linux-kernel - Re: [PATCH v7 4/5] sched: Handle set_cpus_allowed_ptr() & sched

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Message-ID: <0735a6bf-7d92-5564-ae57-d6c05d0a7a4c@redhat.com>
Date:   Thu, 8 Sep 2022 10:54:42 -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 v7 4/5] sched: Handle set_cpus_allowed_ptr() &
 sched_setaffinity() race


On 9/8/22 07:53, Peter Zijlstra wrote:
> On Fri, Sep 02, 2022 at 11:25:55AM -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
>> set_cpus_allowed_common().
>>
>> The SCA_USER flag will be used to signify that a user_cpus_ptr update
>> will have to be done. The new user_cpus_ptr will be put into the
>> a percpu variable pending_user_mask at the beginning of the lock
>> crtical section. The pending user mask will then be taken up in
>> set_cpus_allowed_common().
>>
>> 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. A warning will be printed in this
>> corner case.
>>
>> Signed-off-by: Waiman Long <longman@...hat.com>
> OK, so obviously this is terrible :/
>
> What's wrong with something like so ?

Thanks for the suggestion. I have no problem adding an affinity_context 
structure to pass around the functions. Will incorporate your suggestion 
in the next version of the patch.

Thanks,
Longman