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

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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>
This commit is contained in:
Oleg Nesterov 2010-04-23 17:40:40 +02:00 committed by Tejun Heo
parent eef6a7d5c2
commit 4d707b9f48
1 changed files with 11 additions and 1 deletions
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12
kernel/workqueue.c
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@ -229,6 +229,16 @@ static inline void set_wq_data(struct work_struct *work,
atomic_long_set(&work->data, new);
}
/*
* Clear WORK_STRUCT_PENDING and the workqueue on which it was queued.
*/
static inline void clear_wq_data(struct work_struct *work)
{
unsigned long flags = *work_data_bits(work) &
(1UL << WORK_STRUCT_STATIC);
atomic_long_set(&work->data, flags);
}
static inline
struct cpu_workqueue_struct *get_wq_data(struct work_struct *work)
{
@ -671,7 +681,7 @@ static int __cancel_work_timer(struct work_struct *work,
wait_on_work(work);
} while (unlikely(ret < 0));
work_clear_pending(work);
clear_wq_data(work);
return ret;
}