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Linux/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/lglock.h>
 24 
 25 /*
 26  * Structure to determine completion condition and record errors.  May
 27  * be shared by works on different cpus.
 28  */
 29 struct cpu_stop_done {
 30         atomic_t                nr_todo;        /* nr left to execute */
 31         bool                    executed;       /* actually executed? */
 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         spinlock_t              lock;
 39         bool                    enabled;        /* is this stopper enabled? */
 40         struct list_head        works;          /* list of pending works */
 41 };
 42 
 43 static DEFINE_PER_CPU(struct cpu_stopper, cpu_stopper);
 44 static DEFINE_PER_CPU(struct task_struct *, cpu_stopper_task);
 45 static bool stop_machine_initialized = false;
 46 
 47 /*
 48  * Avoids a race between stop_two_cpus and global stop_cpus, where
 49  * the stoppers could get queued up in reverse order, leading to
 50  * system deadlock. Using an lglock means stop_two_cpus remains
 51  * relatively cheap.
 52  */
 53 DEFINE_STATIC_LGLOCK(stop_cpus_lock);
 54 
 55 static void cpu_stop_init_done(struct cpu_stop_done *done, unsigned int nr_todo)
 56 {
 57         memset(done, 0, sizeof(*done));
 58         atomic_set(&done->nr_todo, nr_todo);
 59         init_completion(&done->completion);
 60 }
 61 
 62 /* signal completion unless @done is NULL */
 63 static void cpu_stop_signal_done(struct cpu_stop_done *done, bool executed)
 64 {
 65         if (done) {
 66                 if (executed)
 67                         done->executed = true;
 68                 if (atomic_dec_and_test(&done->nr_todo))
 69                         complete(&done->completion);
 70         }
 71 }
 72 
 73 /* queue @work to @stopper.  if offline, @work is completed immediately */
 74 static void cpu_stop_queue_work(unsigned int cpu, struct cpu_stop_work *work)
 75 {
 76         struct cpu_stopper *stopper = &per_cpu(cpu_stopper, cpu);
 77         struct task_struct *p = per_cpu(cpu_stopper_task, cpu);
 78 
 79         unsigned long flags;
 80 
 81         spin_lock_irqsave(&stopper->lock, flags);
 82 
 83         if (stopper->enabled) {
 84                 list_add_tail(&work->list, &stopper->works);
 85                 wake_up_process(p);
 86         } else
 87                 cpu_stop_signal_done(work->done, false);
 88 
 89         spin_unlock_irqrestore(&stopper->lock, flags);
 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  */
116 int 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         cpu_stop_queue_work(cpu, &work);
123         wait_for_completion(&done.completion);
124         return done.executed ? done.ret : -ENOENT;
125 }
126 
127 /* This controls the threads on each CPU. */
128 enum multi_stop_state {
129         /* Dummy starting state for thread. */
130         MULTI_STOP_NONE,
131         /* Awaiting everyone to be scheduled. */
132         MULTI_STOP_PREPARE,
133         /* Disable interrupts. */
134         MULTI_STOP_DISABLE_IRQ,
135         /* Run the function */
136         MULTI_STOP_RUN,
137         /* Exit */
138         MULTI_STOP_EXIT,
139 };
140 
141 struct multi_stop_data {
142         int                     (*fn)(void *);
143         void                    *data;
144         /* Like num_online_cpus(), but hotplug cpu uses us, so we need this. */
145         unsigned int            num_threads;
146         const struct cpumask    *active_cpus;
147 
148         enum multi_stop_state   state;
149         atomic_t                thread_ack;
150 };
151 
152 static void set_state(struct multi_stop_data *msdata,
153                       enum multi_stop_state newstate)
154 {
155         /* Reset ack counter. */
156         atomic_set(&msdata->thread_ack, msdata->num_threads);
157         smp_wmb();
158         msdata->state = newstate;
159 }
160 
161 /* Last one to ack a state moves to the next state. */
162 static void ack_state(struct multi_stop_data *msdata)
163 {
164         if (atomic_dec_and_test(&msdata->thread_ack))
165                 set_state(msdata, msdata->state + 1);
166 }
167 
168 /* This is the cpu_stop function which stops the CPU. */
169 static int multi_cpu_stop(void *data)
170 {
171         struct multi_stop_data *msdata = data;
172         enum multi_stop_state curstate = MULTI_STOP_NONE;
173         int cpu = smp_processor_id(), err = 0;
174         unsigned long flags;
175         bool is_active;
176 
177         /*
178          * When called from stop_machine_from_inactive_cpu(), irq might
179          * already be disabled.  Save the state and restore it on exit.
