timer: Warn when del_timer_sync() is called in hardirq context

Add explict warning when del_timer_sync() is called in hardirq
context.

Signed-off-by: Yong Zhang <yong.zhang0@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Peter Zijlstra <peterz@infradead.org>
Acked-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

kernel/timer.c

@@ -974,7 +974,11 @@ int del_timer_sync(struct timer_list *timer)
 	lock_map_release(&timer->lockdep_map);
 	local_bh_enable();
 #endif
-
+	/*
+	 * don't use it in hardirq context, because it
+	 * could lead to deadlock.
+	 */
+	WARN_ON(in_irq());
 	for (;;) {
 		int ret = try_to_del_timer_sync(timer);
 		if (ret >= 0)