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