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workqueues: implement flush_work()

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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: Oleg Nesterov <oleg@tv-sign.ru>
Acked-by: Max Krasnyansky <maxk@qualcomm.com>
Acked-by: Jarek Poplawski <jarkao2@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Oleg Nesterov 2008-07-25 01:47:49 -07:00 committed by Linus Torvalds
parent 1a4d9b0aa0
commit db70089722
2 changed files with 48 additions and 0 deletions
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2
include/linux/workqueue.h
View file

@ -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);
/*

46
kernel/workqueue.c
View file

@ -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.