From: Oleg Nesterov <oleg@redhat.com>
Date: Fri, 23 Apr 2010 15:40:40 +0000 (+0200)
Subject: workqueue: change cancel_work_sync() to clear work->data
X-Git-Tag: v2.6.35-rc1~485^2
X-Git-Url: http://git.openpandora.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=4d707b9f48e2c4aa94b96f1133813b73df71fb55;p=pandora-kernel.git

workqueue: change cancel_work_sync() to clear work->data

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@gmail.com>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
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