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