Re: deadlock in scheduler enabling HRTICK feature

[Peter Zijlstra]

On Fri, Jun 28, 2013 at 11:00:21AM +0200, Ingo Molnar wrote:
> I guess we could merge this fix?

---
Subject: sched: Fix HRTICK

David reported that the HRTICK sched feature was borken; which was enough
motivation for me to finally fix it ;-)

We should not allow hrtimer code to do softirq wakeups while holding scheduler
locks. The hrtimer code only needs this when we accidentally try to program an
expired time. We don't much care about those anyway since we have the regular
tick to fall back to.

Reported-by: David Ahern <dsahern@xxxxxxxxx>
Tested-by: David Ahern <dsahern@xxxxxxxxx>
Signed-off-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
---
 kernel/sched/core.c | 20 +++++++++++---------
 1 file changed, 11 insertions(+), 9 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 9b1f2e5..0d8eb45 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -370,13 +370,6 @@ static struct rq *this_rq_lock(void)
 #ifdef CONFIG_SCHED_HRTICK
 /*
  * Use HR-timers to deliver accurate preemption points.
- *
- * Its all a bit involved since we cannot program an hrt while holding the
- * rq->lock. So what we do is store a state in in rq->hrtick_* and ask for a
- * reschedule event.
- *
- * When we get rescheduled we reprogram the hrtick_timer outside of the
- * rq->lock.
  */
 
 static void hrtick_clear(struct rq *rq)
@@ -404,6 +397,15 @@ static enum hrtimer_restart hrtick(struct hrtimer *timer)
 }
 
 #ifdef CONFIG_SMP
+
+static int __hrtick_restart(struct rq *rq)
+{
+	struct hrtimer *timer = &rq->hrtick_timer;
+	ktime_t time = hrtimer_get_softexpires(timer);
+
+	return __hrtimer_start_range_ns(timer, time, 0, HRTIMER_MODE_ABS_PINNED, 0);
+}
+
 /*
  * called from hardirq (IPI) context
  */
@@ -412,7 +414,7 @@ static void __hrtick_start(void *arg)
 	struct rq *rq = arg;
 
 	raw_spin_lock(&rq->lock);
-	hrtimer_restart(&rq->hrtick_timer);
+	__hrtick_restart(rq);
 	rq->hrtick_csd_pending = 0;
 	raw_spin_unlock(&rq->lock);
 }
@@ -430,7 +432,7 @@ void hrtick_start(struct rq *rq, u64 delay)
 	hrtimer_set_expires(timer, time);
 
 	if (rq == this_rq()) {
-		hrtimer_restart(timer);
+		__hrtick_restart(rq);
 	} else if (!rq->hrtick_csd_pending) {
 		__smp_call_function_single(cpu_of(rq), &rq->hrtick_csd, 0);
 		rq->hrtick_csd_pending = 1;

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