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Date:   Thu, 24 Feb 2022 05:38:01 -0800
From:   Christoph Hellwig <hch@infradead.org>
To:     "Wang Jianchao (Kuaishou)" <jianchao.wan9@gmail.com>
Cc:     Jens Axboe <axboe@kernel.dk>, Josef Bacik <jbacik@fb.com>,
        Tejun Heo <tj@kernel.org>,
        Bart Van Assche <bvanassche@acm.org>,
        linux-block@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [RFC V5 01/16] blk: make the whole blk_mq_submit_bio under
 q_usage_counter
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On Thu, Feb 24, 2022 at 05:06:39PM +0800, Wang Jianchao (Kuaishou) wrote:
> This is to protect the rqos list against the rqos open/close. We
> need to drain all of the caller of blk_mq_submit_bio() before
> we can operate the rqos list.

This means ever I/O now has to d an extra refcount roundtrip.