[PATCH] ITIMER_REAL: fix possible deadlock and race

As Steven Rostedt pointed out, there are 2 problems with ITIMER_REAL
timers.

1. do_setitimer() does not call del_timer_sync() in case
   when the timer is not pending (it_real_value() returns 0).
   This is wrong, the timer may still be running, and it can
   rearm itself.

2. It calls del_timer_sync() with tsk->sighand->siglock held.
   This is deadlockable, because timer's handler needs this
   lock too.
Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Acked-by: Steven Rostedt <rostedt@goodmis.org>
Cc: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

kernel/itimer.c

@@ -153,11 +153,15 @@ int do_setitimer(int which, struct itimerval *value, struct itimerval *ovalue)
 
 	switch (which) {
 	case ITIMER_REAL:
+again:
 		spin_lock_irq(&tsk->sighand->siglock);
 		interval = tsk->signal->it_real_incr;
 		val = it_real_value(tsk->signal);
-		if (val)
-			del_timer_sync(&tsk->signal->real_timer);
+		/* We are sharing ->siglock with it_real_fn() */
+		if (try_to_del_timer_sync(&tsk->signal->real_timer) < 0) {
+			spin_unlock_irq(&tsk->sighand->siglock);
+			goto again;
+		}
 		tsk->signal->it_real_incr =
 			timeval_to_jiffies(&value->it_interval);
 		it_real_arm(tsk, timeval_to_jiffies(&value->it_value));