[PATCH] hrtimers: fix posix-timer requeue race

From: Steven Rostedtrostedt@goodmis.org <rostedt@goodmis.org>

CPU0 expires a posix-timer and runs the callback function.  The signal is
queued.

After releasing the posix-timer lock and before returning to hrtimer_run_queue
CPU0 gets interrupted.  CPU1 delivers the queued signal and rearms the timer.
CPU0 comes back to hrtimer_run_queue and sets the timer state to expired.

The next modification of the timer can result in an oops, because the state
information is wrong.

Keep track of state = RUNNING and check if the state has been in the return
path of hrtimer_run_queue.  In case the state has been changed, ignore a
restart request and do not touch the state variable.
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

include/linux/hrtimer.h

@@ -40,6 +40,7 @@ enum hrtimer_restart {
 enum hrtimer_state {
 	HRTIMER_INACTIVE,	/* Timer is inactive */
 	HRTIMER_EXPIRED,		/* Timer is expired */
+	HRTIMER_RUNNING,		/* Timer is running the callback function */
 	HRTIMER_PENDING,		/* Timer is pending */
 };
 

kernel/hrtimer.c

@@ -550,6 +550,7 @@ static inline void run_hrtimer_queue(struct hrtimer_base *base)
 		fn = timer->function;
 		data = timer->data;
 		set_curr_timer(base, timer);
+		timer->state = HRTIMER_RUNNING;
 		__remove_hrtimer(timer, base);
 		spin_unlock_irq(&base->lock);
 
@@ -565,6 +566,10 @@ static inline void run_hrtimer_queue(struct hrtimer_base *base)
 
 		spin_lock_irq(&base->lock);
 
+		/* Another CPU has added back the timer */
+		if (timer->state != HRTIMER_RUNNING)
+			continue;
+
 		if (restart == HRTIMER_RESTART)
 			enqueue_hrtimer(timer, base);
 		else