kernel/stop_machine.c

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