * be shared by works on different cpus.
  29  */
  30 struct cpu_stop_done {
  31         atomic_t                nr_todo;        /* nr left to execute */
  32         int                     ret;            /* collected return value */
  33         struct completion       completion;     /* fired if nr_todo reaches 0 */
  34 };
  35 
  36 /* the actual stopper, one per every possible cpu, enabled on online cpus */
  37 struct cpu_stopper {
  38         struct task_struct      *thread;
  39 
  40         raw_spinlock_t          lock;
  41         bool                    enabled;        /* is this stopper enabled? */
  42         struct list_head        works;          /* list of pending works */
  43 
  44         struct cpu_stop_work    stop_work;      /* for stop_cpus */
  45 };
  46 
  47 static DEFINE_PER_CPU(struct cpu_stopper, cpu_stopper);
  48 static bool stop_machine_initialized = false;
  49 
  50 /* static data for stop_cpus */
  51 static DEFINE_MUTEX(stop_cpus_mutex);
  52 static bool stop_cpus_in_progress;
  53 
  54 static void cpu_stop_init_done(struct cpu_stop_done *done, unsigned int nr_todo)
  55 {
  56         memset(done, 0, sizeof(*done));
  57         atomic_set(&done->nr_todo, nr_todo);
  58         init_completion(&done->completion);
  59 }
  60 
  61 /* signal completion unless @done is NULL */
  62 static void cpu_stop_signal_done(struct cpu_stop_done *done)
  63 {
  64         if (atomic_dec_and_test(&done->nr_todo))
  65                 complete(&done->completion);
  66 }
  67 
  68 static void __cpu_stop_queue_work(struct cpu_stopper *stopper,
  69                                         struct cpu_stop_work *work,
  70                                         struct wake_q_head *wakeq)
  71 {
  72         list_add_tail(&work->list, &stopper->works);
  73         wake_q_add(wakeq, stopper->thread);
  74 }
  75 
  76 /* queue @work to @stopper.  if offline, @work is completed immediately */
  77 static bool cpu_stop_queue_work(unsigned int cpu, struct cpu_stop_work *work)
  78 {
  79         struct cpu_stopper *stopper = &per_cpu(cpu_stopper, cpu);
  80         DEFINE_WAKE_Q(wakeq);
  81         unsigned long flags;
  82         bool enabled;
  83 
  84         preempt_disable();
  85         raw_spin_lock_irqsave(&stopper->lock, flags);
  86         enabled = stopper->enabled;
  87         if (enabled)
  88                 __cpu_stop_queue_work(stopper, work, &wakeq);
  89         else if (work->done)
  90                 cpu_stop_signal_done(work->done);
  91         raw_spin_unlock_irqrestore(&stopper->lock, flags);
  92 
  93         wake_up_q(&wakeq);
  94         preempt_enable();
  95 
  96         return enabled;
  97 }
  98 
  99 /**
 100  * stop_one_cpu - stop a cpu
 101  * @cpu: cpu to stop
 102  * @fn: function to execute
 103  * @arg: argument to @fn
 104  *
 105  * Execute @fn(@arg) on @cpu.  @fn is run in a process context with
 106  * the highest priority preempting any task on the cpu and
 107  * monopolizing it.  This function returns after the execution is
 108  * complete.
 109  *
 110  * This function doesn't guarantee @cpu stays online till @fn
 111  * completes.  If @cpu goes down in the middle, execution may happen
 112  * partially or fully on different cpus.  @fn should either be ready
 113  * for that or the caller should ensure that @cpu stays online until
 114  * this function completes.
 115  *
 116  * CONTEXT:
 117  * Might sleep.
 118  *
 119  * RETURNS:
 120  * -ENOENT if @fn(@arg) was not executed because @cpu was offline;
 121  * otherwise, the return value of @fn.
