Re: [PATCH v4 2/6] nbd: convert to use blk_mq_find_and_get_req()

[Ming Lei <ming.lei@redhat.com>]

On Tue, Sep 07, 2021 at 10:01:50PM +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. However, there are still some problems if the request is
> started, and this will be fixed in next patch.
> 
> Signed-off-by: Yu Kuai <yukuai3@huawei.com>
> ---
>  drivers/block/nbd.c | 10 +++++++---
>  1 file changed, 7 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
> index 5170a630778d..920da390635c 100644
> --- a/drivers/block/nbd.c
> +++ b/drivers/block/nbd.c
> @@ -718,12 +718,13 @@ static struct nbd_cmd *nbd_read_stat(struct nbd_device *nbd, int index)
>  	tag = nbd_handle_to_tag(handle);
>  	hwq = blk_mq_unique_tag_to_hwq(tag);
>  	if (hwq < nbd->tag_set.nr_hw_queues)
> -		req = blk_mq_tag_to_rq(nbd->tag_set.tags[hwq],
> -				       blk_mq_unique_tag_to_tag(tag));
> +		req = blk_mq_find_and_get_req(nbd->tag_set.tags[hwq],
> +					      blk_mq_unique_tag_to_tag(tag));
>  	if (!req || !blk_mq_request_started(req)) {
>  		dev_err(disk_to_dev(nbd->disk), "Unexpected reply (%d) %p\n",
>  			tag, req);
> -		return ERR_PTR(-ENOENT);
> +		ret = -ENOENT;
> +		goto put_req;
>  	}
>  	trace_nbd_header_received(req, handle);
>  	cmd = blk_mq_rq_to_pdu(req);
> @@ -785,6 +786,9 @@ static struct nbd_cmd *nbd_read_stat(struct nbd_device *nbd, int index)
>  out:
>  	trace_nbd_payload_received(req, handle);
>  	mutex_unlock(&cmd->lock);
> +put_req:
> +	if (req)
> +		blk_mq_put_rq_ref(req);
>  	return ret ? ERR_PTR(ret) : cmd;

After the request's refcnt is dropped, it can be freed immediately, then
one stale command is returned to caller.

Thanks,
Ming