kernel/stop_machine.c

   1 /*
   2  * kernel/stop_machine.c
   3  *
   4  * Copyright (C) 2008, 2005     IBM Corporation.
   5  * Copyright (C) 2008, 2005     Rusty Russell rusty@rustcorp.com.au
   6  * Copyright (C) 2010           SUSE Linux Products GmbH
   7  * Copyright (C) 2010           Tejun Heo <tj@kernel.org>
   8  *
   9  * This file is released under the GPLv2 and any later version.
  10  */
  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         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 /* This is the cpu_stop function which stops the CPU. */
 182 static int multi_cpu_stop(void *data)
 183 {
 184         struct multi_stop_data *msdata = data;
 185         enum multi_stop_state curstate = MULTI_STOP_NONE;
 186         int cpu = smp_processor_id(), err = 0;
 187         unsigned long flags;
 188         bool is_active;
 189
 190         /*
 191          * When called from stop_machine_from_inactive_cpu(), irq might
 192          * already be disabled.  Save the state and restore it on exit.
 193          */
 194         local_save_flags(flags);
 195
 196         if (!msdata->active_cpus)
 197                 is_active = cpu == cpumask_first(cpu_online_mask);
 198         else
 199                 is_active = cpumask_test_cpu(cpu, msdata->active_cpus);
 200
 201         /* Simple state machine */
 202         do {
 203                 /* Chill out and ensure we re-read multi_stop_state. */
 204                 cpu_relax_yield();
 205                 if (msdata->state != curstate) {
 206                         curstate = msdata->state;
 207                         switch (curstate) {
 208                         case MULTI_STOP_DISABLE_IRQ:
 209                                 local_irq_disable();
 210                                 hard_irq_disable();
 211                                 break;
 212                         case MULTI_STOP_RUN:
 213                                 if (is_active)
 214                                         err = msdata->fn(msdata->data);
 215                                 break;
 216                         default:
 217                                 break;
 218                         }
 219                         ack_state(msdata);
 220                 } else if (curstate > MULTI_STOP_PREPARE) {
 221                         /*
 222                          * At this stage all other CPUs we depend on must spin
 223                          * in the same loop. Any reason for hard-lockup should
 224                          * be detected and reported on their side.
 225                          */
 226                         touch_nmi_watchdog();
 227                 }
 228         } while (curstate != MULTI_STOP_EXIT);
 229
 230         local_irq_restore(flags);
 231         return err;
 232 }
 233
 234 static int cpu_stop_queue_two_works(int cpu1, struct cpu_stop_work *work1,
 235                                     int cpu2, struct cpu_stop_work *work2)
 236 {
 237         struct cpu_stopper *stopper1 = per_cpu_ptr(&cpu_stopper, cpu1);
 238         struct cpu_stopper *stopper2 = per_cpu_ptr(&cpu_stopper, cpu2);
 239         DEFINE_WAKE_Q(wakeq);
 240         int err;
 241 retry:
 242         raw_spin_lock_irq(&stopper1->lock);
 243         raw_spin_lock_nested(&stopper2->lock, SINGLE_DEPTH_NESTING);
 244
 245         err = -ENOENT;
 246         if (!stopper1->enabled || !stopper2->enabled)
 247                 goto unlock;
 248         /*
 249          * Ensure that if we race with __stop_cpus() the stoppers won't get
 250          * queued up in reverse order leading to system deadlock.
 251          *
 252          * We can't miss stop_cpus_in_progress if queue_stop_cpus_work() has
 253          * queued a work on cpu1 but not on cpu2, we hold both locks.
 254          *
 255          * It can be falsely true but it is safe to spin until it is cleared,
 256          * queue_stop_cpus_work() does everything under preempt_disable().
 257          */
 258         err = -EDEADLK;
 259         if (unlikely(stop_cpus_in_progress))
 260                         goto unlock;
 261
 262         err = 0;
 263         __cpu_stop_queue_work(stopper1, work1, &wakeq);
 264         __cpu_stop_queue_work(stopper2, work2, &wakeq);
 265         /*
 266          * The waking up of stopper threads has to happen
 267          * in the same scheduling context as the queueing.
