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SubjectRe: [PATCH v5 5/6] nbd: convert to use blk_mq_find_and_get_req()
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On 2021/09/14 9:11, Ming Lei wrote:
> On Thu, Sep 09, 2021 at 10:12:55PM +0800, Yu Kuai wrote:
>> blk_mq_tag_to_rq() can only ensure to return valid request in
>> following situation:
>>
>> 1) client send request message to server first
>> submit_bio
>> ...
>> blk_mq_get_tag
>> ...
>> blk_mq_get_driver_tag
>> ...
>> nbd_queue_rq
>> nbd_handle_cmd
>> nbd_send_cmd
>>
>> 2) client receive respond message from server
>> recv_work
>> nbd_read_stat
>> blk_mq_tag_to_rq
>>
>> If step 1) is missing, blk_mq_tag_to_rq() will return a stale
>> request, which might be freed. Thus convert to use
>> blk_mq_find_and_get_req() to make sure the returned request is not
>> freed.
>
> But NBD_CMD_INFLIGHT has been added for checking if the reply is
> expected, do we still need blk_mq_find_and_get_req() for covering
> this issue? BTW, request and its payload is pre-allocated, so there
> isn't real use-after-free.

Hi, Ming

Checking NBD_CMD_INFLIGHT relied on the request founded by tag is valid,
not the other way round.

nbd_read_stat
req = blk_mq_tag_to_rq()
cmd = blk_mq_rq_to_pdu(req)
mutex_lock(cmd->lock)
checking NBD_CMD_INFLIGHT

The checking doesn't have any effect on blk_mq_tag_to_rq().

Thanks,
Kuai

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