 122  */
 123 int stop_one_cpu(unsigned int cpu, cpu_stop_fn_t fn, void *arg)
 124 {
 125         struct cpu_stop_done done;
 126         struct cpu_stop_work work = { .fn = fn, .arg = arg, .done = &done };
 127 
 128         cpu_stop_init_done(&done, 1);
 129         if (!cpu_stop_queue_work(cpu, &work))
 130                 return -ENOENT;
 131         /*
 132          * In case @cpu == smp_proccessor_id() we can avoid a sleep+wakeup
 133          * cycle by doing a preemption:
 134          */
 135         cond_resched();
 136         wait_for_completion(&done.completion);
 137         return done.ret;
 138 }
 139 
 140 /* This controls the threads on each CPU. */
 141 enum multi_stop_state {
 142         /* Dummy starting state for thread. */
 143         MULTI_STOP_NONE,
 144         /* Awaiting everyone to be scheduled. */
 145         MULTI_STOP_PREPARE,
 146         /* Disable interrupts. */
 147         MULTI_STOP_DISABLE_IRQ,
 148         /* Run the function */
 149         MULTI_STOP_RUN,
 150         /* Exit */
 151         MULTI_STOP_EXIT,
 152 };
 153 
 154 struct multi_stop_data {
 155         cpu_stop_fn_t           fn;
 156         void                    *data;
 157         /* Like num_online_cpus(), but hotplug cpu uses us, so we need this. */
 158         unsigned int            num_threads;
 159         const struct cpumask    *active_cpus;
 160 
 161         enum multi_stop_state   state;
 162         atomic_t                thread_ack;
 163 };
 164 
 165 static void set_state(struct multi_stop_data *msdata,
 166                       enum multi_stop_state newstate)
 167 {
 168         /* Reset ack counter. */
 169         atomic_set(&msdata->thread_ack, msdata->num_threads);
 170         smp_wmb();
 171         WRITE_ONCE(msdata->state, newstate);
 172 }
 173 
 174 /* Last one to ack a state moves to the next state. */
 175 static void ack_state(struct multi_stop_data *msdata)
 176 {
 177         if (atomic_dec_and_test(&msdata->thread_ack))
 178                 set_state(msdata, msdata->state + 1);
 179 }
 180 
 181 void __weak stop_machine_yield(const struct cpumask *cpumask)
 182 {
 183         cpu_relax();
 184 }
 185 
 186 /* This is the cpu_stop function which stops the CPU. */
 187 static int multi_cpu_stop(void *data)
 188 {
 189         struct multi_stop_data *msdata = data;
 190         enum multi_stop_state newstate, curstate = MULTI_STOP_NONE;
 191         int cpu = smp_processor_id(), err = 0;
 192         const struct cpumask *cpumask;
 193         unsigned long flags;
 194         bool is_active;
 195 
 196         /*
 197          * When called from stop_machine_from_inactive_cpu(), irq might
 198          * already be disabled.  Save the state and restore it on exit.
 199          */
 200         local_save_flags(flags);
 201 
 202         if (!msdata->active_cpus) {
 203                 cpumask = cpu_online_mask;
 204                 is_active = cpu == cpumask_first(cpumask);
 205         } else {
 206                 cpumask = msdata->active_cpus;
 207                 is_active = cpumask_test_cpu(cpu, cpumask);
 208         }
 209 
 210         /* Simple state machine */
 211         do {
 212                 /* Chill out and ensure we re-read multi_stop_state. */
 213                 stop_machine_yield(cpumask);
 214                 newstate = READ_ONCE(msdata->state);
 215                 if (newstate != curstate) {
 216                         curstate = newstate;
 217                         switch (curstate) {
 218                         case MULTI_STOP_DISABLE_IRQ:
 219                                 local_irq_disable();
 220                                 hard_irq_disable();
 221                                 break;
 222                         case MULTI_STOP_RUN:
 223                                 if (is_active)
 224                                         err = msdata->fn(msdata->data);
 225                                 break;
 226                         default:
 227                                 break;
 228                         }
 229                         ack_state(msdata);
 230                 } else if (curstate > MULTI_STOP_PREPARE) {
 231                         /*
 232                          * At this stage all other CPUs we depend on must spin
 233                          * in the same loop. Any reason for hard-lockup should
 234                          * be detected and reported on their side.
 235                          */
 236                         touch_nmi_watchdog();
 237                 }
 238         } while (curstate != MULTI_STOP_EXIT);
 239 
 240         local_irq_restore(flags);
 241         return err;
 242 }
 243 
 244 static int cpu_stop_queue_two_works(int cpu1, struct cpu_stop_work *work1,
 245                                     int cpu2, struct cpu_stop_work *work2)
 246 {
 247         struct cpu_stopper *stopper1 = per_cpu_ptr(&cpu_stopper, cpu1);
 248         struct cpu_stopper *stopper2 = per_cpu_ptr(&cpu_stopper, cpu2);
 249         DEFINE_WAKE_Q(wakeq);
 250         int err;
 251 
 252 retry:
 253         /*
 254          * The waking up of stopper threads has to happen in the same
 255          * scheduling context as the queueing.  Otherwise, there is a
 256          * possibility of one of the above stoppers being woken up by another
 257          * CPU, and preempting us. This will cause us to not wake up the other
 258          * stopper forever.
