On Mon, Jun 08, 2015 at 11:14:17AM +0200, Peter Zijlstra wrote:
> On Mon, Jun 08, 2015 at 12:33:17AM +0200, Oleg Nesterov wrote:
> > And I think there is a problem. Consider a timer TIMER which always
> > rearms itself using some "default" timeout.
> >
> > In this case __hrtimer_start_range_ns(&TIMER, ...) must preserve
> > hrtimer_active(&TIMER) == T. By definition, and currently this is
> > true.
> >
> > After this patch this is no longer true (afaics). If the timer is
> > pending but not running, __hrtimer_start_range_ns()->remove_hrtimer()
> > will clear ENQUEUED first, then set it again in enqueue_hrtimer().
>
> That is so even with the current code; the current code uses:
>
> hrtimer->state & CALLBACK
>
> for __remove_hrtimer(.state). In the above case of a pending timer,
> that's 0 aka. HRTIMER_STATE_INACTIVE.
>
> > This means that hrtimer_active() returns false in between. And note
> > that it doesn't matter if the timer changes its ->base or not, so
> > that 2nd cpu_base above can't help.
> >
> > I think that __hrtimer_start_range_ns() should preserve ENQUEUED
> > like migrate_hrtimer_list() should do (see the previous email).
>
> I tend to agree, but I think its a pre-existing problem, not one
> introduced by my proposed patch.
Something like this would fix that I think. It fully preserves
timer->state over hrtimer_start_range_ns().
---
kernel/time/hrtimer.c | 23 +++++++++++++----------
1 file changed, 13 insertions(+), 10 deletions(-)
--- a/kernel/time/hrtimer.c
+++ b/kernel/time/hrtimer.c
@@ -891,10 +891,10 @@ static void __remove_hrtimer(struct hrti
* remove hrtimer, called with base lock held
*/
static inline int
-remove_hrtimer(struct hrtimer *timer, struct hrtimer_clock_base *base)
+remove_hrtimer(struct hrtimer *timer, struct hrtimer_clock_base *base, bool
restart)
{
if (hrtimer_is_queued(timer)) {
- unsigned long state;
+ unsigned long state = timer->state;
int reprogram;
/*
@@ -908,12 +908,15 @@ remove_hrtimer(struct hrtimer *timer, st
debug_deactivate(timer);
timer_stats_hrtimer_clear_start_info(timer);
reprogram = base->cpu_base == this_cpu_ptr(&hrtimer_bases);
- /*
- * We must preserve the CALLBACK state flag here,
- * otherwise we could move the timer base in
- * switch_hrtimer_base.
- */
- state = timer->state & HRTIMER_STATE_CALLBACK;
+
+ if (!restart) {
+ /*
+ * We must preserve the CALLBACK state flag here,
+ * otherwise we could move the timer base in
+ * switch_hrtimer_base.
+ */
+ state &= HRTIMER_STATE_CALLBACK;
+ }
__remove_hrtimer(timer, base, state, reprogram);
return 1;
}
@@ -938,7 +941,7 @@ void hrtimer_start_range_ns(struct hrtim
base = lock_hrtimer_base(timer, &flags);
/* Remove an active timer from the queue: */
- remove_hrtimer(timer, base);
+ remove_hrtimer(timer, base, true);
if (mode & HRTIMER_MODE_REL) {
tim = ktime_add_safe(tim, base->get_time());
@@ -1007,7 +1010,7 @@ int hrtimer_try_to_cancel(struct hrtimer
base = lock_hrtimer_base(timer, &flags);
if (!hrtimer_callback_running(timer))
- ret = remove_hrtimer(timer, base);
+ ret = remove_hrtimer(timer, base, false);
unlock_hrtimer_base(timer, &flags);
--
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