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: 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>
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@ -201,6 +201,8 @@ extern int keventd_up(void);
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extern void init_workqueues(void);
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int execute_in_process_context(work_func_t fn, struct execute_work *);
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extern int flush_work(struct work_struct *work);
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extern int cancel_work_sync(struct work_struct *work);
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/*
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@ -423,6 +423,52 @@ void flush_workqueue(struct workqueue_struct *wq)
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}
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EXPORT_SYMBOL_GPL(flush_workqueue);
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/**
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* flush_work - block until a work_struct's callback has terminated
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* @work: the work which is to be flushed
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*
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* It is expected that, prior to calling flush_work(), the caller has
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* arranged for the work to not be requeued, otherwise it doesn't make
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* sense to use this function.
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*/
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int flush_work(struct work_struct *work)
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{
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struct cpu_workqueue_struct *cwq;
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struct list_head *prev;
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struct wq_barrier barr;
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might_sleep();
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cwq = get_wq_data(work);
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if (!cwq)
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return 0;
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prev = NULL;
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spin_lock_irq(&cwq->lock);
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if (!list_empty(&work->entry)) {
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/*
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* See the comment near try_to_grab_pending()->smp_rmb().
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* If it was re-queued under us we are not going to wait.
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*/
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smp_rmb();
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if (unlikely(cwq != get_wq_data(work)))
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goto out;
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prev = &work->entry;
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} else {
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if (cwq->current_work != work)
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goto out;
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prev = &cwq->worklist;
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}
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insert_wq_barrier(cwq, &barr, prev->next);
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out:
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spin_unlock_irq(&cwq->lock);
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if (!prev)
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return 0;
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wait_for_completion(&barr.done);
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return 1;
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}
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EXPORT_SYMBOL_GPL(flush_work);
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/*
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* Upon a successful return (>= 0), the caller "owns" WORK_STRUCT_PENDING bit,
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* so this work can't be re-armed in any way.
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