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Message-ID: <3969ed5a-b422-4b27-bff4-1db38d9d238b@kernel.dk>
Date:   Fri, 1 Sep 2023 09:34:47 -0600
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Subject: Re: [PATCH V4] lib/group_cpus.c: avoid to acquire cpu hotplug lock in
 group_cpus_evenly
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To:     Ming Lei <ming.lei@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>
Cc:     linux-kernel@vger.kernel.org, Keith Busch <kbusch@kernel.org>,
        linux-nvme@lists.infradead.org, linux-block@vger.kernel.org,
        Yi Zhang <yi.zhang@redhat.com>,
        Guangwu Zhang <guazhang@redhat.com>,
        Chengming Zhou <zhouchengming@bytedance.com>
References: <20230831093754.2322955-1-ming.lei@redhat.com>
From:   Jens Axboe <axboe@kernel.dk>
In-Reply-To: <20230831093754.2322955-1-ming.lei@redhat.com>
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On 8/31/23 3:37 AM, Ming Lei wrote:
> group_cpus_evenly() could be part of storage driver's error handler,
> such as nvme driver, when may happen during CPU hotplug, in which
> storage queue has to drain its pending IOs because all CPUs associated
> with the queue are offline and the queue is becoming inactive. And
> handling IO needs error handler to provide forward progress.
> 
> Then dead lock is caused:
> 
> 1) inside CPU hotplug handler, CPU hotplug lock is held, and blk-mq's
> handler is waiting for inflight IO
> 
> 2) error handler is waiting for CPU hotplug lock
> 
> 3) inflight IO can't be completed in blk-mq's CPU hotplug handler because
> error handling can't provide forward progress.
> 
> Solve the deadlock by not holding CPU hotplug lock in group_cpus_evenly(),
> in which two stage spreads are taken: 1) the 1st stage is over all present
> CPUs; 2) the end stage is over all other CPUs.
> 
> Turns out the two stage spread just needs consistent 'cpu_present_mask', and
> remove the CPU hotplug lock by storing it into one local cache. This way
> doesn't change correctness, because all CPUs are still covered.

Reviewed-by: Jens Axboe <axboe@kernel.dk>

-- 
Jens Axboe