Re: [PATCH v3 3/5] nvme-fabrics: avoid double request completion for nvmf_fail_nonready_command

From: Chao Leng <lengchao@huawei.com>
To: Hannes Reinecke <hare@suse.de>, <linux-nvme@lists.infradead.org>
Cc: axboe@kernel.dk, axboe@fb.com, sagi@grimberg.me,
	linux-block@vger.kernel.org, kbusch@kernel.org, hch@lst.de
Subject: Re: [PATCH v3 3/5] nvme-fabrics: avoid double request completion for nvmf_fail_nonready_command
Date: Fri, 22 Jan 2021 09:48:03 +0800	[thread overview]
Message-ID: <a100b5dd-d38b-3158-d000-b84920a4e274@huawei.com> (raw)
In-Reply-To: <fda1fdb8-8a9d-2e95-4d08-8d8ee1df450d@suse.de>


On 2021/1/21 16:58, Hannes Reinecke wrote:
> On 1/21/21 8:03 AM, Chao Leng wrote:
>> When reconnect, the request may be completed with NVME_SC_HOST_PATH_ERROR
>> in nvmf_fail_nonready_command. The state of request will be changed to
>> MQ_RQ_IN_FLIGHT before call nvme_complete_rq. If free the request
>> asynchronously such as in nvme_submit_user_cmd, in extreme scenario
>> the request may be completed again in tear down process.
>> nvmf_fail_nonready_command do not need calling blk_mq_start_request
>> before complete the request. nvmf_fail_nonready_command should set
>> the state of request to MQ_RQ_COMPLETE before complete the request.
>>
> 
> So what you are saying is that there is a race condition between
> blk_mq_start_request()
> and
> nvme_complete_request()
Yes. The race is:
process1:error recovery->tear down->quiesce queue(wait dispatch done)
process2:dispatch->queue_rq->nvmf_fail_nonready_command->
     nvme_complete_rq(if the request is freed asynchronously, wake
	nvme_submit_user_cmd( for example) but have no chance to run).
process1:continue ->cancle suspend request, check the state is not
     MQ_RQ_IDLE and MQ_RQ_COMPLETE, complete(free) the request.
process3: nvme_submit_user_cmd now has chance to run, and the free the
     request again.
Test Injection Method: inject a msleep before call blk_mq_free_request
in nvme_submit_user_cmd.
> 
>> Signed-off-by: Chao Leng <lengchao@huawei.com>
>> ---
>>   drivers/nvme/host/fabrics.c | 4 +---
>>   1 file changed, 1 insertion(+), 3 deletions(-)
>>
>> diff --git a/drivers/nvme/host/fabrics.c b/drivers/nvme/host/fabrics.c
>> index 72ac00173500..874e4320e214 100644
>> --- a/drivers/nvme/host/fabrics.c
>> +++ b/drivers/nvme/host/fabrics.c
>> @@ -553,9 +553,7 @@ blk_status_t nvmf_fail_nonready_command(struct nvme_ctrl *ctrl,
>>           !blk_noretry_request(rq) && !(rq->cmd_flags & REQ_NVME_MPATH))
>>           return BLK_STS_RESOURCE;
>> -    nvme_req(rq)->status = NVME_SC_HOST_PATH_ERROR;
>> -    blk_mq_start_request(rq);
>> -    nvme_complete_rq(rq);
>> +    nvme_complete_failed_req(rq);
>>       return BLK_STS_OK;
>>   }
>>   EXPORT_SYMBOL_GPL(nvmf_fail_nonready_command);
>> I'd rather have 'nvme_complete_failed_req()' accept the status as 
> argument, like
> 
> nvme_complete_failed_request(rq, NVME_SC_HOST_PATH_ERROR)
> 
> that way it's obvious what is happening, and the status isn't hidden in the function.
Ok, good idea. Thank you for your suggestion.
> 
> Cheers,
> 
> Hannes

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