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

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Message-ID: <xm26ppj93c3x.fsf@sword-of-the-dawn.mtv.corp.google.com>
Date:	Mon, 19 May 2014 10:35:46 -0700
From:	bsegall@...gle.com
To:	Roman Gushchin <klamm@...dex-team.ru>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	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

Roman Gushchin <klamm@...dex-team.ru> writes:

> 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.

Yeah, the below patch was the sort of idea I was thinking of. I'll send
a similar patch that also hits unthrottle_offline_cfs_rqs shortly. This
still doesn't save us from the unlock vs become !hrtimer_active race
though.

>
> --
>
> 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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