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

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