 259          */
 260         preempt_disable();
 261         raw_spin_lock_irq(&stopper1->lock);
 262         raw_spin_lock_nested(&stopper2->lock, SINGLE_DEPTH_NESTING);
 263 
 264         if (!stopper1->enabled || !stopper2->enabled) {
 265                 err = -ENOENT;
 266                 goto unlock;
 267         }
 268 
 269         /*
 270          * Ensure that if we race with __stop_cpus() the stoppers won't get
 271          * queued up in reverse order leading to system deadlock.
 272          *
 273          * We can't miss stop_cpus_in_progress if queue_stop_cpus_work() has
 274          * queued a work on cpu1 but not on cpu2, we hold both locks.
 275          *
 276          * It can be falsely true but it is safe to spin until it is cleared,
 277          * queue_stop_cpus_work() does everything under preempt_disable().
 278          */
 279         if (unlikely(stop_cpus_in_progress)) {
 280                 err = -EDEADLK;
 281                 goto unlock;
 282         }
 283 
 284         err = 0;
 285         __cpu_stop_queue_work(stopper1, work1, &wakeq);
 286         __cpu_stop_queue_work(stopper2, work2, &wakeq);
 287 
 288 unlock:
 289         raw_spin_unlock(&stopper2->lock);
 290         raw_spin_unlock_irq(&stopper1->lock);
 291 
 292         if (unlikely(err == -EDEADLK)) {
 293                 preempt_enable();
 294 
 295                 while (stop_cpus_in_progress)
 296                         cpu_relax();
 297 
 298                 goto retry;
 299         }
 300 
 301         wake_up_q(&wakeq);
 302         preempt_enable();
 303 
 304         return err;
 305 }
 306 /**
 307  * stop_two_cpus - stops two cpus
 308  * @cpu1: the cpu to stop
 309  * @cpu2: the other cpu to stop
 310  * @fn: function to execute
 311  * @arg: argument to @fn
 312  *
 313  * Stops both the current and specified CPU and runs @fn on one of them.
 314  *
 315  * returns when both are completed.
 316  */
 317 int stop_two_cpus(unsigned int cpu1, unsigned int cpu2, cpu_stop_fn_t fn, void *arg)
 318 {
 319         struct cpu_stop_done done;
 320         struct cpu_stop_work work1, work2;
 321         struct multi_stop_data msdata;
 322 
 323         msdata = (struct multi_stop_data){
 324                 .fn = fn,
 325                 .data = arg,
 326                 .num_threads = 2,
 327                 .active_cpus = cpumask_of(cpu1),
 328         };
 329 
 330         work1 = work2 = (struct cpu_stop_work){
 331                 .fn = multi_cpu_stop,
 332                 .arg = &msdata,
 333                 .done = &done
 334         };
 335 
 336         cpu_stop_init_done(&done, 2);
 337         set_state(&msdata, MULTI_STOP_PREPARE);
 338 
 339         if (cpu1 > cpu2)
 340                 swap(cpu1, cpu2);
 341         if (cpu_stop_queue_two_works(cpu1, &work1, cpu2, &work2))
 342                 return -ENOENT;
 343 
 344         wait_for_completion(&done.completion);
 345         return done.ret;
 346 }
 347 
 348 /**
 349  * stop_one_cpu_nowait - stop a cpu but don't wait for completion
 350  * @cpu: cpu to stop
 351  * @fn: function to execute
 352  * @arg: argument to @fn
 353  * @work_buf: pointer to cpu_stop_work structure
 354  *
 355  * Similar to stop_one_cpu() but doesn't wait for completion.  The
 356  * caller is responsible for ensuring @work_buf is currently unused
 357  * and will remain untouched until stopper starts executing @fn.
 358  *
 359  * CONTEXT:
 360  * Don't care.
 361  *
 362  * RETURNS:
 363  * true if cpu_stop_work was queued successfully and @fn will be called,
 364  * false otherwise.
