Root/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/module.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
22#include <asm/atomic.h>
23
24/*
25 * Structure to determine completion condition and record errors. May
26 * be shared by works on different cpus.
27 */
28struct cpu_stop_done {
29    atomic_t nr_todo; /* nr left to execute */
30    bool executed; /* actually executed? */
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    spinlock_t lock;
38    bool enabled; /* is this stopper enabled? */
39    struct list_head works; /* list of pending works */
40    struct task_struct *thread; /* stopper thread */
41};
42
43static DEFINE_PER_CPU(struct cpu_stopper, cpu_stopper);
44
45static void cpu_stop_init_done(struct cpu_stop_done *done, unsigned int nr_todo)
46{
47    memset(done, 0, sizeof(*done));
48    atomic_set(&done->nr_todo, nr_todo);
49    init_completion(&done->completion);
50}
51
52/* signal completion unless @done is NULL */
53static void cpu_stop_signal_done(struct cpu_stop_done *done, bool executed)
54{
55    if (done) {
56        if (executed)
57            done->executed = true;
58        if (atomic_dec_and_test(&done->nr_todo))
59            complete(&done->completion);
60    }
61}
62
63/* queue @work to @stopper. if offline, @work is completed immediately */
64static void cpu_stop_queue_work(struct cpu_stopper *stopper,
65                struct cpu_stop_work *work)
66{
67    unsigned long flags;
68
69    spin_lock_irqsave(&stopper->lock, flags);
70
71    if (stopper->enabled) {
72        list_add_tail(&work->list, &stopper->works);
73        wake_up_process(stopper->thread);
74    } else
75        cpu_stop_signal_done(work->done, false);
76
77    spin_unlock_irqrestore(&stopper->lock, flags);
78}
79
80/**
81 * stop_one_cpu - stop a cpu
82 * @cpu: cpu to stop
83 * @fn: function to execute
84 * @arg: argument to @fn
85 *
86 * Execute @fn(@arg) on @cpu. @fn is run in a process context with
87 * the highest priority preempting any task on the cpu and
88 * monopolizing it. This function returns after the execution is
89 * complete.
90 *
91 * This function doesn't guarantee @cpu stays online till @fn
92 * completes. If @cpu goes down in the middle, execution may happen
93 * partially or fully on different cpus. @fn should either be ready
94 * for that or the caller should ensure that @cpu stays online until
95 * this function completes.
96 *
97 * CONTEXT:
98 * Might sleep.
99 *
100 * RETURNS:
101 * -ENOENT if @fn(@arg) was not executed because @cpu was offline;
102 * otherwise, the return value of @fn.
103 */
104int stop_one_cpu(unsigned int cpu, cpu_stop_fn_t fn, void *arg)
105{
106    struct cpu_stop_done done;
107    struct cpu_stop_work work = { .fn = fn, .arg = arg, .done = &done };
108
109    cpu_stop_init_done(&done, 1);
110    cpu_stop_queue_work(&per_cpu(cpu_stopper, cpu), &work);
111    wait_for_completion(&done.completion);
112    return done.executed ? done.ret : -ENOENT;
113}
114
115/**
116 * stop_one_cpu_nowait - stop a cpu but don't wait for completion
117 * @cpu: cpu to stop
118 * @fn: function to execute
119 * @arg: argument to @fn
120 *
121 * Similar to stop_one_cpu() but doesn't wait for completion. The
122 * caller is responsible for ensuring @work_buf is currently unused
123 * and will remain untouched until stopper starts executing @fn.
124 *
125 * CONTEXT:
126 * Don't care.