 268          * Otherwise, there is a possibility of one of the
 269          * above stoppers being woken up by another CPU,
 270          * and preempting us. This will cause us to n ot
 271          * wake up the other stopper forever.
 272          */
 273         preempt_disable();
 274 unlock:
 275         raw_spin_unlock(&stopper2->lock);
 276         raw_spin_unlock_irq(&stopper1->lock);
 277
 278         if (unlikely(err == -EDEADLK)) {
 279                 while (stop_cpus_in_progress)
 280                         cpu_relax();
 281                 goto retry;
 282         }
 283
 284         if (!err) {
 285                 wake_up_q(&wakeq);
 286                 preempt_enable();
 287         }
 288
 289         return err;
 290 }
 291 /**
 292  * stop_two_cpus - stops two cpus
 293  * @cpu1: the cpu to stop
 294  * @cpu2: the other cpu to stop
 295  * @fn: function to execute
 296  * @arg: argument to @fn
 297  *
 298  * Stops both the current and specified CPU and runs @fn on one of them.
 299  *
 300  * returns when both are completed.
 301  */
 302 int stop_two_cpus(unsigned int cpu1, unsigned int cpu2, cpu_stop_fn_t fn, void *arg)
 303 {
 304         struct cpu_stop_done done;
 305         struct cpu_stop_work work1, work2;
 306         struct multi_stop_data msdata;
 307
 308         msdata = (struct multi_stop_data){
 309                 .fn = fn,
 310                 .data = arg,
 311                 .num_threads = 2,
 312                 .active_cpus = cpumask_of(cpu1),
 313         };
 314
 315         work1 = work2 = (struct cpu_stop_work){
 316                 .fn = multi_cpu_stop,
 317                 .arg = &msdata,
 318                 .done = &done
 319         };
 320
 321         cpu_stop_init_done(&done, 2);
 322         set_state(&msdata, MULTI_STOP_PREPARE);
 323
 324         if (cpu1 > cpu2)
 325                 swap(cpu1, cpu2);
 326         if (cpu_stop_queue_two_works(cpu1, &work1, cpu2, &work2))
 327                 return -ENOENT;
 328
 329         wait_for_completion(&done.completion);
 330         return done.ret;
 331 }
 332
 333 /**
 334  * stop_one_cpu_nowait - stop a cpu but don't wait for completion
 335  * @cpu: cpu to stop
 336  * @fn: function to execute
 337  * @arg: argument to @fn
 338  * @work_buf: pointer to cpu_stop_work structure
 339  *
 340  * Similar to stop_one_cpu() but doesn't wait for completion.  The
 341  * caller is responsible for ensuring @work_buf is currently unused
 342  * and will remain untouched until stopper starts executing @fn.
 343  *
 344  * CONTEXT:
 345  * Don't care.
 346  *
 347  * RETURNS:
 348  * true if cpu_stop_work was queued successfully and @fn will be called,
 349  * false otherwise.
 350  */
 351 bool stop_one_cpu_nowait(unsigned int cpu, cpu_stop_fn_t fn, void *arg,
 352                         struct cpu_stop_work *work_buf)
 353 {
 354         *work_buf = (struct cpu_stop_work){ .fn = fn, .arg = arg, };
 355         return cpu_stop_queue_work(cpu, work_buf);
 356 }
 357
 358 static bool queue_stop_cpus_work(const struct cpumask *cpumask,
 359                                  cpu_stop_fn_t fn, void *arg,
 360                                  struct cpu_stop_done *done)
 361 {
 362         struct cpu_stop_work *work;
 363         unsigned int cpu;
 364         bool queued = false;
 365
 366         /*
 367          * Disable preemption while queueing to avoid getting
 368          * preempted by a stopper which might wait for other stoppers
 369          * to enter @fn which can lead to deadlock.
