workqueues: implement flush_work() (db700897) · Commits · 戴 / test

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>

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.

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