[WORKQUEUE]: cancel_delayed_work: use del_timer() instead of del_timer_sync()

del_timer_sync() buys nothing for cancel_delayed_work(), but it is less
efficient since it locks the timer unconditionally, and may wait for the
completion of the delayed_work_timer_fn().

cancel_delayed_work() == 0 means:

	before this patch:
		work->func may still be running or queued

	after this patch:
		work->func may still be running or queued, or
		delayed_work_timer_fn->__queue_work() in progress.

		The latter doesn't differ from the caller's POV,
		delayed_work_timer_fn() is called with _PENDING
		bit set.

cancel_delayed_work() == 1 with this patch adds a new possibility:

	delayed_work->work was cancelled, but delayed_work_timer_fn
	is still running (this is only possible for the re-arming
	works on single-threaded workqueue).

	In this case the timer was re-started by work->func(), nobody
	else can do this. This in turn means that delayed_work_timer_fn
	has already passed __queue_work() (and wont't touch delayed_work)
	because nobody else can queue delayed_work->work.
Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Signed-Off-By: David Howells <dhowells@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/workqueue.h

@@ -191,14 +191,15 @@ int execute_in_process_context(work_func_t fn, struct execute_work *);
 
 /*
  * Kill off a pending schedule_delayed_work().  Note that the work callback
- * function may still be running on return from cancel_delayed_work().  Run
- * flush_scheduled_work() to wait on it.
+ * function may still be running on return from cancel_delayed_work(), unless
+ * it returns 1 and the work doesn't re-arm itself. Run flush_workqueue() or
+ * cancel_work_sync() to wait on it.
  */
 static inline int cancel_delayed_work(struct delayed_work *work)
 {
 	int ret;
 
-	ret = del_timer_sync(&work->timer);
+	ret = del_timer(&work->timer);
 	if (ret)
 		work_release(&work->work);
 	return ret;