Re: [PATCH] nbd: make starting request more reasonable

[Yufen Yu <yuyufen@huawei.com>]

Ping and Cc to more expert in blk-mq.

On 2020/3/3 21:08, Yufen Yu wrote:
> Our test robot reported a warning for refcount_dec trying to decrease
> value '0'. The reason is that blk_mq_dispatch_rq_list() try to complete
> the failed request from nbd driver, while the request have finished in
> nbd timeout handle function. The race as following:
> 
> CPU1                             CPU2
> 
> //req->ref = 1
> blk_mq_dispatch_rq_list
> nbd_queue_rq
>    nbd_handle_cmd
>      blk_mq_start_request
>                                   blk_mq_check_expired
>                                     //req->ref = 2
>                                     blk_mq_rq_timed_out
>                                       nbd_xmit_timeout
>                                         blk_mq_complete_request
>                                           //req->ref = 1
>                                           refcount_dec_and_test(&req->ref)
> 
>                                     refcount_dec_and_test(&req->ref)
>                                     //req->ref = 0
>                                       __blk_mq_free_request(req)
>    ret = BLK_STS_IOERR
> blk_mq_end_request
> // req->ref = 0, req have been free
> refcount_dec_and_test(&rq->ref)
> 
> In fact, the bug also have been reported by syzbot:
>    https://lkml.org/lkml/2018/12/5/1308
> 
> Since the request have been freed by timeout handle, it can be reused
> by others. Then, blk_mq_end_request() may get the re-initialized request
> and free it, which is unexpected.
> 
> To fix the problem, we move blk_mq_start_request() down until the driver
> will handle the request actully. If .queue_rq return something error in
> preparation phase, timeout handle may don't need. Thus, moving start
> request down may be more reasonable. Then, nbd_queue_rq() will not return
> BLK_STS_IOERR after starting request.
> 
> Reported-by: Hulk Robot <hulkci@huawei.com>
> Signed-off-by: Yufen Yu <yuyufen@huawei.com>
> ---
>   drivers/block/nbd.c | 6 ++----
>   1 file changed, 2 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
> index 78181908f0df..5256e9d02a03 100644
> --- a/drivers/block/nbd.c
> +++ b/drivers/block/nbd.c
> @@ -541,6 +541,8 @@ static int nbd_send_cmd(struct nbd_device *nbd, struct nbd_cmd *cmd, int index)
>   		return -EIO;
>   	}
>   
> +	blk_mq_start_request(req);
> +
>   	if (req->cmd_flags & REQ_FUA)
>   		nbd_cmd_flags |= NBD_CMD_FLAG_FUA;
>   
> @@ -879,7 +881,6 @@ static int nbd_handle_cmd(struct nbd_cmd *cmd, int index)
>   	if (!refcount_inc_not_zero(&nbd->config_refs)) {
>   		dev_err_ratelimited(disk_to_dev(nbd->disk),
>   				    "Socks array is empty\n");
> -		blk_mq_start_request(req);
>   		return -EINVAL;
>   	}
>   	config = nbd->config;
> @@ -888,7 +889,6 @@ static int nbd_handle_cmd(struct nbd_cmd *cmd, int index)
>   		dev_err_ratelimited(disk_to_dev(nbd->disk),
>   				    "Attempted send on invalid socket\n");
>   		nbd_config_put(nbd);
> -		blk_mq_start_request(req);
>   		return -EINVAL;
>   	}
>   	cmd->status = BLK_STS_OK;
> @@ -912,7 +912,6 @@ static int nbd_handle_cmd(struct nbd_cmd *cmd, int index)
>   			 */
>   			sock_shutdown(nbd);
>   			nbd_config_put(nbd);
> -			blk_mq_start_request(req);
>   			return -EIO;
>   		}
>   		goto again;
> @@ -923,7 +922,6 @@ static int nbd_handle_cmd(struct nbd_cmd *cmd, int index)
>   	 * here so that it gets put _after_ the request that is already on the
>   	 * dispatch list.
>   	 */
> -	blk_mq_start_request(req);
>   	if (unlikely(nsock->pending && nsock->pending != req)) {
>   		nbd_requeue_cmd(cmd);
>   		ret = 0;
>