 365  */
 366 bool stop_one_cpu_nowait(unsigned int cpu, cpu_stop_fn_t fn, void *arg,
 367                         struct cpu_stop_work *work_buf)
 368 {
 369         *work_buf = (struct cpu_stop_work){ .fn = fn, .arg = arg, };
 370         return cpu_stop_queue_work(cpu, work_buf);
 371 }
 372 
 373 static bool queue_stop_cpus_work(const struct cpumask *cpumask,
 374                                  cpu_stop_fn_t fn, void *arg,
 375                                  struct cpu_stop_done *done)
 376 {
 377         struct cpu_stop_work *work;
 378         unsigned int cpu;
 379         bool queued = false;
 380 
 381         /*
 382          * Disable preemption while queueing to avoid getting
 383          * preempted by a stopper which might wait for other stoppers
 384          * to enter @fn which can lead to deadlock.
 385          */
 386         preempt_disable();
 387         stop_cpus_in_progress = true;
 388         barrier();
 389         for_each_cpu(cpu, cpumask) {
 390                 work = &per_cpu(cpu_stopper.stop_work, cpu);
 391                 work->fn = fn;
 392                 work->arg = arg;
 393                 work->done = done;
 394                 if (cpu_stop_queue_work(cpu, work))
 395                         queued = true;
 396         }
 397         barrier();
 398         stop_cpus_in_progress = false;
 399         preempt_enable();
 400 
 401         return queued;
 402 }
 403 
 404 static int __stop_cpus(const struct cpumask *cpumask,
 405                        cpu_stop_fn_t fn, void *arg)
 406 {
 407         struct cpu_stop_done done;
 408 
 409         cpu_stop_init_done(&done, cpumask_weight(cpumask));
 410         if (!queue_stop_cpus_work(cpumask, fn, arg, &done))
 411                 return -ENOENT;
 412         wait_for_completion(&done.completion);
 413         return done.ret;
 414 }
 415 
 416 /**
 417  * stop_cpus - stop multiple cpus
 418  * @cpumask: cpus to stop
 419  * @fn: function to execute
 420  * @arg: argument to @fn
 421  *
 422  * Execute @fn(@arg) on online cpus in @cpumask.  On each target cpu,
 423  * @fn is run in a process context with the highest priority
 424  * preempting any task on the cpu and monopolizing it.  This function
 425  * returns after all executions are complete.
 426  *
 427  * This function doesn't guarantee the cpus in @cpumask stay online
 428  * till @fn completes.  If some cpus go down in the middle, execution
 429  * on the cpu may happen partially or fully on different cpus.  @fn
 430  * should either be ready for that or the caller should ensure that
 431  * the cpus stay online until this function completes.
 432  *
 433  * All stop_cpus() calls are serialized making it safe for @fn to wait
 434  * for all cpus to start executing it.
 435  *
 436  * CONTEXT:
 437  * Might sleep.
 438  *
 439  * RETURNS:
 440  * -ENOENT if @fn(@arg) was not executed at all because all cpus in
 441  * @cpumask were offline; otherwise, 0 if all executions of @fn
 442  * returned 0, any non zero return value if any returned non zero.
 443  */
 444 int stop_cpus(const struct cpumask *cpumask, cpu_stop_fn_t fn, void *arg)
 445 {
 446         int ret;
 447 
 448         /* static works are used, process one request at a time */
 449         mutex_lock(&stop_cpus_mutex);
 450         ret = __stop_cpus(cpumask, fn, arg);
 451         mutex_unlock(&stop_cpus_mutex);
 452         return ret;
 453 }
 454 
 455 /**
 456  * try_stop_cpus - try to stop multiple cpus
 457  * @cpumask: cpus to stop
 458  * @fn: function to execute
 459  * @arg: argument to @fn
 460  *
 461  * Identical to stop_cpus() except that it fails with -EAGAIN if
 462  * someone else is already using the facility.
 463  *
 464  * CONTEXT:
 465  * Might sleep.
 466  *
 467  * RETURNS:
 468  * -EAGAIN if someone else is already stopping cpus, -ENOENT if
 469  * @fn(@arg) was not executed at all because all cpus in @cpumask were
 470  * offline; otherwise, 0 if all executions of @fn returned 0, any non
 471  * zero return value if any returned non zero.
