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Subject: Re: [PATCH] blk-mq: fix a hung issue when fsync
To:     Jianchao Wang <jianchao.w.wang@oracle.com>
Cc:     m19@florianstecker.de, linux-block@vger.kernel.org,
        linux-kernel@vger.kernel.org
References: <1548838916-25051-1-git-send-email-jianchao.w.wang@oracle.com>
From:   Jens Axboe <axboe@kernel.dk>
Message-ID: <319fffef-2fa8-afff-8f93-1ce8fd721581@kernel.dk>
Date:   Wed, 30 Jan 2019 08:54:09 -0700
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On 1/30/19 2:01 AM, Jianchao Wang wrote:
> Florian reported a io hung issue when fsync(). It should be
> triggered by following race condition.
> 
> data + post flush         a flush
> 
> blk_flush_complete_seq
>   case REQ_FSEQ_DATA
>     blk_flush_queue_rq
>     issued to driver      blk_mq_dispatch_rq_list
>                             try to issue a flush req
>                             failed due to NON-NCQ command
>                             .queue_rq return BLK_STS_DEV_RESOURCE
> 
> request completion
>   req->end_io // doesn't check RESTART
>   mq_flush_data_end_io
>     case REQ_FSEQ_POSTFLUSH
>       blk_kick_flush
>         do nothing because previous flush
>         has not been completed
>      blk_mq_run_hw_queue
>                               insert rq to hctx->dispatch
>                               due to RESTART is still set, do nothing
> 
> To fix this, replace the blk_mq_run_hw_queue in mq_flush_data_end_io
> with blk_mq_sched_restart to check and clear the RESTART flag.

Applied, thanks.

-- 
Jens Axboe