linux-kernel - Re: [PATCH 2/3] workqueues: implement flush

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Message-ID: <20080701210300.GA3272@ami.dom.local>
Date:	Tue, 1 Jul 2008 23:03:00 +0200
From:	Jarek Poplawski <jarkao2@...il.com>
To:	Oleg Nesterov <oleg@...sign.ru>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Jarek Poplawski <jarkao2@...pl>,
	Max Krasnyansky <maxk@...lcomm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] workqueues: implement flush_work()

On Tue, Jul 01, 2008 at 04:50:18PM +0400, Oleg Nesterov wrote:
...
> 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.

Right! The question is how "stale" this cwq could be when read without
any lock or barrier. Of course, there can't be the false positive, but
I wonder if we really do enough, to check if a work isn't current on
some other cwq, even without any immediate re-queuing.

Thanks for the explanation,
Jarek P.
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