 370          */
 371         preempt_disable();
 372         stop_cpus_in_progress = true;
 373         for_each_cpu(cpu, cpumask) {
 374                 work = &per_cpu(cpu_stopper.stop_work, cpu);
 375                 work->fn = fn;
 376                 work->arg = arg;
 377                 work->done = done;
 378                 if (cpu_stop_queue_work(cpu, work))
 379                         queued = true;
 380         }
 381         stop_cpus_in_progress = false;
 382         preempt_enable();
 383
 384         return queued;
 385 }
 386
 387 static int __stop_cpus(const struct cpumask *cpumask,
 388                        cpu_stop_fn_t fn, void *arg)
 389 {
 390         struct cpu_stop_done done;
 391
 392         cpu_stop_init_done(&done, cpumask_weight(cpumask));
 393         if (!queue_stop_cpus_work(cpumask, fn, arg, &done))
 394                 return -ENOENT;
 395         wait_for_completion(&done.completion);
 396         return done.ret;
 397 }
 398
 399 /**
 400  * stop_cpus - stop multiple cpus
 401  * @cpumask: cpus to stop
 402  * @fn: function to execute
 403  * @arg: argument to @fn
 404  *
 405  * Execute @fn(@arg) on online cpus in @cpumask.  On each target cpu,
 406  * @fn is run in a process context with the highest priority
 407  * preempting any task on the cpu and monopolizing it.  This function
 408  * returns after all executions are complete.
 409  *
 410  * This function doesn't guarantee the cpus in @cpumask stay online
 411  * till @fn completes.  If some cpus go down in the middle, execution
 412  * on the cpu may happen partially or fully on different cpus.  @fn
 413  * should either be ready for that or the caller should ensure that
 414  * the cpus stay online until this function completes.
 415  *
 416  * All stop_cpus() calls are serialized making it safe for @fn to wait
 417  * for all cpus to start executing it.
 418  *
 419  * CONTEXT:
 420  * Might sleep.
 421  *
 422  * RETURNS:
 423  * -ENOENT if @fn(@arg) was not executed at all because all cpus in
 424  * @cpumask were offline; otherwise, 0 if all executions of @fn
 425  * returned 0, any non zero return value if any returned non zero.
 426  */
 427 int stop_cpus(const struct cpumask *cpumask, cpu_stop_fn_t fn, void *arg)
 428 {
 429         int ret;
 430
 431         /* static works are used, process one request at a time */
 432         mutex_lock(&stop_cpus_mutex);
 433         ret = __stop_cpus(cpumask, fn, arg);
 434         mutex_unlock(&stop_cpus_mutex);
 435         return ret;
 436 }
 437
 438 /**
 439  * try_stop_cpus - try to stop multiple cpus
 440  * @cpumask: cpus to stop
 441  * @fn: function to execute
 442  * @arg: argument to @fn
 443  *
 444  * Identical to stop_cpus() except that it fails with -EAGAIN if
 445  * someone else is already using the facility.
 446  *
 447  * CONTEXT:
 448  * Might sleep.
 449  *
 450  * RETURNS:
 451  * -EAGAIN if someone else is already stopping cpus, -ENOENT if
 452  * @fn(@arg) was not executed at all because all cpus in @cpumask were
 453  * offline; otherwise, 0 if all executions of @fn returned 0, any non
 454  * zero return value if any returned non zero.