127 */
128void stop_one_cpu_nowait(unsigned int cpu, cpu_stop_fn_t fn, void *arg,
129            struct cpu_stop_work *work_buf)
130{
131    *work_buf = (struct cpu_stop_work){ .fn = fn, .arg = arg, };
132    cpu_stop_queue_work(&per_cpu(cpu_stopper, cpu), work_buf);
133}
134
135/* static data for stop_cpus */
136static DEFINE_MUTEX(stop_cpus_mutex);
137static DEFINE_PER_CPU(struct cpu_stop_work, stop_cpus_work);
138
139int __stop_cpus(const struct cpumask *cpumask, cpu_stop_fn_t fn, void *arg)
140{
141    struct cpu_stop_work *work;
142    struct cpu_stop_done done;
143    unsigned int cpu;
144
145    /* initialize works and done */
146    for_each_cpu(cpu, cpumask) {
147        work = &per_cpu(stop_cpus_work, cpu);
148        work->fn = fn;
149        work->arg = arg;
150        work->done = &done;
151    }
152    cpu_stop_init_done(&done, cpumask_weight(cpumask));
153
154    /*
155     * Disable preemption while queueing to avoid getting
156     * preempted by a stopper which might wait for other stoppers
157     * to enter @fn which can lead to deadlock.
158     */
159    preempt_disable();
160    for_each_cpu(cpu, cpumask)
161        cpu_stop_queue_work(&per_cpu(cpu_stopper, cpu),
162                    &per_cpu(stop_cpus_work, cpu));
163    preempt_enable();
164
165    wait_for_completion(&done.completion);
166    return done.executed ? done.ret : -ENOENT;
167}
168
169/**
170 * stop_cpus - stop multiple cpus
171 * @cpumask: cpus to stop
172 * @fn: function to execute
173 * @arg: argument to @fn
174 *
175 * Execute @fn(@arg) on online cpus in @cpumask. On each target cpu,
176 * @fn is run in a process context with the highest priority
177 * preempting any task on the cpu and monopolizing it. This function
178 * returns after all executions are complete.
179 *
180 * This function doesn't guarantee the cpus in @cpumask stay online
181 * till @fn completes. If some cpus go down in the middle, execution
182 * on the cpu may happen partially or fully on different cpus. @fn
183 * should either be ready for that or the caller should ensure that
184 * the cpus stay online until this function completes.
185 *
186 * All stop_cpus() calls are serialized making it safe for @fn to wait
187 * for all cpus to start executing it.
188 *
189 * CONTEXT:
190 * Might sleep.
191 *
192 * RETURNS:
193 * -ENOENT if @fn(@arg) was not executed at all because all cpus in
194 * @cpumask were offline; otherwise, 0 if all executions of @fn
195 * returned 0, any non zero return value if any returned non zero.
196 */
197int stop_cpus(const struct cpumask *cpumask, cpu_stop_fn_t fn, void *arg)
198{
199    int ret;
200
201    /* static works are used, process one request at a time */
202    mutex_lock(&stop_cpus_mutex);
203    ret = __stop_cpus(cpumask, fn, arg);
204    mutex_unlock(&stop_cpus_mutex);
205    return ret;
206}
207
208/**
209 * try_stop_cpus - try to stop multiple cpus
210 * @cpumask: cpus to stop
211 * @fn: function to execute
212 * @arg: argument to @fn
213 *
214 * Identical to stop_cpus() except that it fails with -EAGAIN if
215 * someone else is already using the facility.
216 *
217 * CONTEXT:
218 * Might sleep.
219 *
220 * RETURNS:
221 * -EAGAIN if someone else is already stopping cpus, -ENOENT if
222 * @fn(@arg) was not executed at all because all cpus in @cpumask were
223 * offline; otherwise, 0 if all executions of @fn returned 0, any non
224 * zero return value if any returned non zero.
