posix-timers: Clarify posix_timer_fn() comments

Make the issues vs. SIG_IGN understandable and remove the 15 years old
promise that a proper solution is already on the horizon.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/874jnrdmrq.ffs@tglx

kernel/time/posix-timers.c

@@ -326,11 +326,11 @@ int posix_timer_event(struct k_itimer *timr, int si_private)
 }
 
 /*
- * This function gets called when a POSIX.1b interval timer expires.  It
- * is used as a callback from the kernel internal timer.  The
- * run_timer_list code ALWAYS calls with interrupts on.
-
- * This code is for CLOCK_REALTIME* and CLOCK_MONOTONIC* timers.
+ * This function gets called when a POSIX.1b interval timer expires from
+ * the HRTIMER interrupt (soft interrupt on RT kernels).
+ *
+ * Handles CLOCK_REALTIME, CLOCK_MONOTONIC, CLOCK_BOOTTIME and CLOCK_TAI
+ * based timers.
  */
 static enum hrtimer_restart posix_timer_fn(struct hrtimer *timer)
 {
@@ -348,9 +348,10 @@ static enum hrtimer_restart posix_timer_fn(struct hrtimer *timer)
 
 	if (posix_timer_event(timr, si_private)) {
 		/*
-		 * signal was not sent because of sig_ignor
-		 * we will not get a call back to restart it AND
-		 * it should be restarted.
+		 * The signal was not queued due to SIG_IGN. As a
+		 * consequence the timer is not going to be rearmed from
+		 * the signal delivery path. But as a real signal handler
+		 * can be installed later the timer must be rearmed here.
 		 */
 		if (timr->it_interval != 0) {
 			ktime_t now = hrtimer_cb_get_time(timer);
@@ -359,34 +360,35 @@ static enum hrtimer_restart posix_timer_fn(struct hrtimer *timer)
 			 * FIXME: What we really want, is to stop this
 			 * timer completely and restart it in case the
 			 * SIG_IGN is removed. This is a non trivial
-			 * change which involves sighand locking
-			 * (sigh !), which we don't want to do late in
-			 * the release cycle.
+			 * change to the signal handling code.
+			 *
+			 * For now let timers with an interval less than a
+			 * jiffie expire every jiffie and recheck for a
+			 * valid signal handler.
+			 *
+			 * This avoids interrupt starvation in case of a
+			 * very small interval, which would expire the
+			 * timer immediately again.
 			 *
-			 * For now we just let timers with an interval
-			 * less than a jiffie expire every jiffie to
-			 * avoid softirq starvation in case of SIG_IGN
-			 * and a very small interval, which would put
-			 * the timer right back on the softirq pending
-			 * list. By moving now ahead of time we trick
-			 * hrtimer_forward() to expire the timer
-			 * later, while we still maintain the overrun
-			 * accuracy, but have some inconsistency in
-			 * the timer_gettime() case. This is at least
-			 * better than a starved softirq. A more
-			 * complex fix which solves also another related
-			 * inconsistency is already in the pipeline.
+			 * Moving now ahead of time by one jiffie tricks
+			 * hrtimer_forward() to expire the timer later,
+			 * while it still maintains the overrun accuracy
+			 * for the price of a slight inconsistency in the
+			 * timer_gettime() case. This is at least better
+			 * than a timer storm.
+			 *
+			 * Only required when high resolution timers are
+			 * enabled as the periodic tick based timers are
+			 * automatically aligned to the next tick.
 			 */
-#ifdef CONFIG_HIGH_RES_TIMERS
-			{
-				ktime_t kj = NSEC_PER_SEC / HZ;
+			if (IS_ENABLED(CONFIG_HIGHRES_TIMERS)) {
+				ktime_t kj = TICK_NSEC;
 
 				if (timr->it_interval < kj)
 					now = ktime_add(now, kj);
 			}
-#endif
-			timr->it_overrun += hrtimer_forward(timer, now,
-							    timr->it_interval);
+
+			timr->it_overrun += hrtimer_forward(timer, now, timr->it_interval);
 			ret = HRTIMER_RESTART;
 			++timr->it_requeue_pending;
 			timr->it_active = 1;