Re: [PATCH v5 5/6] nbd: convert to use blk_mq_find_and_get_req()

[yukuai (C)]

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