On 2021/1/21 17:27, Hannes Reinecke wrote:

On 1/21/21 10:00 AM, Christoph Hellwig wrote:

quoted

On Thu, Jan 21, 2021 at 09:58:37AM +0100, Hannes Reinecke wrote:

quoted

On 1/21/21 8:03 AM, Chao Leng wrote:

quoted

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()

Between those to a teardown that cancels all requests can come in.

Doesn't nvme_complete_request() insulate against a double completion?

nvme_complete_request can not insulate against double completion.
Setting the state of request to MQ_RQ_COMPLETE avoid double completion.
tear down(nvme_cancel_request) check the state of the request, if the
state is MQ_RQ_COMPLETE, it will skip completion.

I seem to remember we've gone through great lengths ensuring that.

And if this is just about setting the correct error code on completion I'd really prefer to stick with the current code. Moving that into a helper is fine, but I'd rather not introduce our own code modifying request state.

If there really is a race condition this feels like a more generic problem; calling blk_mq_start_request() followed by blk_mq_end_request() is a quite common pattern, and from my impression the recommended way.
So if there is an issue it would need to be addressed for all drivers, not just some nvme-specific way.

Currently, it is not safe for nvme. The probability is very low.
I am not sure whether similar occurs in other scenarios.

Plus I'd like to have Jens' opinion here.

Cheers,

Hannes

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