posix-cpu-timers: Handle interval timers correctly in timer_get()

timer_gettime() must return the remaining time to the next expiry of a
timer or 0 if the timer is not armed and no signal pending, but posix CPU
timers fail to forward a timer which is already expired.

Add the required logic to address that.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>

kernel/time/posix-cpu-timers.c

@@ -787,8 +787,24 @@ static int posix_cpu_timer_set(struct k_itimer *timer, int timer_flags,
 
 static void __posix_cpu_timer_get(struct k_itimer *timer, struct itimerspec64 *itp, u64 now)
 {
-	u64 expires = cpu_timer_getexpires(&timer->it.cpu);
+	u64 expires, iv = timer->it_interval;
 
+	/*
+	 * Make sure that interval timers are moved forward for the
+	 * following cases:
+	 *  - Timers which expired, but the signal has not yet been
+	 *    delivered
+	 */
+	if (iv && (timer->it_requeue_pending & REQUEUE_PENDING))
+		expires = bump_cpu_timer(timer, now);
+	else
+		expires = cpu_timer_getexpires(&timer->it.cpu);
+
+	/*
+	 * Expired interval timers cannot have a remaining time <= 0.
+	 * The kernel has to move them forward so that the next
+	 * timer expiry is > @now.
+	 */
 	if (now < expires) {
 		itp->it_value = ns_to_timespec64(expires - now);
 	} else {