linux-kernel - [patch 18/20] posix-timers: Clarify posix_timer

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Message-ID: <20230425183313.777610259@linutronix.de>
Date:   Tue, 25 Apr 2023 20:49:24 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     LKML <linux-kernel@...r.kernel.org>
Cc:     Frederic Weisbecker <frederic@...nel.org>,
        Anna-Maria Behnsen <anna-maria@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Sebastian Siewior <bigeasy@...utronix.de>,
        syzbot+5c54bd3eb218bb595aa9@...kaller.appspotmail.com,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Michael Kerrisk <mtk.manpages@...il.com>
Subject: [patch 18/20] 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@...utronix.de>
---
 kernel/time/posix-timers.c |   56 +++++++++++++++++++++------------------------
 1 file changed, 27 insertions(+), 29 deletions(-)

--- a/kernel/time/posix-timers.c
+++ b/kernel/time/posix-timers.c
@@ -325,11 +325,11 @@ int posix_timer_event(struct k_itimer *t
 }
 
 /*
- * 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 soft interrupt with interrupts enabled.
+ *
+ * Handles CLOCK_REALTIME, CLOCK_MONOTONIC, CLOCK_BOOTTIME and CLOCK_TAI
+ * based timers.
  */
 static enum hrtimer_restart posix_timer_fn(struct hrtimer *timer)
 {
@@ -347,9 +347,10 @@ static enum hrtimer_restart posix_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);
@@ -358,34 +359,31 @@ static enum hrtimer_restart posix_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 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. Moving now ahead of
+			 * time tricks hrtimer_forward() to expire the
+			 * timer later, while it still maintains the
+			 * overrun accuracy for the price of a slightly
+			 * inconsistency in the timer_gettime() case. This
+			 * is at least better than a starved softirq.
 			 *
-			 * 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.
+			 * 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
-			{
+			if (IS_ENABLED(CONFIG_HIGHRES_TIMERS)) {
 				ktime_t kj = NSEC_PER_SEC / HZ;
 
 				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;