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

diff --git a/kernel/time/posix-timers.c b/kernel/time/posix-timers.c
index d8d2169408f4ad25bbdb03354ae09948a1afaa05..ae8799e97fcd9382d4f54d02b0d5fed7cfad8b67 100644 (file)
--- a/kernel/time/posix-timers.c
+++ b/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;