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From: Oleg Nesterov on 24 Feb 2010 15:30
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(a)gmail.com>
Signed-off-by: Oleg Nesterov <oleg(a)redhat.com>
---

kernel/workqueue.c | 12 +++++++++++-
1 file changed, 11 insertions(+), 1 deletion(-)

--- wq/kernel/workqueue.c~1_CANCEL_CLEAR_WQ 2010-02-24 20:43:32.000000000 +0100
+++ wq/kernel/workqueue.c 2010-02-24 20:55:53.000000000 +0100
@@ -229,6 +229,16 @@ static inline void set_wq_data(struct wo
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 wo
wait_on_work(work);
} while (unlikely(ret < 0));

- work_clear_pending(work);
+ clear_wq_data(work);
return ret;
}


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Prev: [PATCH 2/3] workqueues: microoptimize set_wq_data()
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