 455  */
 456 int try_stop_cpus(const struct cpumask *cpumask, cpu_stop_fn_t fn, void *arg)
 457 {
 458         int ret;
 459
 460         /* static works are used, process one request at a time */
 461         if (!mutex_trylock(&stop_cpus_mutex))
 462                 return -EAGAIN;
 463         ret = __stop_cpus(cpumask, fn, arg);
 464         mutex_unlock(&stop_cpus_mutex);
 465         return ret;
 466 }
 467
 468 static int cpu_stop_should_run(unsigned int cpu)
 469 {
 470         struct cpu_stopper *stopper = &per_cpu(cpu_stopper, cpu);
 471         unsigned long flags;
 472         int run;
 473
 474         raw_spin_lock_irqsave(&stopper->lock, flags);
 475         run = !list_empty(&stopper->works);
 476         raw_spin_unlock_irqrestore(&stopper->lock, flags);
 477         return run;
 478 }
 479
 480 static void cpu_stopper_thread(unsigned int cpu)
 481 {
 482         struct cpu_stopper *stopper = &per_cpu(cpu_stopper, cpu);
 483         struct cpu_stop_work *work;
 484
 485 repeat:
 486         work = NULL;
 487         raw_spin_lock_irq(&stopper->lock);
 488         if (!list_empty(&stopper->works)) {
 489                 work = list_first_entry(&stopper->works,
 490                                         struct cpu_stop_work, list);
 491                 list_del_init(&work->list);
 492         }
 493         raw_spin_unlock_irq(&stopper->lock);
 494
 495         if (work) {
 496                 cpu_stop_fn_t fn = work->fn;
 497                 void *arg = work->arg;
 498                 struct cpu_stop_done *done = work->done;
 499                 int ret;
 500
 501                 /* cpu stop callbacks must not sleep, make in_atomic() == T */
 502                 preempt_count_inc();
 503                 ret = fn(arg);
 504                 if (done) {
 505                         if (ret)
 506                                 done->ret = ret;
 507                         cpu_stop_signal_done(done);
 508                 }
 509                 preempt_count_dec();
 510                 WARN_ONCE(preempt_count(),
 511                           "cpu_stop: %pf(%p) leaked preempt count\n", fn, arg);
 512                 goto repeat;
 513         }
 514 }
 515
 516 void stop_machine_park(int cpu)
 517 {
 518         struct cpu_stopper *stopper = &per_cpu(cpu_stopper, cpu);
 519         /*
 520          * Lockless. cpu_stopper_thread() will take stopper->lock and flush
 521          * the pending works before it parks, until then it is fine to queue
 522          * the new works.
 523          */
 524         stopper->enabled = false;
 525         kthread_park(stopper->thread);
 526 }
 527
 528 extern void sched_set_stop_task(int cpu, struct task_struct *stop);
 529
 530 static void cpu_stop_create(unsigned int cpu)
 531 {
 532         sched_set_stop_task(cpu, per_cpu(cpu_stopper.thread, cpu));
 533 }
 534
 535 static void cpu_stop_park(unsigned int cpu)
 536 {
 537         struct cpu_stopper *stopper = &per_cpu(cpu_stopper, cpu);
 538
 539         WARN_ON(!list_empty(&stopper->works));
 540 }
 541
 542 void stop_machine_unpark(int cpu)
 543 {
 544         struct cpu_stopper *stopper = &per_cpu(cpu_stopper, cpu);
 545
 546         stopper->enabled = true;
 547         kthread_unpark(stopper->thread);
 548 }
 549
 550 static struct smp_hotplug_thread cpu_stop_threads = {
 551         .store                  = &cpu_stopper.thread,
 552         .thread_should_run      = cpu_stop_should_run,
 553         .thread_fn              = cpu_stopper_thread,
 554         .thread_comm            = "migration/%u",
 555         .create                 = cpu_stop_create,
 556         .park                   = cpu_stop_park,
 557         .selfparking            = true,
 558 };
 559
 560 static int __init cpu_stop_init(void)
 561 {
 562         unsigned int cpu;
 563
 564         for_each_possible_cpu(cpu) {
 565                 struct cpu_stopper *stopper = &per_cpu(cpu_stopper, cpu);
 566
 567                 raw_spin_lock_init(&stopper->lock);
 568                 INIT_LIST_HEAD(&stopper->works);
 569         }
 570
 571         BUG_ON(smpboot_register_percpu_thread(&cpu_stop_threads));
 572         stop_machine_unpark(raw_smp_processor_id());
 573         stop_machine_initialized = true;
 574         return 0;
 575 }
 576 early_initcall(cpu_stop_init);
 577
 578 int stop_machine_cpuslocked(cpu_stop_fn_t fn, void *data,
 579                             const struct cpumask *cpus)
 580 {
 581         struct multi_stop_data msdata = {
 582                 .fn = fn,
 583                 .data = data,
 584                 .num_threads = num_online_cpus(),
 585                 .active_cpus = cpus,
 586         };
 587
 588         lockdep_assert_cpus_held();
 589
 590         if (!stop_machine_initialized) {
 591                 /*
 592                  * Handle the case where stop_machine() is called
 593                  * early in boot before stop_machine() has been
 594                  * initialized.
