[PATCH] posix-timers: fix posix_cpu_timer_set() vs run_posix_cpu_timers() race

This might be harmless, but looks like a race from code inspection (I
was unable to trigger it).  I must admit, I don't understand why we
can't return TIMER_RETRY after 'spin_unlock(&p->sighand->siglock)'
without doing bump_cpu_timer(), but this is what original code does.

posix_cpu_timer_set:

	read_lock(&tasklist_lock);

	spin_lock(&p->sighand->siglock);
	list_del_init(&timer->it.cpu.entry);
	spin_unlock(&p->sighand->siglock);

We are probaly deleting the timer from run_posix_cpu_timers's 'firing'
local list_head while run_posix_cpu_timers() does list_for_each_safe.

Various bad things can happen, for example we can just delete this timer
so that list_for_each() will not notice it and run_posix_cpu_timers()
will not reset '->firing' flag. In that case,

	....

	if (timer->it.cpu.firing) {
		read_unlock(&tasklist_lock);
		timer->it.cpu.firing = -1;
		return TIMER_RETRY;
	}

sys_timer_settime() goes to 'retry:', calls posix_cpu_timer_set() again,
it returns TIMER_RETRY ...
Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

kernel/posix-cpu-timers.c

@@ -730,9 +730,15 @@ int posix_cpu_timer_set(struct k_itimer *timer, int flags,
 	 * Disarm any old timer after extracting its expiry time.
 	 */
 	BUG_ON(!irqs_disabled());
+
+	ret = 0;
 	spin_lock(&p->sighand->siglock);
 	old_expires = timer->it.cpu.expires;
-	list_del_init(&timer->it.cpu.entry);
+	if (unlikely(timer->it.cpu.firing)) {
+		timer->it.cpu.firing = -1;
+		ret = TIMER_RETRY;
+	} else
+		list_del_init(&timer->it.cpu.entry);
 	spin_unlock(&p->sighand->siglock);
 
 	/*
@@ -780,7 +786,7 @@ int posix_cpu_timer_set(struct k_itimer *timer, int flags,
 		}
 	}
 
-	if (unlikely(timer->it.cpu.firing)) {
+	if (unlikely(ret)) {
 		/*
 		 * We are colliding with the timer actually firing.
 		 * Punt after filling in the timer's old value, and
@@ -788,8 +794,6 @@ int posix_cpu_timer_set(struct k_itimer *timer, int flags,
 		 * it as an overrun (thanks to bump_cpu_timer above).
 		 */
 		read_unlock(&tasklist_lock);
-		timer->it.cpu.firing = -1;
-		ret = TIMER_RETRY;
 		goto out;
 	}