linux-kernel - Re: [PATCH v2] blk-mq: release scheduler resource when request complete

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Date:   Sun, 13 Aug 2023 09:34:59 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     chengming.zhou@...ux.dev, hch@....de, chuck.lever@...cle.com
Cc:     bvanassche@....org, cel@...nel.org, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, zhouchengming@...edance.com
Subject: Re: [PATCH v2] blk-mq: release scheduler resource when request
 complete

On 8/13/23 9:23 AM, chengming.zhou@...ux.dev wrote:
> From: Chengming Zhou <zhouchengming@...edance.com>
> 
> Chuck reported [1] a IO hang problem on NFS exports that reside on SATA
> devices and bisected to commit 615939a2ae73 ("blk-mq: defer to the normal
> submission path for post-flush requests").
> 
> We analysed the IO hang problem, found there are two postflush requests
> are waiting for each other.
> 
> The first postflush request completed the REQ_FSEQ_DATA sequence, so go to
> the REQ_FSEQ_POSTFLUSH sequence and added in the flush pending list, but
> failed to blk_kick_flush() because of the second postflush request which
> is inflight waiting in scheduler queue.
> 
> The second postflush waiting in scheduler queue can't be dispatched because
> the first postflush hasn't released scheduler resource even though it has
> completed by itself.
> 
> Fix it by releasing scheduler resource when the first postflush request
> completed, so the second postflush can be dispatched and completed, then
> make blk_kick_flush() succeed.

Thanks, applied with a bit of commit message massaging and adding a
comment for the newly added WARN_ON_ONCE().

-- 
Jens Axboe