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Subject[GIT PULL] timer fix
Linus,

Please pull the latest timers-urgent-for-linus git tree from:

git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git timers-urgent-for-linus

# HEAD: b9023b91dd020ad7e093baa5122b6968c48cc9e0 tick: broadcast-hrtimer: Fix a race in bc_set_next

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.

Thanks,

Ingo

------------------>
Balasubramani Vivekanandan (1):
tick: broadcast-hrtimer: Fix a race in bc_set_next


kernel/time/tick-broadcast-hrtimer.c | 62 +++++++++++++++++-------------------
1 file changed, 29 insertions(+), 33 deletions(-)

diff --git a/kernel/time/tick-broadcast-hrtimer.c b/kernel/time/tick-broadcast-hrtimer.c
index c1f5bb590b5e..b5a65e212df2 100644
--- a/kernel/time/tick-broadcast-hrtimer.c
+++ b/kernel/time/tick-broadcast-hrtimer.c
@@ -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;
}

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