225 */
226int try_stop_cpus(const struct cpumask *cpumask, cpu_stop_fn_t fn, void *arg)
227{
228    int ret;
229
230    /* static works are used, process one request at a time */
231    if (!mutex_trylock(&stop_cpus_mutex))
232        return -EAGAIN;
233    ret = __stop_cpus(cpumask, fn, arg);
234    mutex_unlock(&stop_cpus_mutex);
235    return ret;
236}
237
238static int cpu_stopper_thread(void *data)
239{
240    struct cpu_stopper *stopper = data;
241    struct cpu_stop_work *work;
242    int ret;
243
244repeat:
245    set_current_state(TASK_INTERRUPTIBLE); /* mb paired w/ kthread_stop */
246
247    if (kthread_should_stop()) {
248        __set_current_state(TASK_RUNNING);
249        return 0;
250    }
251
252    work = NULL;
253    spin_lock_irq(&stopper->lock);
254    if (!list_empty(&stopper->works)) {
255        work = list_first_entry(&stopper->works,
256                    struct cpu_stop_work, list);
257        list_del_init(&work->list);
258    }
259    spin_unlock_irq(&stopper->lock);
260
261    if (work) {
262        cpu_stop_fn_t fn = work->fn;
263        void *arg = work->arg;
264        struct cpu_stop_done *done = work->done;
265        char ksym_buf[KSYM_NAME_LEN];
266
267        __set_current_state(TASK_RUNNING);
268
269        /* cpu stop callbacks are not allowed to sleep */
270        preempt_disable();
271
272        ret = fn(arg);
273        if (ret)
274            done->ret = ret;
275
276        /* restore preemption and check it's still balanced */
277        preempt_enable();
278        WARN_ONCE(preempt_count(),
279              "cpu_stop: %s(%p) leaked preempt count\n",
280              kallsyms_lookup((unsigned long)fn, NULL, NULL, NULL,
281                      ksym_buf), arg);
282
283        cpu_stop_signal_done(done, true);
284    } else
285        schedule();
286
287    goto repeat;
288}
289
290/* manage stopper for a cpu, mostly lifted from sched migration thread mgmt */
291static int __cpuinit cpu_stop_cpu_callback(struct notifier_block *nfb,
292                       unsigned long action, void *hcpu)
293{
294    struct sched_param param = { .sched_priority = MAX_RT_PRIO - 1 };
295    unsigned int cpu = (unsigned long)hcpu;
296    struct cpu_stopper *stopper = &per_cpu(cpu_stopper, cpu);
297    struct task_struct *p;
298
299    switch (action & ~CPU_TASKS_FROZEN) {
300    case CPU_UP_PREPARE:
301        BUG_ON(stopper->thread || stopper->enabled ||
302               !list_empty(&stopper->works));
303        p = kthread_create(cpu_stopper_thread, stopper, "migration/%d",
304                   cpu);
305        if (IS_ERR(p))
306            return NOTIFY_BAD;
307        sched_setscheduler_nocheck(p, SCHED_FIFO, &param);
308        get_task_struct(p);
309        stopper->thread = p;
310        break;
311
312    case CPU_ONLINE:
313        kthread_bind(stopper->thread, cpu);
314        /* strictly unnecessary, as first user will wake it */
315        wake_up_process(stopper->thread);
316        /* mark enabled */
317        spin_lock_irq(&stopper->lock);
318        stopper->enabled = true;
319        spin_unlock_irq(&stopper->lock);
320        break;
321
322#ifdef CONFIG_HOTPLUG_CPU
323    case CPU_UP_CANCELED:
324    case CPU_POST_DEAD:
325    {
326        struct cpu_stop_work *work;
327
328        /* kill the stopper */
329        kthread_stop(stopper->thread);
330        /* drain remaining works */
331        spin_lock_irq(&stopper->lock);
332        list_for_each_entry(work, &stopper->works, list)
333            cpu_stop_signal_done(work->done, false);
334        stopper->enabled = false;
335        spin_unlock_irq(&stopper->lock);
336        /* release the stopper */
337        put_task_struct(stopper->thread);
338        stopper->thread = NULL;
339        break;
340    }
341#endif
342    }
343
344    return NOTIFY_OK;
345}
346
347/*
348 * Give it a higher priority so that cpu stopper is available to other
349 * cpu notifiers. It currently shares the same priority as sched
350 * migration_notifier.
