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From:   Daniel Jordan <daniel.m.jordan@oracle.com>
To:     Alexey Klimov <aklimov@redhat.com>
Cc:     linux-kernel@vger.kernel.org, cgroups@vger.kernel.org,
        Peter Zijlstra <peterz@infradead.org>, yury.norov@gmail.com,
        tglx@linutronix.de, Joshua Baker <jobaker@redhat.com>,
        audralmitchel@gmail.com, arnd@arndb.de, gregkh@linuxfoundation.org,
        rafael@kernel.org, tj@kernel.org,
        Qais Yousef <qais.yousef@arm.com>, hannes@cmpxchg.org,
        Alexey Klimov <klimov.linux@gmail.com>
Subject: Re: [PATCH v2] cpu/hotplug: wait for cpuset_hotplug_work to finish
 on cpu onlining
In-Reply-To: <CAFBcO+99Ax5MuOtzNx=NrmnUN=+913Sc-DV83ObOi01A=kkN3w@mail.gmail.com>
References: <20210212003032.2037750-1-aklimov@redhat.com>
 <87tuqhrs3d.fsf@oracle.com>
 <CAFBcO+99Ax5MuOtzNx=NrmnUN=+913Sc-DV83ObOi01A=kkN3w@mail.gmail.com>
Date:   Thu, 18 Mar 2021 15:12:25 -0400
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Alexey Klimov <aklimov@redhat.com> writes:
> The first section of comment seems problematic to me with regards to such move:
>
>                  * As this needs to hold the cpu maps lock it's impossible
>                  * to call device_offline() because that ends up calling
>                  * cpu_down() which takes cpu maps lock. cpu maps lock
>                  * needs to be held as this might race against in kernel
>                  * abusers of the hotplug machinery (thermal management).
>
> Cpu maps lock is released in cpu_maps_update_done() hence we will move
> dev->offline out of cpu maps lock. Maybe I misunderstood the comment
> and it relates to calling cpu_down_maps_locked() under lock to avoid
> race?

Yes, that's what I take from the comment, the cpu maps lock protects
against racing hotplug operations.