 472  */
 473 int try_stop_cpus(const struct cpumask *cpumask, cpu_stop_fn_t fn, void *arg)
 474 {
 475         int ret;
 476 
 477         /* static works are used, process one request at a time */
 478         if (!mutex_trylock(&stop_cpus_mutex))
 479                 return -EAGAIN;
 480         ret = __stop_cpus(cpumask, fn, arg);
 481         mutex_unlock(&stop_cpus_mutex);
 482         return ret;
 483 }
 484 
 485 static int cpu_stop_should_run(unsigned int cpu)
 486 {
 487         struct cpu_stopper *stopper = &per_cpu(cpu_stopper, cpu);
 488         unsigned long flags;
 489         int run;
 490 
 491         raw_spin_lock_irqsave(&stopper->lock, flags);
 492         run = !list_empty(&stopper->works);
 493         raw_spin_unlock_irqrestore(&stopper->lock, flags);
 494         return run;
 495 }
 496 
 497 static void cpu_stopper_thread(unsigned int cpu)
 498 {
 499         struct cpu_stopper *stopper = &per_cpu(cpu_stopper, cpu);
 500         struct cpu_stop_work *work;
 501 
 502 repeat:
 503         work = NULL;
 504         raw_spin_lock_irq(&stopper->lock);
 505         if (!list_empty(&stopper->works)) {
 506                 work = list_first_entry(&stopper->works,
 507                                         struct cpu_stop_work, list);
 508                 list_del_init(&work->list);
 509         }
 510         raw_spin_unlock_irq(&stopper->lock);
 511 
 512         if (work) {
 513                 cpu_stop_fn_t fn = work->fn;
 514                 void *arg = work->arg;
 515                 struct cpu_stop_done *done = work->done;
 516                 int ret;
 517 
 518                 /* cpu stop callbacks must not sleep, make in_atomic() == T */
 519                 preempt_count_inc();
 520                 ret = fn(arg);
 521                 if (done) {
 522                         if (ret)
 523                                 done->ret = ret;
 524                         cpu_stop_signal_done(done);
 525                 }
 526                 preempt_count_dec();
 527                 WARN_ONCE(preempt_count(),
 528                           "cpu_stop: %ps(%p) leaked preempt count\n", fn, arg);
 529                 goto repeat;
 530         }
 531 }
 532 
 533 void stop_machine_park(int cpu)
 534 {
 535         struct cpu_stopper *stopper = &per_cpu(cpu_stopper, cpu);
 536         /*
 537          * Lockless. cpu_stopper_thread() will take stopper->lock and flush
 538          * the pending works before it parks, until then it is fine to queue
 539          * the new works.
 540          */
 541         stopper->enabled = false;
 542         kthread_park(stopper->thread);
 543 }
 544 
 545 extern void sched_set_stop_task(int cpu, struct task_struct *stop);
 546 
 547 static void cpu_stop_create(unsigned int cpu)
 548 {
 549         sched_set_stop_task(cpu, per_cpu(cpu_stopper.thread, cpu));
 550 }
 551 
 552 static void cpu_stop_park(unsigned int cpu)
 553 {
 554         struct cpu_stopper *stopper = &per_cpu(cpu_stopper, cpu);
 555 
 556         WARN_ON(!list_empty(&stopper->works));
 557 }
 558 
 559 void stop_machine_unpark(int cpu)
 560 {
 561         struct cpu_stopper *stopper = &per_cpu(cpu_stopper, cpu);
 562 
 563         stopper->enabled = true;
 564         kthread_unpark(stopper->thread);
 565 }
 566 
 567 static struct smp_hotplug_thread cpu_stop_threads = {
 568         .store                  = &cpu_stopper.thread,
 569         .thread_should_run      = cpu_stop_should_run,
 570         .thread_fn              = cpu_stopper_thread,
 571         .thread_comm            = "migration/%u",
 572         .create                 = cpu_stop_create,
 573         .park                   = cpu_stop_park,
 574         .selfparking            = true,
 575 };
 576 
 577 static int __init cpu_stop_init(void)
 578 {
 579         unsigned int cpu;
 580 
 581         for_each_possible_cpu(cpu) {
 582                 struct cpu_stopper *stopper = &per_cpu(cpu_stopper, cpu);
 583 
 584                 raw_spin_lock_init(&stopper->lock);
 585                 INIT_LIST_HEAD(&stopper->works);
 586         }
 587 
 588         BUG_ON(smpboot_register_percpu_thread(&cpu_stop_threads));
 589         stop_machine_unpark(raw_smp_processor_id());
 590         stop_machine_initialized = true;
 591         return 0;
 592 }
 593 early_initcall(cpu_stop_init);
 594 
 595 int stop_machine_cpuslocked(cpu_stop_fn_t fn, void *data,
 596                             const struct cpumask *cpus)
 597 {
 598         struct multi_stop_data msdata = {
 599                 .fn = fn,
 600                 .data = data,
 601                 .num_threads = num_online_cpus(),
 602                 .active_cpus = cpus,
 603         };
 604 
 605         lockdep_assert_cpus_held();
 606 
 607         if (!stop_machine_initialized) {
 608                 /*
 609                  * Handle the case where stop_machine() is called
 610                  * early in boot before stop_machine() has been
 611                  * initialized.
