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Merge branch 'timers-urgent-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

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Pull timer fix from Ingo Molnar:
 "Fix a broadcast-timer handling race that can result in spuriously and
  indefinitely delayed hrtimers and even RCU stalls if the system is
  otherwise quiet"

* 'timers-urgent-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  tick: broadcast-hrtimer: Fix a race in bc_set_next
This commit is contained in:
Linus Torvalds 2019-10-02 15:54:19 -07:00
commit 5021b9182e
1 changed files with 29 additions and 33 deletions
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62
kernel/time/tick-broadcast-hrtimer.c
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@ -42,39 +42,39 @@ static int bc_shutdown(struct clock_event_device *evt)
*/
static int bc_set_next(ktime_t expires, struct clock_event_device *bc)
{
int bc_moved;
/*
* We try to cancel the timer first. If the callback is on
* flight on some other cpu then we let it handle it. If we
* were able to cancel the timer nothing can rearm it as we
* own broadcast_lock.
* This is called either from enter/exit idle code or from the
* broadcast handler. In all cases tick_broadcast_lock is held.
*
* However we can also be called from the event handler of
* ce_broadcast_hrtimer itself when it expires. We cannot
* restart the timer because we are in the callback, but we
* can set the expiry time and let the callback return
* HRTIMER_RESTART.
* hrtimer_cancel() cannot be called here neither from the
* broadcast handler nor from the enter/exit idle code. The idle
* code can run into the problem described in bc_shutdown() and the
* broadcast handler cannot wait for itself to complete for obvious
* reasons.
*
* Since we are in the idle loop at this point and because
* hrtimer_{start/cancel} functions call into tracing,
* calls to these functions must be bound within RCU_NONIDLE.
* Each caller tries to arm the hrtimer on its own CPU, but if the
* hrtimer callbback function is currently running, then
* hrtimer_start() cannot move it and the timer stays on the CPU on
* which it is assigned at the moment.
*
* As this can be called from idle code, the hrtimer_start()
* invocation has to be wrapped with RCU_NONIDLE() as
* hrtimer_start() can call into tracing.
*/
RCU_NONIDLE(
{
bc_moved = hrtimer_try_to_cancel(&bctimer) >= 0;
if (bc_moved) {
hrtimer_start(&bctimer, expires,
HRTIMER_MODE_ABS_PINNED_HARD);
}
}
);
if (bc_moved) {
/* Bind the "device" to the cpu */
bc->bound_on = smp_processor_id();
} else if (bc->bound_on == smp_processor_id()) {
hrtimer_set_expires(&bctimer, expires);
}
RCU_NONIDLE( {
hrtimer_start(&bctimer, expires, HRTIMER_MODE_ABS_PINNED_HARD);
/*
* The core tick broadcast mode expects bc->bound_on to be set
* correctly to prevent a CPU which has the broadcast hrtimer
* armed from going deep idle.
*
* As tick_broadcast_lock is held, nothing can change the cpu
* base which was just established in hrtimer_start() above. So
* the below access is safe even without holding the hrtimer
* base lock.
*/
bc->bound_on = bctimer.base->cpu_base->cpu;
} );
return 0;
}
@ -100,10 +100,6 @@ static enum hrtimer_restart bc_handler(struct hrtimer *t)
{
ce_broadcast_hrtimer.event_handler(&ce_broadcast_hrtimer);
if (clockevent_state_oneshot(&ce_broadcast_hrtimer))
if (ce_broadcast_hrtimer.next_event != KTIME_MAX)
return HRTIMER_RESTART;
return HRTIMER_NORESTART;
}