linux-kernel - Re: [PATCH] sched: tg_set_cfs_bandwidth() causes rq->lock deadlock

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Date:	Mon, 19 May 2014 15:37:58 +0400
From:	Roman Gushchin <klamm@...dex-team.ru>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Roman Gushchin <klamm@...dex-team.ru>, bsegall@...gle.com,
	linux-kernel@...r.kernel.org, pjt@...gle.com,
	chris.j.arges@...onical.com, gregkh@...uxfoundation.org
Subject: Re: [PATCH] sched: tg_set_cfs_bandwidth() causes rq->lock deadlock

At Mon, 19 May 2014 12:32:47 +0200,
Peter Zijlstra wrote:
> 
> OK, so can someone explain this ->timer_active thing? esp. what's the
> 'obvious' difference with hrtimer_active()?

A good question :)

> Ideally we'd change the lot to not have this, but if we have to keep it
> we'll need to make it lockdep visible because all this stinks

If the patch below is what Ben means, timer_active will become even more unused.
It seems to me now, that it will be perfectly possible to just drop it.
I'll try to prepare a patch for.

--

index 7570dd9..be7865e 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -3408,7 +3408,6 @@ static int do_sched_cfs_period_timer(struct cfs_bandwidth *cfs_b, int overrun)
 	u64 runtime, runtime_expires;
 	int idle = 1, throttled;
 
-	raw_spin_lock(&cfs_b->lock);
 	/* no need to continue the timer with no bandwidth constraint */
 	if (cfs_b->quota == RUNTIME_INF)
 		goto out_unlock;
@@ -3477,7 +3476,6 @@ static int do_sched_cfs_period_timer(struct cfs_bandwidth *cfs_b, int overrun)
 out_unlock:
 	if (idle)
 		cfs_b->timer_active = 0;
-	raw_spin_unlock(&cfs_b->lock);
 
 	return idle;
 }
@@ -3656,6 +3654,8 @@ static enum hrtimer_restart sched_cfs_period_timer(struct hrtimer *timer)
 	int overrun;
 	int idle = 0;
 
+	raw_spin_lock(&cfs_b->lock);
+
 	for (;;) {
 		now = hrtimer_cb_get_time(timer);
 		overrun = hrtimer_forward(timer, now, cfs_b->period);
@@ -3666,6 +3666,8 @@ static enum hrtimer_restart sched_cfs_period_timer(struct hrtimer *timer)
 		idle = do_sched_cfs_period_timer(cfs_b, overrun);
 	}
 
+	raw_spin_unlock(&cfs_b->lock);
+
 	return idle ? HRTIMER_NORESTART : HRTIMER_RESTART;
 }

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