351 */
352static struct notifier_block __cpuinitdata cpu_stop_cpu_notifier = {
353    .notifier_call = cpu_stop_cpu_callback,
354    .priority = 10,
355};
356
357static int __init cpu_stop_init(void)
358{
359    void *bcpu = (void *)(long)smp_processor_id();
360    unsigned int cpu;
361    int err;
362
363    for_each_possible_cpu(cpu) {
364        struct cpu_stopper *stopper = &per_cpu(cpu_stopper, cpu);
365
366        spin_lock_init(&stopper->lock);
367        INIT_LIST_HEAD(&stopper->works);
368    }
369
370    /* start one for the boot cpu */
371    err = cpu_stop_cpu_callback(&cpu_stop_cpu_notifier, CPU_UP_PREPARE,
372                    bcpu);
373    BUG_ON(err == NOTIFY_BAD);
374    cpu_stop_cpu_callback(&cpu_stop_cpu_notifier, CPU_ONLINE, bcpu);
375    register_cpu_notifier(&cpu_stop_cpu_notifier);
376
377    return 0;
378}
379early_initcall(cpu_stop_init);
380
381#ifdef CONFIG_STOP_MACHINE
382
383/* This controls the threads on each CPU. */
384enum stopmachine_state {
385    /* Dummy starting state for thread. */
386    STOPMACHINE_NONE,
387    /* Awaiting everyone to be scheduled. */
388    STOPMACHINE_PREPARE,
389    /* Disable interrupts. */
390    STOPMACHINE_DISABLE_IRQ,
391    /* Run the function */
392    STOPMACHINE_RUN,
393    /* Exit */
394    STOPMACHINE_EXIT,
395};
396
397struct stop_machine_data {
398    int (*fn)(void *);
399    void *data;
400    /* Like num_online_cpus(), but hotplug cpu uses us, so we need this. */
401    unsigned int num_threads;
402    const struct cpumask *active_cpus;
403
404    enum stopmachine_state state;
405    atomic_t thread_ack;
406};
407
408static void set_state(struct stop_machine_data *smdata,
409              enum stopmachine_state newstate)
410{
411    /* Reset ack counter. */
412    atomic_set(&smdata->thread_ack, smdata->num_threads);
413    smp_wmb();
414    smdata->state = newstate;
415}
416
417/* Last one to ack a state moves to the next state. */
418static void ack_state(struct stop_machine_data *smdata)
419{
420    if (atomic_dec_and_test(&smdata->thread_ack))
421        set_state(smdata, smdata->state + 1);
422}
423
424/* This is the cpu_stop function which stops the CPU. */
425static int stop_machine_cpu_stop(void *data)
426{
427    struct stop_machine_data *smdata = data;
428    enum stopmachine_state curstate = STOPMACHINE_NONE;
429    int cpu = smp_processor_id(), err = 0;
430    bool is_active;
431
432    if (!smdata->active_cpus)
433        is_active = cpu == cpumask_first(cpu_online_mask);
434    else
435        is_active = cpumask_test_cpu(cpu, smdata->active_cpus);
436
437    /* Simple state machine */
438    do {
439        /* Chill out and ensure we re-read stopmachine_state. */
440        cpu_relax();
441        if (smdata->state != curstate) {
442            curstate = smdata->state;
443            switch (curstate) {
444            case STOPMACHINE_DISABLE_IRQ:
445                local_irq_disable();
446                hard_irq_disable();
447                break;
448            case STOPMACHINE_RUN:
449                if (is_active)
450                    err = smdata->fn(smdata->data);
451                break;
452            default:
453                break;
454            }
455            ack_state(smdata);
456        }
457    } while (curstate != STOPMACHINE_EXIT);
458
459    local_irq_enable();
460    return err;
461}
462
463int __stop_machine(int (*fn)(void *), void *data, const struct cpumask *cpus)
464{
465    struct stop_machine_data smdata = { .fn = fn, .data = data,
466                        .num_threads = num_online_cpus(),
467                        .active_cpus = cpus };
468
469    /* Set the initial state and stop all online cpus. */
470    set_state(&smdata, STOPMACHINE_PREPARE);
471    return stop_cpus(cpu_online_mask, stop_machine_cpu_stop, &smdata);
472}
473
474int stop_machine(int (*fn)(void *), void *data, const struct cpumask *cpus)
475{
476    int ret;
477
478    /* No CPUs can come up or down during this. */
479    get_online_cpus();
480    ret = __stop_machine(fn, data, cpus);
481    put_online_cpus();
482    return ret;
483}
484EXPORT_SYMBOL_GPL(stop_machine);
485
486#endif /* CONFIG_STOP_MACHINE */
487

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