180          */
181         local_save_flags(flags);
182 
183         if (!msdata->active_cpus)
184                 is_active = cpu == cpumask_first(cpu_online_mask);
185         else
186                 is_active = cpumask_test_cpu(cpu, msdata->active_cpus);
187 
188         /* Simple state machine */
189         do {
190                 /* Chill out and ensure we re-read multi_stop_state. */
191                 cpu_relax();
192                 if (msdata->state != curstate) {
193                         curstate = msdata->state;
194                         switch (curstate) {
195                         case MULTI_STOP_DISABLE_IRQ:
196                                 local_irq_disable();
197                                 hard_irq_disable();
198                                 break;
199                         case MULTI_STOP_RUN:
200                                 if (is_active)
201                                         err = msdata->fn(msdata->data);
202                                 break;
203                         default:
204                                 break;
205                         }
206                         ack_state(msdata);
207                 }
208         } while (curstate != MULTI_STOP_EXIT);
209 
210         local_irq_restore(flags);
211         return err;
212 }
213 
214 struct irq_cpu_stop_queue_work_info {
215         int cpu1;
216         int cpu2;
217         struct cpu_stop_work *work1;
218         struct cpu_stop_work *work2;
219 };
220 
221 /*
222  * This function is always run with irqs and preemption disabled.
223  * This guarantees that both work1 and work2 get queued, before
224  * our local migrate thread gets the chance to preempt us.
225  */
226 static void irq_cpu_stop_queue_work(void *arg)
227 {
228         struct irq_cpu_stop_queue_work_info *info = arg;
229         cpu_stop_queue_work(info->cpu1, info->work1);
230         cpu_stop_queue_work(info->cpu2, info->work2);
231 }
232 
233 /**
234  * stop_two_cpus - stops two cpus
235  * @cpu1: the cpu to stop
236  * @cpu2: the other cpu to stop
237  * @fn: function to execute
238  * @arg: argument to @fn
239  *
240  * Stops both the current and specified CPU and runs @fn on one of them.
241  *
242  * returns when both are completed.
243  */
244 int stop_two_cpus(unsigned int cpu1, unsigned int cpu2, cpu_stop_fn_t fn, void *arg)
245 {
246         struct cpu_stop_done done;
247         struct cpu_stop_work work1, work2;
248         struct irq_cpu_stop_queue_work_info call_args;
249         struct multi_stop_data msdata;
250 
251         preempt_disable();
252         msdata = (struct multi_stop_data){
253                 .fn = fn,
254                 .data = arg,
255                 .num_threads = 2,
256                 .active_cpus = cpumask_of(cpu1),
257         };
258 
259         work1 = work2 = (struct cpu_stop_work){
260                 .fn = multi_cpu_stop,
261                 .arg = &msdata,
262                 .done = &done
263         };
264 
265         call_args = (struct irq_cpu_stop_queue_work_info){
266                 .cpu1 = cpu1,
267                 .cpu2 = cpu2,
268                 .work1 = &work1,
269                 .work2 = &work2,
270         };
271 
272         cpu_stop_init_done(&done, 2);
273         set_state(&msdata, MULTI_STOP_PREPARE);
274 
275         /*
276          * If we observe both CPUs active we know _cpu_down() cannot yet have
277          * queued its stop_machine works and therefore ours will get executed
278          * first. Or its not either one of our CPUs that's getting unplugged,
279          * in which case we don't care.
280          *
281          * This relies on the stopper workqueues to be FIFO.
282          */
283         if (!cpu_active(cpu1) || !cpu_active(cpu2)) {
284                 preempt_enable();
285                 return -ENOENT;
286         }
287 
288         lg_local_lock(&stop_cpus_lock);
289         /*
290          * Queuing needs to be done by the lowest numbered CPU, to ensure
291          * that works are always queued in the same order on every CPU.
292          * This prevents deadlocks.
