posix-cpu-timers: Simplify posix_cpu_timer_set()

Avoid the late sighand lock/unlock dance when a timer is not armed to
enforce reevaluation of the timer base so that the process wide CPU timer
sampling can be disabled.

Do it right at the point where the arming decision is made which already
has sighand locked.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Reviewed-by: Anna-Maria Behnsen <anna-maria@linutronix.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>

kernel/time/posix-cpu-timers.c

@@ -705,10 +705,16 @@ static int posix_cpu_timer_set(struct k_itimer *timer, int timer_flags,
 	/*
 	 * Arm the timer if it is not disabled, the new expiry value has
 	 * not yet expired and the timer requires signal delivery.
-	 * SIGEV_NONE timers are never armed.
+	 * SIGEV_NONE timers are never armed. In case the timer is not
+	 * armed, enforce the reevaluation of the timer base so that the
+	 * process wide cputime counter can be disabled eventually.
 	 */
-	if (!sigev_none && new_expires && now < new_expires)
-		arm_timer(timer, p);
+	if (likely(!sigev_none)) {
+		if (new_expires && now < new_expires)
+			arm_timer(timer, p);
+		else
+			trigger_base_recalc_expires(timer, p);
+	}
 
 	unlock_task_sighand(p, &flags);
 	/*
@@ -727,30 +733,14 @@ static int posix_cpu_timer_set(struct k_itimer *timer, int timer_flags,
 	timer->it_overrun_last = 0;
 	timer->it_overrun = -1;
 
-	if (!sigev_none && now >= new_expires) {
-		if (new_expires != 0) {
-			/*
-			 * The designated time already passed, so we notify
-			 * immediately, even if the thread never runs to
-			 * accumulate more time on this clock.
-			 */
-			cpu_timer_fire(timer);
-		}
-
-		/*
-		 * Make sure we don't keep around the process wide cputime
-		 * counter or the tick dependency if they are not necessary.
-		 */
-		sighand = lock_task_sighand(p, &flags);
-		if (!sighand)
-			goto out;
-
-		if (!cpu_timer_queued(ctmr))
-			trigger_base_recalc_expires(timer, p);
-
-		unlock_task_sighand(p, &flags);
-	}
- out:
+	/*
+	 * If the new expiry time was already in the past the timer was not
+	 * queued. Fire it immediately even if the thread never runs to
+	 * accumulate more time on this clock.
+	 */
+	if (!sigev_none && new_expires && now >= new_expires)
+		cpu_timer_fire(timer);
+out:
 	rcu_read_unlock();
 	if (old)
 		old->it_interval = ns_to_timespec64(old_incr);