 612                  */
 613                 unsigned long flags;
 614                 int ret;
 615 
 616                 WARN_ON_ONCE(msdata.num_threads != 1);
 617 
 618                 local_irq_save(flags);
 619                 hard_irq_disable();
 620                 ret = (*fn)(data);
 621                 local_irq_restore(flags);
 622 
 623                 return ret;
 624         }
 625 
 626         /* Set the initial state and stop all online cpus. */
 627         set_state(&msdata, MULTI_STOP_PREPARE);
 628         return stop_cpus(cpu_online_mask, multi_cpu_stop, &msdata);
 629 }
 630 
 631 int stop_machine(cpu_stop_fn_t fn, void *data, const struct cpumask *cpus)
 632 {
 633         int ret;
 634 
 635         /* No CPUs can come up or down during this. */
 636         cpus_read_lock();
 637         ret = stop_machine_cpuslocked(fn, data, cpus);
 638         cpus_read_unlock();
 639         return ret;
 640 }
 641 EXPORT_SYMBOL_GPL(stop_machine);
 642 
 643 /**
 644  * stop_machine_from_inactive_cpu - stop_machine() from inactive CPU
 645  * @fn: the function to run
 646  * @data: the data ptr for the @fn()
 647  * @cpus: the cpus to run the @fn() on (NULL = any online cpu)
 648  *
 649  * This is identical to stop_machine() but can be called from a CPU which
 650  * is not active.  The local CPU is in the process of hotplug (so no other
 651  * CPU hotplug can start) and not marked active and doesn't have enough
 652  * context to sleep.
 653  *
 654  * This function provides stop_machine() functionality for such state by
 655  * using busy-wait for synchronization and executing @fn directly for local
 656  * CPU.
 657  *
 658  * CONTEXT:
 659  * Local CPU is inactive.  Temporarily stops all active CPUs.
 660  *
 661  * RETURNS:
 662  * 0 if all executions of @fn returned 0, any non zero return value if any
 663  * returned non zero.
 664  */
 665 int stop_machine_from_inactive_cpu(cpu_stop_fn_t fn, void *data,
 666                                   const struct cpumask *cpus)
 667 {
 668         struct multi_stop_data msdata = { .fn = fn, .data = data,
 669                                             .active_cpus = cpus };
 670         struct cpu_stop_done done;
 671         int ret;
 672 
 673         /* Local CPU must be inactive and CPU hotplug in progress. */
 674         BUG_ON(cpu_active(raw_smp_processor_id()));
 675         msdata.num_threads = num_active_cpus() + 1;     /* +1 for local */
 676 
 677         /* No proper task established and can't sleep - busy wait for lock. */
 678         while (!mutex_trylock(&stop_cpus_mutex))
 679                 cpu_relax();
 680 
 681         /* Schedule work on other CPUs and execute directly for local CPU */
 682         set_state(&msdata, MULTI_STOP_PREPARE);
 683         cpu_stop_init_done(&done, num_active_cpus());
 684         queue_stop_cpus_work(cpu_active_mask, multi_cpu_stop, &msdata,
 685                              &done);
 686         ret = multi_cpu_stop(&msdata);
 687 
 688         /* Busy wait for completion. */
 689         while (!completion_done(&done.completion))
 690                 cpu_relax();
 691 
 692         mutex_unlock(&stop_cpus_mutex);
 693         return ret ?: done.ret;
 694 }