[Devel] [PATCH rh7] ms/hrtimer: Allow concurrent hrtimer_start() for self restarting timers

[Andrey Ryabinin]

From: Peter Zijlstra <peterz at infradead.org>

Because we drop cpu_base->lock around calling hrtimer::function, it is
possible for hrtimer_start() to come in between and enqueue the timer.

If hrtimer::function then returns HRTIMER_RESTART we'll hit the BUG_ON
because HRTIMER_STATE_ENQUEUED will be set.

Since the above is a perfectly valid scenario, remove the BUG_ON and
make the enqueue_hrtimer() call conditional on the timer not being
enqueued already.

NOTE: in that concurrent scenario its entirely common for both sites
to want to modify the hrtimer, since hrtimers don't provide
serialization themselves be sure to provide some such that the
hrtimer::function and the hrtimer_start() caller don't both try and
fudge the expiration state at the same time.

To that effect, add a WARN when someone tries to forward an already
enqueued timer, the most common way to change the expiry of self
restarting timers. Ideally we'd put the WARN in everything modifying
the expiry but most of that is inlines and we don't need the bloat.

Fixes: 2d44ae4d7135 ("hrtimer: clean up cpu->base locking tricks")
Signed-off-by: Peter Zijlstra (Intel) <peterz at infradead.org>
Cc: Ben Segall <bsegall at google.com>
Cc: Roman Gushchin <klamm at yandex-team.ru>
Cc: Paul Turner <pjt at google.com>
Link: http://lkml.kernel.org/r/20150415113105.GT5029@twins.programming.kicks-ass.net
Signed-off-by: Thomas Gleixner <tglx at linutronix.de>

https://jira.sw.ru/browse/PSBM-88818
(cherry picked from commit 5de2755c8c8b3a6b8414870e2c284914a2b42e4d)
Signed-off-by: Andrey Ryabinin <aryabinin at virtuozzo.com>
---
 kernel/hrtimer.c | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/kernel/hrtimer.c b/kernel/hrtimer.c
index 70c7747aa61a..30bf706bc211 100644
--- a/kernel/hrtimer.c
+++ b/kernel/hrtimer.c
@@ -843,6 +843,9 @@ u64 hrtimer_forward(struct hrtimer *timer, ktime_t now, ktime_t interval)
 	if (delta.tv64 < 0)
 		return 0;
 
+	if (WARN_ON(timer->state & HRTIMER_STATE_ENQUEUED))
+		return 0;
+
 	if (interval.tv64 < timer->base->resolution.tv64)
 		interval.tv64 = timer->base->resolution.tv64;
 
@@ -1222,6 +1225,10 @@ static void __run_hrtimer(struct hrtimer_cpu_base *cpu_base,
 	 * Because we run timers from hardirq context, there is no chance
 	 * they get migrated to another cpu, therefore its safe to unlock
 	 * the timer base.
+	 *
+	 * Note: Because we dropped the cpu_base->lock above,
+	 * hrtimer_start_range_ns() can have popped in and enqueued the timer
+	 * for us already
 	 */
 	raw_spin_unlock(&cpu_base->lock);
 	trace_hrtimer_expire_entry(timer, now);
@@ -1234,10 +1241,9 @@ static void __run_hrtimer(struct hrtimer_cpu_base *cpu_base,
 	 * we do not reprogramm the event hardware. Happens either in
 	 * hrtimer_start_range_ns() or in hrtimer_interrupt()
 	 */
-	if (restart != HRTIMER_NORESTART) {
-		BUG_ON(timer->state != HRTIMER_STATE_CALLBACK);
+	if (restart != HRTIMER_NORESTART &&
+		!(timer->state & HRTIMER_STATE_ENQUEUED))
 		enqueue_hrtimer(timer, base);
-	}
 
 	WARN_ON_ONCE(!(timer->state & HRTIMER_STATE_CALLBACK));
 
-- 
2.16.4