posix-timers: Rework cancel retry loops

As a preparatory step for adding the PREEMPT RT specific synchronization
mechanism to wait for a running timer callback, rework the timer cancel
retry loops so they call a common function. This allows trivial
substitution in one place.
Originally-by: Anna-Maria Gleixner <anna-maria@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20190730223828.874901027@linutronix.de

kernel/time/posix-timers.c

@@ -805,6 +805,17 @@ static int common_hrtimer_try_to_cancel(struct k_itimer *timr)
 	return hrtimer_try_to_cancel(&timr->it.real.timer);
 }
 
+static struct k_itimer *timer_wait_running(struct k_itimer *timer,
+					   unsigned long *flags)
+{
+	timer_t timer_id = READ_ONCE(timer->it_id);
+
+	unlock_timer(timer, *flags);
+	cpu_relax();
+	/* Relock the timer. It might be not longer hashed. */
+	return lock_timer(timer_id, flags);
+}
+
 /* Set a POSIX.1b interval timer. */
 int common_timer_set(struct k_itimer *timr, int flags,
 		     struct itimerspec64 *new_setting,
@@ -859,8 +870,9 @@ static int do_timer_settime(timer_t timer_id, int tmr_flags,
 
 	if (old_spec64)
 		memset(old_spec64, 0, sizeof(*old_spec64));
-retry:
+
 	timr = lock_timer(timer_id, &flags);
+retry:
 	if (!timr)
 		return -EINVAL;
 
@@ -870,11 +882,14 @@ static int do_timer_settime(timer_t timer_id, int tmr_flags,
 	else
 		error = kc->timer_set(timr, tmr_flags, new_spec64, old_spec64);
 
-	unlock_timer(timr, flags);
 	if (error == TIMER_RETRY) {
-		old_spec64 = NULL;	// We already got the old time...
+		// We already got the old time...
+		old_spec64 = NULL;
+		/* Unlocks and relocks the timer if it still exists */
+		timr = timer_wait_running(timr, &flags);
 		goto retry;
 	}
+	unlock_timer(timr, flags);
 
 	return error;
 }
@@ -951,13 +966,15 @@ SYSCALL_DEFINE1(timer_delete, timer_t, timer_id)
 	struct k_itimer *timer;
 	unsigned long flags;
 
-retry_delete:
 	timer = lock_timer(timer_id, &flags);
+
+retry_delete:
 	if (!timer)
 		return -EINVAL;
 
-	if (timer_delete_hook(timer) == TIMER_RETRY) {
-		unlock_timer(timer, flags);
+	if (unlikely(timer_delete_hook(timer) == TIMER_RETRY)) {
+		/* Unlocks and relocks the timer if it still exists */
+		timer = timer_wait_running(timer, &flags);
 		goto retry_delete;
 	}