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>

kernel/workqueue.c

@@ -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;
 }