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

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Message-ID: <CAJhGHyD050bnK4eP4sxgQKY22hzPM_cW74s20YC9dYJbXdprWQ@mail.gmail.com>
Date:   Tue, 30 Aug 2022 22:16:46 +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 Tue, Aug 2, 2022 at 4:42 PM Valentin Schneider <vschneid@...hat.com> 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.
>

Hello Valentin,

Updating wq_unbound_cpumask requires cpus_read_lock() and
unbind_workers() is in the CPU hotplug path and so it is sufficient to
access to wq_unbound_cpumask in unbind_workers().

The extra protection is only required when the logic is also moved to
destroy_worker().

Thanks
Lai