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

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

On Mon, Mar 16, 2020 at 08:26:35PM +0800, Yufen Yu wrote:
> 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

This shouldn't happen in reality, given rq->deadline is just updated
in blk_mq_start_request(), suppose you use the default 30 sec timeout.
How can the race be triggered in so short time?

Could you explain a bit your test case?

> >                                         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);

I think it is fine to not start request in case of failure, given 
__blk_mq_end_request() doesn't check rq's state.



Thanks,
Ming