linux-kernel - Re: [PATCH 1/2] workqueues: implement flush

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Date:	Fri, 13 Jun 2008 18:46:05 +0400
From:	Oleg Nesterov <oleg@...sign.ru>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Jarek Poplawski <jarkao2@...pl>,
	Max Krasnyansky <maxk@...lcomm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] workqueues: implement flush_work()

On 06/13, Oleg Nesterov wrote:
>
> Add the new helper, flush_work(work), which waits for the completion of the
> specific work_struct.
>
> By its nature it requires that this work must not be re-queued, and thus its
> usage is limited. For example, this code
>
> 	queue_work(wq, work);
> 	/* WINDOW */
> 	queue_work(wq, work);
>
> 	flush_work(work);
>
> is not right. 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.

To clarify, in the scenario above, flush_work() behaves correctly. It "flushes"
the result of the last queue_work() which is visible to the caller. We can add
another simple helper,

	int flush_work_sync(work)
	{
		return flush_work(work) || wait_on_work(work);
	}

but I think it is not needed.

In short: flush_work() should be use with care when it is really needed.

Oleg.

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