 595                  */
 596                 unsigned long flags;
 597                 int ret;
 598
 599                 WARN_ON_ONCE(msdata.num_threads != 1);
 600
 601                 local_irq_save(flags);
 602                 hard_irq_disable();
 603                 ret = (*fn)(data);
 604                 local_irq_restore(flags);
 605
 606                 return ret;
 607         }
 608
 609         /* Set the initial state and stop all online cpus. */
 610         set_state(&msdata, MULTI_STOP_PREPARE);
 611         return stop_cpus(cpu_online_mask, multi_cpu_stop, &msdata);
 612 }
 613
 614 int stop_machine(cpu_stop_fn_t fn, void *data, const struct cpumask *cpus)
 615 {
 616         int ret;
 617
 618         /* No CPUs can come up or down during this. */
 619         cpus_read_lock();
 620         ret = stop_machine_cpuslocked(fn, data, cpus);
 621         cpus_read_unlock();
 622         return ret;
 623 }
 624 EXPORT_SYMBOL_GPL(stop_machine);
 625
 626 /**
 627  * stop_machine_from_inactive_cpu - stop_machine() from inactive CPU
 628  * @fn: the function to run
 629  * @data: the data ptr for the @fn()
 630  * @cpus: the cpus to run the @fn() on (NULL = any online cpu)
 631  *
 632  * This is identical to stop_machine() but can be called from a CPU which
 633  * is not active.  The local CPU is in the process of hotplug (so no other
 634  * CPU hotplug can start) and not marked active and doesn't have enough
 635  * context to sleep.
 636  *
 637  * This function provides stop_machine() functionality for such state by
 638  * using busy-wait for synchronization and executing @fn directly for local
 639  * CPU.
 640  *
 641  * CONTEXT:
 642  * Local CPU is inactive.  Temporarily stops all active CPUs.
 643  *
 644  * RETURNS:
 645  * 0 if all executions of @fn returned 0, any non zero return value if any
 646  * returned non zero.
 647  */
 648 int stop_machine_from_inactive_cpu(cpu_stop_fn_t fn, void *data,
 649                                   const struct cpumask *cpus)
 650 {
 651         struct multi_stop_data msdata = { .fn = fn, .data = data,
 652                                             .active_cpus = cpus };
 653         struct cpu_stop_done done;
 654         int ret;
 655
 656         /* Local CPU must be inactive and CPU hotplug in progress. */
 657         BUG_ON(cpu_active(raw_smp_processor_id()));
 658         msdata.num_threads = num_active_cpus() + 1;     /* +1 for local */
 659
 660         /* No proper task established and can't sleep - busy wait for lock. */
 661         while (!mutex_trylock(&stop_cpus_mutex))
 662                 cpu_relax();
 663
 664         /* Schedule work on other CPUs and execute directly for local CPU */
 665         set_state(&msdata, MULTI_STOP_PREPARE);
 666         cpu_stop_init_done(&done, num_active_cpus());
 667         queue_stop_cpus_work(cpu_active_mask, multi_cpu_stop, &msdata,
 668                              &done);
 669         ret = multi_cpu_stop(&msdata);
 670
 671         /* Busy wait for completion. */
 672         while (!completion_done(&done.completion))
 673                 cpu_relax();
 674
 675         mutex_unlock(&stop_cpus_mutex);
 676         return ret ?: done.ret;
 677 }