Messages in this thread | | | From | Kirill Tkhai <> | Subject | Re: [PATCH 8/9] hrtimer: Allow hrtimer::function() to free the timer | Date | Thu, 04 Jun 2015 12:07:03 +0300 |
| |
В Ср, 03/06/2015 в 23:13 +0200, Peter Zijlstra пишет: On Wed, Jun 03, 2015 at 07:26:00PM +0300, Kirill Tkhai wrote: > > > @@ -402,7 +394,8 @@ extern u64 hrtimer_get_next_event(void); > > > */ > > > static inline int hrtimer_active(const struct hrtimer *timer) > > > { > > > - return timer->state != HRTIMER_STATE_INACTIVE; > > > + return timer->state != HRTIMER_STATE_INACTIVE || > > > + timer->base->running == timer; > > > } > > > > It seems to be not good, because hrtimer_active() check stops > > to be atomic. So the things like hrtimer_try_to_cancel() race > > with a callback of self-rearming timer and may return a false > > positive result. > > Hurm.. the below isn't really pretty either I suppose. The best I can > say about it is that's its not too expensive on x86. > > I should probably go sleep.. > > --- a/include/linux/hrtimer.h > +++ b/include/linux/hrtimer.h > @@ -391,11 +391,25 @@ extern u64 hrtimer_get_next_event(void); > * A timer is active, when it is enqueued into the rbtree or the > * callback function is running or it's in the state of being migrated > * to another cpu. > + * > + * See __run_hrtimer(). > */ > -static inline int hrtimer_active(const struct hrtimer *timer) > +static inline bool hrtimer_active(const struct hrtimer *timer) > { > - return timer->state != HRTIMER_STATE_INACTIVE || > - timer->base->running == timer; > + if (timer->state != HRTIMER_STATE_INACTIVE) > + return true; > + > + smp_rmb(); /* C matches A */ > + > + if (timer->base->running == timer) > + return true; > + > + smp_rmb(); /* D matches B */ > + > + if (timer->state != HRTIMER_STATE_INACTIVE) > + return true; > + > + return false;
This races with two sequential timer handlers. hrtimer_active() is preemptible everywhere, and no guarantees that all three "if" conditions check the same timer tick.
How about transformation of hrtimer_bases.lock: raw_spinlock_t --> seqlock_t?
> } > > /* > --- a/kernel/time/hrtimer.c > +++ b/kernel/time/hrtimer.c > @@ -1122,6 +1122,20 @@ static void __run_hrtimer(struct hrtimer > > debug_deactivate(timer); > base->running = timer; > + > + /* > + * Pairs with hrtimer_active(). > + * > + * [S] base->running = timer [L] timer->state > + * WMB RMB > + * [S] timer->state = INACTIVE [L] base->running > + * > + * BUG_ON(base->running != timer && timer->state != INACTIVE) > + * > + * If we observe INACTIVE we must observe base->running == timer. > + */ > + smp_wmb(); /* A matches C */ > + > __remove_hrtimer(timer, base, HRTIMER_STATE_INACTIVE, 0); > timer_stats_account_hrtimer(timer); > fn = timer->function; > @@ -1150,6 +1164,20 @@ static void __run_hrtimer(struct hrtimer > !(timer->state & HRTIMER_STATE_ENQUEUED)) > enqueue_hrtimer(timer, base); > > + /* > + * Pairs with hrtimer_active(). > + * > + * [S] timer->state = ENQUEUED [L] base->running > + * WMB RMB > + * [S] base->running = NULL [L] timer->state > + * > + * BUG_ON(base->running == NULL && timer->state == INACTIVE) > + * > + * If we observe base->running == NULL, we must observe any preceding > + * enqueue. > + */ > + smp_wmb(); /* B matches D */ > + > WARN_ON_ONCE(base->running != timer); > base->running = NULL; > } >
| |