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Date:   Tue, 1 Dec 2020 16:56:57 +0000
From:   Will Deacon <will@kernel.org>
To:     Qais Yousef <qais.yousef@arm.com>
Cc:     linux-arm-kernel@lists.infradead.org, linux-arch@vger.kernel.org,
        linux-kernel@vger.kernel.org,
        Catalin Marinas <catalin.marinas@arm.com>,
        Marc Zyngier <maz@kernel.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Morten Rasmussen <morten.rasmussen@arm.com>,
        Suren Baghdasaryan <surenb@google.com>,
        Quentin Perret <qperret@google.com>, Tejun Heo <tj@kernel.org>,
        Li Zefan <lizefan@huawei.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Ingo Molnar <mingo@redhat.com>,
        Juri Lelli <juri.lelli@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        kernel-team@android.com
Subject: Re: [PATCH v4 07/14] sched: Introduce restrict_cpus_allowed_ptr() to
 limit task CPU affinity
Message-ID: <20201201165656.GE27783@willie-the-truck>
References: <20201124155039.13804-1-will@kernel.org>
 <20201124155039.13804-8-will@kernel.org>
 <20201127131916.ncoqmg62dselezyl@e107158-lin.cambridge.arm.com>
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On Fri, Nov 27, 2020 at 01:19:16PM +0000, Qais Yousef wrote:
> On 11/24/20 15:50, Will Deacon wrote:
> 
> [...]
> 
> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > index d2003a7d5ab5..818c8f7bdf2a 100644
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -1860,24 +1860,18 @@ void do_set_cpus_allowed(struct task_struct *p, const struct cpumask *new_mask)
> >  }
> >  
> >  /*
> > - * Change a given task's CPU affinity. Migrate the thread to a
> > - * proper CPU and schedule it away if the CPU it's executing on
> > - * is removed from the allowed bitmask.
> > - *
> > - * NOTE: the caller must have a valid reference to the task, the
> > - * task must not exit() & deallocate itself prematurely. The
> > - * call is not atomic; no spinlocks may be held.
> > + * Called with both p->pi_lock and rq->lock held; drops both before returning.
> 
> nit: wouldn't it be better for the caller to acquire and release the locks?
> Not a big deal but it's always confusing when half of the work done outside the
> function and the other half done inside.

That came up in the last version of the patches iirc, but the problem is
that __set_cpus_allowed_ptr_locked() can trigger migration, which can
drop the lock and take another one for the new runqueue.

Given that this function is internal to the scheduler, I think we can
probably live with it.

Will