linux-kernel - Re: [External] Re: [syzbot] [block?] WARNING in blk_mq_start

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Message-ID: <d9f1ee3b-c303-44af-ba30-b52137fc29cb@bytedance.com>
Date:   Mon, 13 Nov 2023 23:05:14 +0800
From:   Chengming Zhou <zhouchengming@...edance.com>
To:     Bart Van Assche <bvanassche@....org>,
        syzbot <syzbot+fcc47ba2476570cbbeb0@...kaller.appspotmail.com>,
        axboe@...nel.dk, chaitanyak@...dia.com, eadavis@...com,
        hch@...radead.org, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, ming.lei@...hat.com,
        syzkaller-bugs@...glegroups.com
Subject: Re: [External] Re: [syzbot] [block?] WARNING in blk_mq_start_request

On 2023/11/10 02:13, Bart Van Assche wrote:
> On 11/8/23 17:27, Chengming Zhou wrote:
>> CONFIG_BLK_DEV_NULL_BLK_FAULT_INJECTION is enabled in the kernel config,
>> so null_queue_rq() will return BLK_STS_RESOURCE or BLK_STS_DEV_RESOURCE
>> for some requests, which have been marked as IN_FLIGHT status.
>>
>> Then null_queue_rqs() put these requests in the rqlist and return back,
>> blk-mq will try to queue them individually once again, caused the warning
>> "WARN_ON_ONCE(blk_mq_rq_state(rq) != MQ_RQ_IDLE)" in blk_mq_start_request().
>>
>> So handling of return value of null_queue_rq() in null_queue_rqs() is wrong,
>> maybe we should __blk_mq_requeue_request() for these requests, before
>> adding them in the rqlist?
> 
> Please follow the example of virtio_queue_rqs() and send any requests
> that need to be requeued back to the block layer core instead of
> handling these directly in null_queue_rqs().
> 

Ok, I reviewed the code of virtio_queue_rqs(), found the main difference
is that request won't fail after blk_mq_start_request().

But in null_blk case, the request will fail after blk_mq_start_request(),
return BLK_STS_RESOURCE or BLK_STS_DEV_RESOURCE. If we return these rqs
back to the block layer core, they will be queued individually once again.
So caused the warning.

Thanks!