Messages in this thread | | | Date | Tue, 1 Jul 2008 16:50:18 +0400 | From | Oleg Nesterov <> | Subject | Re: [PATCH 2/3] workqueues: implement flush_work() |
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On 06/30, Jarek Poplawski wrote: > > On Sun, Jun 29, 2008 at 06:49:26PM +0400, Oleg Nesterov wrote: > ... > > --- 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. > > + */ > > I missed this before, and probably it's not required, but "Returns..." > could be added here.
Agreed, I'll update the comment later, together with other changes in workqueue.c
> > + 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 { > > Probably it doesn't matter too much, but one little doubt: don't we > need (for consistency) smp_rmb() for this branch as well? It seems > this cwq could be read out of order here too. > > > + if (cwq->current_work != work) > > + goto out;
Yes, cwq can be "stale", but this doesn't matter and we can't have the false positive here.
cwq->current_work is always changed under cwq->lock, and we hold this lock. If we see "cwq->current_work == work" we can safely insert the barrier and wait. Even if this work was already re-queued on another CPU or another workqueue_struct.
Note also that rmb() can't really help here.
> Otherwise, all looks correct to me as before.
Thanks!
Oleg.
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