linux-kernel - Re: [PATCH] sched/fair: Use rq->lock when checking cfs

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Message-ID: <20211013142643.GA48428@blackbody.suse.cz>
Date:   Wed, 13 Oct 2021 16:26:43 +0200
From:   Michal Koutný <mkoutny@...e.com>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     linux-kernel <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        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>,
        Odin Ugedal <odin@...d.al>
Subject: Re: [PATCH] sched/fair: Use rq->lock when checking cfs_rq list
 presence

On Wed, Oct 13, 2021 at 09:57:17AM +0200, Vincent Guittot <vincent.guittot@...aro.org> wrote:
> Furthermore, list_del_leaf_cfs_rq() starts with the same test on of
> cfs_rq->on_list.

Yes, the same check but synchronized with rq->lock.

> The problem is that the cfs_rq can be added during or
> after the test. Removing it should not be enough because we do the
> same test under rq lock which only ensures that both the test and the
> add on the list will not happen simultaneously.

This is what I overlooked when I was looking for explanation of the UAF
on the leaf list.

> This seems to closes the race window in your case but this could still
> happen AFAICT.

You seem to be right.
Hopefully, I'll be able to collect more data evaluating this.

> What about  your patchset about adding a cfs in the list only when
> there is a runnable task ?

The patches I had sent previously [1] avoid adding cfs_rq to the list
when it's under a throttled ancestor (namely 4/5). The runnable
condition is rather orthogonal. (Not sure it's the patchset you were
referring to.)

> Wouldn't this fix the problem ?

FWIW, the "reliable" fix so far is a revert of the commit a7b359fc6a37
("sched/fair: Correctly insert cfs_rq's to list on
unthrottle"). Therefore my hypothesis about racy adding from
tg_unthrottle_up(), so I think the other patches won't affect the issue.

Thanks for your feedback. Let me examine the problem some more before
continuing with this patch.

Michal

[1] https://lore.kernel.org/all/20210819175034.4577-1-mkoutny@suse.com/