[tip: timers/core] posix-timers: Clarify posix_timer_fn() comments

tip-bot2 for Thomas Gleixner
2023-06-18T20:50:22Z

The following commit has been merged into the timers/core branch of tip:

Commit-ID: 84999b8bdb4969816c7bb7c14c3a55ed42aa4b94
Gitweb: https://git.kernel.org/tip/84999b8bdb4969816c7bb7c14c3a55ed42aa4b94
Author: Thomas Gleixner <tglx@linutronix.de>
AuthorDate: Thu, 01 Jun 2023 21:07:37 +02:00
Committer: Thomas Gleixner <tglx@linutronix.de>
CommitterDate: Sun, 18 Jun 2023 22:41:52 +02:00

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 | 62 +++++++++++++++++++------------------
1 file changed, 32 insertions(+), 30 deletions(-)

diff --git a/kernel/time/posix-timers.c b/kernel/time/posix-timers.c
index d8d2169..ae8799e 100644
--- 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;