linux-kernel - Re: [PATCH] sched/deadline: Fix BUG

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Date:   Thu, 14 Jul 2022 09:28:08 +0200
From:   Juri Lelli <juri.lelli@...hat.com>
To:     "Srivatsa S. Bhat" <srivatsa@...il.mit.edu>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        linux-rt-users <linux-rt-users@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Valentin Schneider <vschneid@...hat.com>,
        Sharan Turlapati <sturlapati@...are.com>, bordoloih@...are.com,
        ankitja@...are.com, Keerthana K <keerthanak@...are.com>,
        Anish Swaminathan <anishs@...are.com>,
        Srivatsa Bhat <srivatsab@...are.com>
Subject: Re: [PATCH] sched/deadline: Fix BUG_ON condition for deboosted tasks

Hi,

On 13/07/22 14:31, Srivatsa S. Bhat wrote:
> 
> Hi Juri,
> 
> On 7/13/22 12:50 AM, Juri Lelli wrote:
> > Tasks the are being deboosted from SCHED_DEADLINE might enter
> > enqueue_task_dl() one last time and hit an erroneous BUG_ON condition:
> > since they are not boosted anymore, the if (is_dl_boosted()) branch is
> > not taken, but the else if (!dl_prio) is and inside this one we
> > BUG_ON(!is_dl_boosted), which is of course false (BUG_ON triggered)
> > otherwise we had entered the if branch above. Long story short, the
> > current condition doesn't make sense and always leads to triggering of a
> > BUG.
> > 
> > Fix this by only checking enqueue flags, properly: ENQUEUE_REPLENISH has
> > to be present, but additional flags are not a problem.
> > 
> > Fixes: 2279f540ea7d ("sched/deadline: Fix priority inheritance with multiple scheduling classes")
> 
> It looks like this problem goes further back than the above commit
> (which was merged in v5.10).
> 
> Even the oldest LTS kernel (4.9) has code like this:
> 
> if (... && p->dl.dl_boosted && ...)) {
> 	/* code */
> 
> } else if (!dl_prio(p->normal_prio)) {
> 
> 	BUG_ON(!p->dl.dl_boosted || flags != ENQUEUE_REPLENISH);
> 	return;
> } 
> 
> And we have observed crashes in the 4.19 kernel series too (CC'ed
> Ankit Jain and Him Kalyan who have reproduced this issue).
> 
> I believe commit 64be6f1f5f71 ("sched/deadline: Don't replenish from a
> !SCHED_DEADLINE entity") introduced the problem, which dates back to
> v3.18.
> 
> Would you mind updating the Fixes: tag and adding a CC: stable tag as
> well, when you respin the patch, please?

I think you are right. Will do.

Thanks for taking a look!

Best,
Juri