linux-kernel - Re: [PATCH v11 07/16] sched: Split the guts of sched

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Date:   Wed, 18 Aug 2021 11:50:30 +0100
From:   Will Deacon <will@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-arch@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Catalin Marinas <catalin.marinas@....com>,
        Marc Zyngier <maz@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Qais Yousef <qais.yousef@....com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Quentin Perret <qperret@...gle.com>, Tejun Heo <tj@...nel.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Valentin Schneider <valentin.schneider@....com>,
        Mark Rutland <mark.rutland@....com>, kernel-team@...roid.com
Subject: Re: [PATCH v11 07/16] sched: Split the guts of sched_setaffinity()
 into a helper function

On Tue, Aug 17, 2021 at 05:40:24PM +0200, Peter Zijlstra wrote:
> On Fri, Jul 30, 2021 at 12:24:34PM +0100, Will Deacon wrote:
> > In preparation for replaying user affinity requests using a saved mask,
> > split sched_setaffinity() up so that the initial task lookup and
> > security checks are only performed when the request is coming directly
> > from userspace.
> > 
> > Reviewed-by: Valentin Schneider <Valentin.Schneider@....com>
> > Signed-off-by: Will Deacon <will@...nel.org>
> 
> Should not sched_setaffinity() update user_cpus_ptr when it isn't NULL,
> such that the upcoming relax_compatible_cpus_allowed_ptr() preserve the
> full user mask?

The idea is that force_compatible_cpus_allowed_ptr() and
relax_compatible_cpus_allowed_ptr() are used as a pair, with the former
setting ->user_cpus_ptr and the latter restoring it. An intervening call
to sched_setaffinity() must _clear_ the saved mask, as we discussed
before at:

https://lore.kernel.org/r/YK53kDtczHIYumDC@hirez.programming.kicks-ass.net

Will