293          */
294         smp_call_function_single(min(cpu1, cpu2),
295                                  &irq_cpu_stop_queue_work,
296                                  &call_args, 1);
297         lg_local_unlock(&stop_cpus_lock);
298         preempt_enable();
299 
300         wait_for_completion(&done.completion);
301 
302         return done.executed ? done.ret : -ENOENT;
303 }
304 
305 /**
306  * stop_one_cpu_nowait - stop a cpu but don't wait for completion
307  * @cpu: cpu to stop
308  * @fn: function to execute
309  * @arg: argument to @fn
310  *
311  * Similar to stop_one_cpu() but doesn't wait for completion.  The
312  * caller is responsible for ensuring @work_buf is currently unused
313  * and will remain untouched until stopper starts executing @fn.
314  *
315  * CONTEXT:
316  * Don't care.
317  */
318 void stop_one_cpu_nowait(unsigned int cpu, cpu_stop_fn_t fn, void *arg,
319                         struct cpu_stop_work *work_buf)
320 {
321         *work_buf = (struct cpu_stop_work){ .fn = fn, .arg = arg, };
322         cpu_stop_queue_work(cpu, work_buf);
323 }
324 
325 /* static data for stop_cpus */
326 static DEFINE_MUTEX(stop_cpus_mutex);
327 static DEFINE_PER_CPU(struct cpu_stop_work, stop_cpus_work);
328 
329 static void queue_stop_cpus_work(const struct cpumask *cpumask,
330                                  cpu_stop_fn_t fn, void *arg,
331                                  struct cpu_stop_done *done)
332 {
333         struct cpu_stop_work *work;
334         unsigned int cpu;
335 
336         /* initialize works and done */
337         for_each_cpu(cpu, cpumask) {
338                 work = &per_cpu(stop_cpus_work, cpu);
339                 work->fn = fn;
340                 work->arg = arg;
341                 work->done = done;
342         }
343 
344         /*
345          * Disable preemption while queueing to avoid getting
346          * preempted by a stopper which might wait for other stoppers
347          * to enter @fn which can lead to deadlock.
348          */
349         lg_global_lock(&stop_cpus_lock);
350         for_each_cpu(cpu, cpumask)
351                 cpu_stop_queue_work(cpu, &per_cpu(stop_cpus_work, cpu));
352         lg_global_unlock(&stop_cpus_lock);
353 }
354 
355 static int __stop_cpus(const struct cpumask *cpumask,
356                        cpu_stop_fn_t fn, void *arg)
357 {
358         struct cpu_stop_done done;
359 
360         cpu_stop_init_done(&done, cpumask_weight(cpumask));
361         queue_stop_cpus_work(cpumask, fn, arg, &done);
362         wait_for_completion(&done.completion);
363         return done.executed ? done.ret : -ENOENT;
364 }
365 
366 /**
367  * stop_cpus - stop multiple cpus
368  * @cpumask: cpus to stop
369  * @fn: function to execute
370  * @arg: argument to @fn
371  *
372  * Execute @fn(@arg) on online cpus in @cpumask.  On each target cpu,
373  * @fn is run in a process context with the highest priority
374  * preempting any task on the cpu and monopolizing it.  This function
375  * returns after all executions are complete.
376  *
377  * This function doesn't guarantee the cpus in @cpumask stay online
378  * till @fn completes.  If some cpus go down in the middle, execution
379  * on the cpu may happen partially or fully on different cpus.  @fn
380  * should either be ready for that or the caller should ensure that
381  * the cpus stay online until this function completes.
382  *
383  * All stop_cpus() calls are serialized making it safe for @fn to wait
384  * for all cpus to start executing it.
385  *
386  * CONTEXT:
387  * Might sleep.
388  *
389  * RETURNS:
390  * -ENOENT if @fn(@arg) was not executed at all because all cpus in
391  * @cpumask were offline; otherwise, 0 if all executions of @fn
392  * returned 0, any non zero return value if any returned non zero.
393  */
394 int stop_cpus(const struct cpumask *cpumask, cpu_stop_fn_t fn, void *arg)
395 {
396         int ret;
397 
398         /* static works are used, process one request at a time */
399         mutex_lock(&stop_cpus_mutex);
400         ret = __stop_cpus(cpumask, fn, arg);
401         mutex_unlock(&stop_cpus_mutex);
402         return ret;
403 }
404 
405 /**
406  * try_stop_cpus - try to stop multiple cpus
407  * @cpumask: cpus to stop
408  * @fn: function to execute
409  * @arg: argument to @fn
410  *
411  * Identical to stop_cpus() except that it fails with -EAGAIN if
412  * someone else is already using the facility.
