workqueues: introduce __cancel_delayed_work()

cancel_delayed_work() has to use del_timer_sync() to guarantee the timer
function is not running after return.  But most users doesn't actually
need this, and del_timer_sync() has problems: it is not useable from
interrupt, and it depends on every lock which could be taken from irq.

Introduce __cancel_delayed_work() which calls del_timer() instead.

The immediate reason for this patch is
http://bugzilla.kernel.org/show_bug.cgi?id=13757
but hopefully this helper makes sense anyway.

As for 13757 bug, actually we need requeue_delayed_work(), but its
semantics are not yet clear.

Merge this patch early to resolves cross-tree interdependencies between
input and infiniband.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Cc: Roland Dreier <rdreier@cisco.com>
Cc: Stefan Richter <stefanr@s5r6.in-berlin.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/workqueue.h

@@ -240,6 +240,21 @@ static inline int cancel_delayed_work(struct delayed_work *work)
 	return ret;
 }
 
+/*
+ * Like above, but uses del_timer() instead of del_timer_sync(). This means,
+ * if it returns 0 the timer function may be running and the queueing is in
+ * progress.
+ */
+static inline int __cancel_delayed_work(struct delayed_work *work)
+{
+	int ret;
+
+	ret = del_timer(&work->timer);
+	if (ret)
+		work_clear_pending(&work->work);
+	return ret;
+}
+
 extern int cancel_delayed_work_sync(struct delayed_work *work);
 
 /* Obsolete. use cancel_delayed_work_sync() */