workqueue: convert BUG_ON()s in __queue_delayed_work() to WARN_ON_ONCE()s

commit fc4b514f upstream.

8852aac2 ("workqueue: mod_delayed_work_on() shouldn't queue timer on
0 delay") unexpectedly uncovered a very nasty abuse of delayed_work in
megaraid - it allocated work_struct, casted it to delayed_work and
then pass that into queue_delayed_work().

Previously, this was okay because 0 @delay short-circuited to
queue_work() before doing anything with delayed_work.  8852aac2
moved 0 @delay test into __queue_delayed_work() after sanity check on
delayed_work making megaraid trigger BUG_ON().

Although megaraid is already fixed by c1d390d8 ("megaraid: fix
BUG_ON() from incorrect use of delayed work"), this patch converts
BUG_ON()s in __queue_delayed_work() to WARN_ON_ONCE()s so that such
abusers, if there are more, trigger warning but don't crash the
machine.
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Xiaotian Feng <xtfeng@gmail.com>
[Shuah Khan: This change is back-ported from upstream change that
 converted BUG_ON()s in __queue_delayed_work() to WARN_ON_ONCE()s.]
Tested on Stable Trees: 3.0.x, 3.4.x, 3.6.x
Signed-off-by: Shuah Khan <shuah.khan@hp.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

kernel/workqueue.c

@@ -1146,8 +1146,8 @@ int queue_delayed_work_on(int cpu, struct workqueue_struct *wq,
 	if (!test_and_set_bit(WORK_STRUCT_PENDING_BIT, work_data_bits(work))) {
 		unsigned int lcpu;
 
-		BUG_ON(timer_pending(timer));
-		BUG_ON(!list_empty(&work->entry));
+		WARN_ON_ONCE(timer_pending(timer));
+		WARN_ON_ONCE(!list_empty(&work->entry));
 
 		timer_stats_timer_set_start_info(&dwork->timer);