413  *
414  * CONTEXT:
415  * Might sleep.
416  *
417  * RETURNS:
418  * -EAGAIN if someone else is already stopping cpus, -ENOENT if
419  * @fn(@arg) was not executed at all because all cpus in @cpumask were
420  * offline; otherwise, 0 if all executions of @fn returned 0, any non
421  * zero return value if any returned non zero.
422  */
423 int try_stop_cpus(const struct cpumask *cpumask, cpu_stop_fn_t fn, void *arg)
424 {
425         int ret;
426 
427         /* static works are used, process one request at a time */
428         if (!mutex_trylock(&stop_cpus_mutex))
429                 return -EAGAIN;
430         ret = __stop_cpus(cpumask, fn, arg);
431         mutex_unlock(&stop_cpus_mutex);
432         return ret;
433 }
434 
435 static int cpu_stop_should_run(unsigned int cpu)
436 {
437         struct cpu_stopper *stopper = &per_cpu(cpu_stopper, cpu);
438         unsigned long flags;
439         int run;
440 
441         spin_lock_irqsave(&stopper->lock, flags);
442         run = !list_empty(&stopper->works);
443         spin_unlock_irqrestore(&stopper->lock, flags);
444         return run;
445 }
446 
447 static void cpu_stopper_thread(unsigned int cpu)
448 {
449         struct cpu_stopper *stopper = &per_cpu(cpu_stopper, cpu);
450         struct cpu_stop_work *work;
451         int ret;
452 
453 repeat:
454         work = NULL;
455         spin_lock_irq(&stopper->lock);
456         if (!list_empty(&stopper->works)) {
457                 work = list_first_entry(&stopper->works,
458                                         struct cpu_stop_work, list);
459                 list_del_init(&work->list);
460         }
461         spin_unlock_irq(&stopper->lock);
462 
463         if (work) {
464                 cpu_stop_fn_t fn = work->fn;
465                 void *arg = work->arg;
466                 struct cpu_stop_done *done = work->done;
467                 char ksym_buf[KSYM_NAME_LEN] __maybe_unused;
468 
469                 /* cpu stop callbacks are not allowed to sleep */
470                 preempt_disable();
471 
472                 ret = fn(arg);
473                 if (ret)
474                         done->ret = ret;
475 
476                 /* restore preemption and check it's still balanced */
477                 preempt_enable();
478                 WARN_ONCE(preempt_count(),
479                           "cpu_stop: %s(%p) leaked preempt count\n",
480                           kallsyms_lookup((unsigned long)fn, NULL, NULL, NULL,
481                                           ksym_buf), arg);
482 
483                 cpu_stop_signal_done(done, true);
484                 goto repeat;
485         }
486 }
487 
488 extern void sched_set_stop_task(int cpu, struct task_struct *stop);
489 
490 static void cpu_stop_create(unsigned int cpu)
491 {
492         sched_set_stop_task(cpu, per_cpu(cpu_stopper_task, cpu));
493 }
494 
495 static void cpu_stop_park(unsigned int cpu)
496 {
497         struct cpu_stopper *stopper = &per_cpu(cpu_stopper, cpu);
498         struct cpu_stop_work *work;
499         unsigned long flags;
500 
501         /* drain remaining works */
502         spin_lock_irqsave(&stopper->lock, flags);
503         list_for_each_entry(work, &stopper->works, list)
504                 cpu_stop_signal_done(work->done, false);
505         stopper->enabled = false;
506         spin_unlock_irqrestore(&stopper->lock, flags);
507 }
508 
509 static void cpu_stop_unpark(unsigned int cpu)
510 {
511         struct cpu_stopper *stopper = &per_cpu(cpu_stopper, cpu);
512 
513         spin_lock_irq(&stopper->lock);
514         stopper->enabled = true;
515         spin_unlock_irq(&stopper->lock);
516 }
517 
518 static struct smp_hotplug_thread cpu_stop_threads = {
519         .store                  = &cpu_stopper_task,
520         .thread_should_run      = cpu_stop_should_run,
521         .thread_fn              = cpu_stopper_thread,
522         .thread_comm            = "migration/%u",
523         .create                 = cpu_stop_create,
524         .setup                  = cpu_stop_unpark,
525         .park                   = cpu_stop_park,
526         .pre_unpark             = cpu_stop_unpark,
527         .selfparking            = true,
528 };
529 
530 static int __init cpu_stop_init(void)
531 {
532         unsigned int cpu;
533 
534         for_each_possible_cpu(cpu) {
535                 struct cpu_stopper *stopper = &per_cpu(cpu_stopper, cpu);
536 
537                 spin_lock_init(&stopper->lock);
538                 INIT_LIST_HEAD(&stopper->works);
539         }
540 
541         BUG_ON(smpboot_register_percpu_thread(&cpu_stop_threads));
542         stop_machine_initialized = true;
543         return 0;
544 }
545 early_initcall(cpu_stop_init);
546 
547 #ifdef CONFIG_STOP_MACHINE
548 
549 int __stop_machine(int (*fn)(void *), void *data, const struct cpumask *cpus)
550 {
551         struct multi_stop_data msdata = {
552                 .fn = fn,
553                 .data = data,
554                 .num_threads = num_online_cpus(),
555                 .active_cpus = cpus,
556         };
557 
558         if (!stop_machine_initialized) {
559                 /*
560                  * Handle the case where stop_machine() is called
561                  * early in boot before stop_machine() has been
562                  * initialized.
