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References: <20220802084146.3922640-1-vschneid@redhat.com> <20220802084146.3922640-2-vschneid@redhat.com>
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From:   Lai Jiangshan <jiangshanlai@gmail.com>
Date:   Tue, 30 Aug 2022 22:16:46 +0800
Message-ID: <CAJhGHyD050bnK4eP4sxgQKY22hzPM_cW74s20YC9dYJbXdprWQ@mail.gmail.com>
Subject: Re: [RFC PATCH v3 1/3] workqueue: Hold wq_pool_mutex while affining
 tasks to wq_unbound_cpumask
To:     Valentin Schneider <vschneid@redhat.com>
Cc:     LKML <linux-kernel@vger.kernel.org>, Tejun Heo <tj@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Frederic Weisbecker <frederic@kernel.org>,
        Juri Lelli <juri.lelli@redhat.com>,
        Phil Auld <pauld@redhat.com>,
        Marcelo Tosatti <mtosatti@redhat.com>
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On Tue, Aug 2, 2022 at 4:42 PM Valentin Schneider <vschneid@redhat.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