linux-kernel - Re: [PATCH-tip] sched: Don't call kfree() in do_set_cpus

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Message-ID: <Y3tVPjAjE9OFRLaP@hirez.programming.kicks-ass.net>
Date:   Mon, 21 Nov 2022 11:38:54 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Waiman Long <longman@...hat.com>
Cc:     Ingo Molnar <mingo@...hat.com>, Juri Lelli <juri.lelli@...hat.com>,
        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>,
        Phil Auld <pauld@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH-tip] sched: Don't call kfree() in do_set_cpus_allowed()

On Fri, Nov 18, 2022 at 02:33:02PM -0500, Waiman Long wrote:
> Commit 851a723e45d1 ("sched: Always clear user_cpus_ptr in
> do_set_cpus_allowed()") may call kfree() if user_cpus_ptr was previously
> set. Unfortunately, some of the callers of do_set_cpus_allowed()

'some' ? There's only 3 or so, which one triggers this?

> may not be in a context where kfree() can be safely called. So the
> following splats may be printed:
> 
>    WARNING: possible circular locking dependency detected
>    BUG: sleeping function called from invalid context
> 
> To avoid these problems without leaking memory, the free cpumask is now
> put into a lockless list to be reused in a later sched_setaffinity()
> call instead.

Urgh.. depending on which of the callsites it is, it's probably simpler
to just rework the caller to not use do_set_cpus_allowed(), no?