[tip: timers/core] posix-timers: Document common_clock_get() correctly

tip-bot2 for Thomas Gleixner
2023-06-05T15:09:13Z

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

Commit-ID: 13a5dd88bb4c8aec6c3f808f75b73b1fe4705244
Gitweb: https://git.kernel.org/tip/13a5dd88bb4c8aec6c3f808f75b73b1fe4705244
Author: Thomas Gleixner <tglx@linutronix.de>
AuthorDate: Tue, 25 Apr 2023 20:49:12 +02:00
Committer: Thomas Gleixner <tglx@linutronix.de>
CommitterDate: Mon, 05 Jun 2023 17:03:38 +02:00

posix-timers: Document common_clock_get() correctly

Replace another confusing and inaccurate set of comments.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Link: https://lore.kernel.org/r/20230425183313.409169321@linutronix.de

---
kernel/time/posix-timers.c | 50 ++++++++++++++++++++++---------------
1 file changed, 30 insertions(+), 20 deletions(-)

diff --git a/kernel/time/posix-timers.c b/kernel/time/posix-timers.c
index c9896ff..663d3c6 100644
--- a/kernel/time/posix-timers.c
+++ b/kernel/time/posix-timers.c
@@ -660,20 +660,16 @@ static s64 common_hrtimer_forward(struct k_itimer *timr, ktime_t now)
}

/*
- * Get the time remaining on a POSIX.1b interval timer. This function
- * is ALWAYS called with spin_lock_irq on the timer, thus it must not
- * mess with irq.
+ * Get the time remaining on a POSIX.1b interval timer.
*
- * We have a couple of messes to clean up here. First there is the case
- * of a timer that has a requeue pending. These timers should appear to
- * be in the timer list with an expiry as if we were to requeue them
- * now.
+ * Two issues to handle here:
*
- * The second issue is the SIGEV_NONE timer which may be active but is
- * not really ever put in the timer list (to save system resources).
- * This timer may be expired, and if so, we will do it here. Otherwise
- * it is the same as a requeue pending timer WRT to what we should
- * report.
+ * 1) The timer has a requeue pending. The return value must appear as
+ * if the timer has been requeued right now.
+ *
+ * 2) The timer is a SIGEV_NONE timer. These timers are never enqueued
+ * into the hrtimer queue and therefore never expired. Emulate expiry
+ * here taking #1 into account.
*/
void common_timer_get(struct k_itimer *timr, struct itimerspec64 *cur_setting)
{
@@ -689,8 +685,12 @@ void common_timer_get(struct k_itimer *timr, struct itimerspec64 *cur_setting)
cur_setting->it_interval = ktime_to_timespec64(iv);
} else if (!timr->it_active) {
/*
- * SIGEV_NONE oneshot timers are never queued. Check them
- * below.
+ * SIGEV_NONE oneshot timers are never queued and therefore
+ * timr->it_active is always false. The check below
+ * vs. remaining time will handle this case.
+ *
+ * For all other timers there is nothing to update here, so
+ * return.
*/
if (!sig_none)
return;
@@ -699,18 +699,29 @@ void common_timer_get(struct k_itimer *timr, struct itimerspec64 *cur_setting)
now = kc->clock_get_ktime(timr->it_clock);

/*
- * When a requeue is pending or this is a SIGEV_NONE timer move the
- * expiry time forward by intervals, so expiry is > now.
+ * If this is an interval timer and either has requeue pending or
+ * is a SIGEV_NONE timer move the expiry time forward by intervals,
+ * so expiry is > now.
*/
if (iv && (timr->it_requeue_pending & REQUEUE_PENDING || sig_none))
timr->it_overrun += kc->timer_forward(timr, now);

remaining = kc->timer_remaining(timr, now);
- /* Return 0 only, when the timer is expired and not pending */
+ /*
+ * As @now is retrieved before a possible timer_forward() and
+ * cannot be reevaluated by the compiler @remaining is based on the
+ * same @now value. Therefore @remaining is consistent vs. @now.
+ *
+ * Consequently all interval timers, i.e. @iv > 0, cannot have a
+ * remaining time <= 0 because timer_forward() guarantees to move
+ * them forward so that the next timer expiry is > @now.
+ */
if (remaining <= 0) {
/*
- * A single shot SIGEV_NONE timer must return 0, when
- * it is expired !
+ * A single shot SIGEV_NONE timer must return 0, when it is
+ * expired! Timers which have a real signal delivery mode
+ * must return a remaining time greater than 0 because the
+ * signal has not yet been delivered.
*/
if (!sig_none)
cur_setting->it_value.tv_nsec = 1;
@@ -719,7 +730,6 @@ void common_timer_get(struct k_itimer *timr, struct itimerspec64 *cur_setting)
}
}

-/* Get the time remaining on a POSIX.1b interval timer. */
static int do_timer_gettime(timer_t timer_id, struct itimerspec64 *setting)
{
struct k_itimer *timr;