563                  */
564                 unsigned long flags;
565                 int ret;
566 
567                 WARN_ON_ONCE(msdata.num_threads != 1);
568 
569                 local_irq_save(flags);
570                 hard_irq_disable();
571                 ret = (*fn)(data);
572                 local_irq_restore(flags);
573 
574                 return ret;
575         }
576 
577         /* Set the initial state and stop all online cpus. */
578         set_state(&msdata, MULTI_STOP_PREPARE);
579         return stop_cpus(cpu_online_mask, multi_cpu_stop, &msdata);
580 }
581 
582 int stop_machine(int (*fn)(void *), void *data, const struct cpumask *cpus)
583 {
584         int ret;
585 
586         /* No CPUs can come up or down during this. */
587         get_online_cpus();
588         ret = __stop_machine(fn, data, cpus);
589         put_online_cpus();
590         return ret;
591 }
592 EXPORT_SYMBOL_GPL(stop_machine);
593 
594 /**
595  * stop_machine_from_inactive_cpu - stop_machine() from inactive CPU
596  * @fn: the function to run
597  * @data: the data ptr for the @fn()
598  * @cpus: the cpus to run the @fn() on (NULL = any online cpu)
599  *
600  * This is identical to stop_machine() but can be called from a CPU which
601  * is not active.  The local CPU is in the process of hotplug (so no other
602  * CPU hotplug can start) and not marked active and doesn't have enough
603  * context to sleep.
604  *
605  * This function provides stop_machine() functionality for such state by
606  * using busy-wait for synchronization and executing @fn directly for local
607  * CPU.
608  *
609  * CONTEXT:
610  * Local CPU is inactive.  Temporarily stops all active CPUs.
611  *
612  * RETURNS:
613  * 0 if all executions of @fn returned 0, any non zero return value if any
614  * returned non zero.
615  */
616 int stop_machine_from_inactive_cpu(int (*fn)(void *), void *data,
617                                   const struct cpumask *cpus)
618 {
619         struct multi_stop_data msdata = { .fn = fn, .data = data,
620                                             .active_cpus = cpus };
621         struct cpu_stop_done done;
622         int ret;
623 
624         /* Local CPU must be inactive and CPU hotplug in progress. */
625         BUG_ON(cpu_active(raw_smp_processor_id()));
626         msdata.num_threads = num_active_cpus() + 1;     /* +1 for local */
627 
628         /* No proper task established and can't sleep - busy wait for lock. */
629         while (!mutex_trylock(&stop_cpus_mutex))
630                 cpu_relax();
631 
632         /* Schedule work on other CPUs and execute directly for local CPU */
633         set_state(&msdata, MULTI_STOP_PREPARE);
634         cpu_stop_init_done(&done, num_active_cpus());
635         queue_stop_cpus_work(cpu_active_mask, multi_cpu_stop, &msdata,
636                              &done);
637         ret = multi_cpu_stop(&msdata);
638 
639         /* Busy wait for completion. */
640         while (!completion_done(&done.completion))
641                 cpu_relax();
642 
643         mutex_unlock(&stop_cpus_mutex);
644         return ret ?: done.ret;
645 }
646 
647 #endif  /* CONFIG_STOP_MACHINE */
648 

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