Re: [PATCH v2 2/2] nbd: convert to use blk_mq_get_rq_by_tag()

[Ming Lei]

On Mon, Aug 09, 2021 at 11:09:27AM +0800, Yu Kuai wrote:
> blk_mq_tag_to_rq() might return freed request, use
> blk_mq_get_rq_by_tag() instead.
> 
> Signed-off-by: Yu Kuai <yukuai3@xxxxxxxxxx>
> ---
>  drivers/block/nbd.c | 11 ++++++-----
>  1 file changed, 6 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
> index c38317979f74..9e56975a8eee 100644
> --- a/drivers/block/nbd.c
> +++ b/drivers/block/nbd.c
> @@ -713,11 +713,10 @@ 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));
> -	if (!req || !blk_mq_request_started(req)) {
> -		dev_err(disk_to_dev(nbd->disk), "Unexpected reply (%d) %p\n",
> -			tag, req);
> +		req = blk_mq_get_rq_by_tag(nbd->tag_set.tags[hwq],
> +					   blk_mq_unique_tag_to_tag(tag));
> +	if (!req) {
> +		dev_err(disk_to_dev(nbd->disk), "Unexpected reply %d\n", tag);
>  		return ERR_PTR(-ENOENT);
>  	}
>  	trace_nbd_header_received(req, handle);
> @@ -779,6 +778,8 @@ static struct nbd_cmd *nbd_read_stat(struct nbd_device *nbd, int index)
>  	}
>  out:
>  	trace_nbd_payload_received(req, handle);
> +	if (req)
> +		blk_mq_put_rq_ref(req);
>  	mutex_unlock(&cmd->lock);
>  	return ret ? ERR_PTR(ret) : cmd;

After blk_mq_put_rq_ref() returns, this request may have been freed,
so the returned 'cmd' may have been freed too.

As I replied in your another thread, it is driver's responsibility to
cover race between normal completion and timeout/error handling, that
means the caller of blk_mq_tag_to_rq need to make sure that the request
represented by the passed 'tag' can't be freed.

I'd suggest to understand why nbd_read_stat()/blk_mq_tag_to_rq() may return
one freed request first, who frees the request and how when calling
blk_mq_tag_to_rq() before figuring out solutions.



Thanks,
Ming