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

[Josef Bacik <josef@toxicpanda.com>]

On 3/3/20 8:08 AM, 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.
> 

This won't work, you have to have the request started if you return an error 
because of this in blk_mq_dispatch_rq_list

                 if (unlikely(ret != BLK_STS_OK)) {
                         errors++;
                         blk_mq_end_request(rq, BLK_STS_IOERR);
                         continue;
                 }

The request has to be started before we return an error, pushing it down means 
we have all of these error cases where we haven't started the request.  Thanks,

Josef