Donate to e Foundation | Murena handsets with /e/OS | Own a part of Murena! Learn more

Commit db700897 authored by Oleg Nesterov's avatar Oleg Nesterov Committed by Linus Torvalds
Browse files

workqueues: implement flush_work() 



Most of users of flush_workqueue() can be changed to use cancel_work_sync(),
but sometimes we really need to wait for the completion and cancelling is not
an option. schedule_on_each_cpu() is good example.

Add the new helper, flush_work(work), which waits for the completion of the
specific work_struct. More precisely, it "flushes" the result of of the last
queue_work() which is visible to the caller.

For example, this code

	queue_work(wq, work);
	/* WINDOW */
	queue_work(wq, work);

	flush_work(work);

doesn't necessary work "as expected". What can happen in the WINDOW above is

	- wq starts the execution of work->func()

	- the caller migrates to another CPU

now, after the 2nd queue_work() this work is active on the previous CPU, and
at the same time it is queued on another. In this case flush_work(work) may
return before the first work->func() completes.

It is trivial to add another helper

	int flush_work_sync(struct work_struct *work)
	{
		return flush_work(work) || wait_on_work(work);
	}

which works "more correctly", but it has to iterate over all CPUs and thus
it much slower than flush_work().

Signed-off-by: default avatarOleg Nesterov <oleg@tv-sign.ru>
Acked-by: default avatarMax Krasnyansky <maxk@qualcomm.com>
Acked-by: default avatarJarek Poplawski <jarkao2@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 1a4d9b0a

include/linux/workqueue.h

+2 −0
Original line number Diff line number Diff line
@@ -201,6 +201,8 @@ extern int keventd_up(void); 
extern void init_workqueues(void);

int execute_in_process_context(work_func_t fn, struct execute_work *);



extern int flush_work(struct work_struct *work);



extern int cancel_work_sync(struct work_struct *work);



/*

kernel/workqueue.c

+46 −0
Original line number Diff line number Diff line
@@ -423,6 +423,52 @@ void flush_workqueue(struct workqueue_struct *wq) 
}

EXPORT_SYMBOL_GPL(flush_workqueue);



/**

 * flush_work - block until a work_struct's callback has terminated

 * @work: the work which is to be flushed

 *

 * It is expected that, prior to calling flush_work(), the caller has

 * arranged for the work to not be requeued, otherwise it doesn't make

 * sense to use this function.

 */

int flush_work(struct work_struct *work)

{

	struct cpu_workqueue_struct *cwq;

	struct list_head *prev;

	struct wq_barrier barr;



	might_sleep();

	cwq = get_wq_data(work);

	if (!cwq)

		return 0;



	prev = NULL;

	spin_lock_irq(&cwq->lock);

	if (!list_empty(&work->entry)) {

		/*

		 * See the comment near try_to_grab_pending()->smp_rmb().

		 * If it was re-queued under us we are not going to wait.

		 */

		smp_rmb();

		if (unlikely(cwq != get_wq_data(work)))

			goto out;

		prev = &work->entry;

	} else {

		if (cwq->current_work != work)

			goto out;

		prev = &cwq->worklist;

	}

	insert_wq_barrier(cwq, &barr, prev->next);

out:

	spin_unlock_irq(&cwq->lock);

	if (!prev)

		return 0;



	wait_for_completion(&barr.done);

	return 1;

}

EXPORT_SYMBOL_GPL(flush_work);



/*

 * Upon a successful return (>= 0), the caller "owns" WORK_STRUCT_PENDING bit,

 * so this work can't be re-armed in any way.