linux-kernel - Re: [RFC PATCH v3 1/3] workqueue: Hold wq_pool_mutex while affining tasks to wq_unbound

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Date:   Fri, 5 Aug 2022 10:43:07 +0800
From:   Lai Jiangshan <jiangshanlai@...il.com>
To:     Valentin Schneider <vschneid@...hat.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, Tejun Heo <tj@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Phil Auld <pauld@...hat.com>,
        Marcelo Tosatti <mtosatti@...hat.com>
Subject: Re: [RFC PATCH v3 1/3] workqueue: Hold wq_pool_mutex while affining
 tasks to wq_unbound_cpumask

On Thu, Aug 4, 2022 at 7:40 PM Valentin Schneider <vschneid@...hat.com> wrote:
>
> On 03/08/22 11:40, Lai Jiangshan wrote:
> > On 2022/8/2 16:41, Valentin Schneider wrote:
> >> When unbind_workers() reads wq_unbound_cpumask to set the affinity of
> >> freshly-unbound kworkers, it only holds wq_pool_attach_mutex. This isn't
> >> sufficient as wq_unbound_cpumask is only protected by wq_pool_mutex.
> >>
> >> This is made more obvious as of recent commit
> >>
> >>    46a4d679ef88 ("workqueue: Avoid a false warning in unbind_workers()")
> >>
> >> e.g.
> >>
> >> unbind_workers()                             workqueue_set_unbound_cpumask()
> >>    kthread_set_per_cpu(p, -1);
> >>    if (cpumask_intersects(wq_unbound_cpumask, cpu_active_mask))
> >>                                             cpumask_copy(wq_unbound_cpumask, cpumask);
> >>      WARN_ON_ONCE(set_cpus_allowed_ptr(worker->task, wq_unbound_cpumask) < 0);
> >>
> >> Make workqueue_offline_cpu() invoke unbind_workers() with wq_pool_mutex
> >> held.
> >
> > I would prefer to protect wq_unbound_cpumask with wq_pool_attach_mutex.
>
> That looks alright to me, do you want to push that separately as it's a
> standalone patch, or should I carry it with this series?
>

I'm Okay with both.

It needs review from Tejun.  If Tejun has not queued it before you send
a new update of this series, I will be glad if you carry it.