Re: [PATCH 08/14] hrtimer: Allow hrtimer::function() to free the timer

[Peter Zijlstra <peterz@infradead.org>]

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);