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

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Message-ID: <8dc0fac7-5451-022f-b3e9-11656c93c34d@redhat.com>
Date:   Wed, 31 Aug 2022 16:53:20 -0400
From:   Waiman Long <longman@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
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 8/31/22 05:26, Peter Zijlstra wrote:
> 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.

This is just to make each patch simpler and easier to read.

Cheers,
Longman