linux-kernel - Re: [PATCH 1/3] workqueues: change cancel_work

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Message-ID: <4B85EAE8.8040104@kernel.org>
Date:	Thu, 25 Feb 2010 12:13:44 +0900
From:	Tejun Heo <tj@...nel.org>
To:	Oleg Nesterov <oleg@...hat.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Dmitry Torokhov <dmitry.torokhov@...il.com>,
	Samu Onkalo <samu.p.onkalo@...ia.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] workqueues: change cancel_work_sync() to clear	work->data

Hello,

On 02/25/2010 05:20 AM, Oleg Nesterov wrote:
> In short: change cancel_work_sync(work) to mark this work as "never
> queued" upon return.
> 
> When cancel_work_sync(work) succeeds, we know that this work can't be
> queued or running, and since we own WORK_STRUCT_PENDING nobody can change
> the bits in work->data under us. This means we can also clear the "cwq"
> part along with _PENDING bit lockless before return, unless the work is
> queued nobody can assume get_wq_data() is stable even under cwq->lock.
> 
> This change can speedup the subsequent cancel/flush requests, and as
> Dmitry pointed out this simplifies the usage of work_struct's which
> can be queued on different workqueues. Consider this pseudo code from
> the input subsystem:
> 
> 	struct workqueue_struct *WQ;
> 	struct work_struct *WORK;
> 
> 	for (;;) {
> 		WQ = create_workqueue();
> 		...
> 		if (condition())
> 			queue_work(WQ, WORK);
> 		...
> 		cancel_work_sync(WORK);
> 		destroy_workqueue(WQ);
> 	}
> 
> If condition() returns T and then F, cancel_work_sync() will crash the
> kernel because WORK->data still points to the already destroyed workqueue.
> With this patch the code like above becomes correct.
> 
> Suggested-by: Dmitry Torokhov <dmitry.torokhov@...il.com>
> Signed-off-by: Oleg Nesterov <oleg@...hat.com>

Yeap, generally looks good to me and I've been doing similar things to
implement non-reentrant workqueues (records the last cpu a work was on
in work->dataa after the work is dispatched to a worker thread).  I'll
add this to the series.

Thank you.

-- 
tejun
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