linux-kernel - Re: [PATCH v6 4/5] sched: Handle set_cpus_allowed_ptr() & sched

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Message-ID: <Yw8pOVhOV9XVe9nq@hirez.programming.kicks-ass.net>
Date:   Wed, 31 Aug 2022 11:26:17 +0200
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>,
        Valentin Schneider <vschneid@...hat.com>,
        Tejun Heo <tj@...nel.org>, Zefan Li <lizefan.x@...edance.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Will Deacon <will@...nel.org>, linux-kernel@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Lai Jiangshan <jiangshanlai@...il.com>
Subject: Re: [PATCH v6 4/5] sched: Handle set_cpus_allowed_ptr() &
 sched_setaffinity() race

On Thu, Aug 25, 2022 at 09:01:18PM -0400, Waiman Long wrote:
> Racing is possible between set_cpus_allowed_ptr() and sched_setaffinity()
> or between multiple sched_setaffinity() calls from different CPUs. To
> resolve these race conditions, we need to update both user_cpus_ptr
> and cpus_mask in a single lock critical section instead of separated
> ones. This requires moving the user_cpus_ptr update to
> affine_move_task() before doing task_rq_unlock().
> 
> A new argument puser_mask is added to affine_move_task(),
> __set_cpus_allowed_ptr_locked() and __set_cpus_allowed_ptr() to do that.
> 
> Ideally, user_cpus_ptr should only be updated if the sched_setaffinity()
> is successful. However, this patch will update user_cpus_ptr when the
> first call to __set_cpus_allowed_ptr() is successful. However, if there
> is racing between sched_setaffinity() and cpuset update, the subsequent
> calls to __set_cpus_allowed_ptr() may fail but the user_cpus_ptr will
> still be updated in this corner case.

Urgh, this is a bit weird, to have a fix for a patch in the same series.