Messages in this thread Patch in this message | | | Date | Sun, 29 Jun 2008 18:49:26 +0400 | From | Oleg Nesterov <> | Subject | [PATCH 2/3] 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>
--- 26-rc2/include/linux/workqueue.h~WQ_2_FLUSH_WORK 2008-05-18 15:42:34.000000000 +0400 +++ 26-rc2/include/linux/workqueue.h 2008-05-18 15:42:34.000000000 +0400 @@ -198,6 +198,8 @@ 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); /* --- 26-rc2/kernel/workqueue.c~WQ_2_FLUSH_WORK 2008-06-12 21:28:13.000000000 +0400 +++ 26-rc2/kernel/workqueue.c 2008-06-29 18:20:33.000000000 +0400 @@ -399,6 +399,52 @@ void flush_workqueue(struct workqueue_st } 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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