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Commit d721304d authored by Oleg Nesterov's avatar Oleg Nesterov Committed by Linus Torvalds
Browse files

workqueue: fix flush_workqueue() vs CPU_DEAD race 



Many thanks to Srivatsa Vaddagiri for the helpful discussion and for spotting
the bug in my previous attempt.

work->func() (and thus flush_workqueue()) must not use workqueue_mutex,
this leads to deadlock when CPU_DEAD does kthread_stop(). However without
this mutex held we can't detect CPU_DEAD in progress, which can move pending
works to another CPU while the dead one is not on cpu_online_map.

Change flush_workqueue() to use for_each_possible_cpu(). This means that
flush_cpu_workqueue() may hit CPU which is already dead. However in that
case

	!list_empty(&cwq->worklist) || cwq->current_work != NULL

means that CPU_DEAD in progress, it will do kthread_stop() + take_over_work()
so we can proceed and insert a barrier. We hold cwq->lock, so we are safe.

Also, add migrate_sequence incremented by take_over_work() under cwq->lock.
If take_over_work() happened before we checked this CPU, we should see the
new value after spin_unlock().

Further possible changes:

	remove CPU_DEAD handling (along with take_over_work, migrate_sequence)
	from workqueue.c. CPU_DEAD just sets cwq->please_exit_after_flush flag.

	CPU_UP_PREPARE->create_workqueue_thread() clears this flag, and creates
	the new thread if cwq->thread == NULL.

This way the workqueue/cpu-hotplug interaction is almost zero, workqueue_mutex
just protects "workqueues" list, CPU_LOCK_ACQUIRE/CPU_LOCK_RELEASE go away.

Signed-off-by: default avatarOleg Nesterov <oleg@tv-sign.ru>
Cc: Srivatsa Vaddagiri <vatsa@in.ibm.com>
Cc: "Pallipadi, Venkatesh" <venkatesh.pallipadi@intel.com>
Cc: Gautham shenoy <ego@in.ibm.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 319c2a98

kernel/workqueue.c

+25 −19
Original line number Diff line number Diff line
@@ -64,6 +64,7 @@ struct workqueue_struct { 


/* All the per-cpu workqueues on the system, for hotplug cpu to add/remove

   threads to each one as cpus come/go. */

static long migrate_sequence __read_mostly;

static DEFINE_MUTEX(workqueue_mutex);

static LIST_HEAD(workqueues);
@@ -421,13 +422,7 @@ static void flush_cpu_workqueue(struct cpu_workqueue_struct *cwq) 
		 * Probably keventd trying to flush its own queue. So simply run

		 * it by hand rather than deadlocking.

		 */

		preempt_enable();

		/*

		 * We can still touch *cwq here because we are keventd, and

		 * hot-unplug will be waiting us to exit.

		 */

		run_workqueue(cwq);

		preempt_disable();

	} else {

		struct wq_barrier barr;

		int active = 0;
@@ -439,11 +434,8 @@ static void flush_cpu_workqueue(struct cpu_workqueue_struct *cwq) 
		}

		spin_unlock_irq(&cwq->lock);



		if (active) {

			preempt_enable();

		if (active)

			wait_for_completion(&barr.done);

			preempt_disable();

		}

	}

}
@@ -462,17 +454,21 @@ static void flush_cpu_workqueue(struct cpu_workqueue_struct *cwq) 
 */

void fastcall flush_workqueue(struct workqueue_struct *wq)

{

	preempt_disable();		/* CPU hotplug */

	if (is_single_threaded(wq)) {

		/* Always use first cpu's area. */

		flush_cpu_workqueue(per_cpu_ptr(wq->cpu_wq, singlethread_cpu));

	} else {

		long sequence;

		int cpu;

again:

		sequence = migrate_sequence;



		for_each_online_cpu(cpu)

		for_each_possible_cpu(cpu)

			flush_cpu_workqueue(per_cpu_ptr(wq->cpu_wq, cpu));



		if (unlikely(sequence != migrate_sequence))

			goto again;

	}

	preempt_enable();

}

EXPORT_SYMBOL_GPL(flush_workqueue);
@@ -544,17 +540,21 @@ void flush_work(struct workqueue_struct *wq, struct work_struct *work) 
}

EXPORT_SYMBOL_GPL(flush_work);



static struct task_struct *create_workqueue_thread(struct workqueue_struct *wq,

							int cpu)

static void init_cpu_workqueue(struct workqueue_struct *wq, int cpu)

{

	struct cpu_workqueue_struct *cwq = per_cpu_ptr(wq->cpu_wq, cpu);

	struct task_struct *p;



	spin_lock_init(&cwq->lock);

	cwq->wq = wq;

	cwq->thread = NULL;

	spin_lock_init(&cwq->lock);

	INIT_LIST_HEAD(&cwq->worklist);

	init_waitqueue_head(&cwq->more_work);

}



static struct task_struct *create_workqueue_thread(struct workqueue_struct *wq,

							int cpu)

{

	struct cpu_workqueue_struct *cwq = per_cpu_ptr(wq->cpu_wq, cpu);

	struct task_struct *p;



	if (is_single_threaded(wq))

		p = kthread_create(worker_thread, cwq, "%s", wq->name);
@@ -589,6 +589,7 @@ struct workqueue_struct *__create_workqueue(const char *name, 
	mutex_lock(&workqueue_mutex);

	if (singlethread) {

		INIT_LIST_HEAD(&wq->list);

		init_cpu_workqueue(wq, singlethread_cpu);

		p = create_workqueue_thread(wq, singlethread_cpu);

		if (!p)

			destroy = 1;
@@ -596,7 +597,11 @@ struct workqueue_struct *__create_workqueue(const char *name, 
			wake_up_process(p);

	} else {

		list_add(&wq->list, &workqueues);

		for_each_online_cpu(cpu) {

		for_each_possible_cpu(cpu) {

			init_cpu_workqueue(wq, cpu);

			if (!cpu_online(cpu))

				continue;



			p = create_workqueue_thread(wq, cpu);

			if (p) {

				kthread_bind(p, cpu);
@@ -831,6 +836,7 @@ static void take_over_work(struct workqueue_struct *wq, unsigned int cpu) 


	spin_lock_irq(&cwq->lock);

	list_replace_init(&cwq->worklist, &list);

	migrate_sequence++;



	while (!list_empty(&list)) {

		printk("Taking work for %s\n", wq->name);