Re: [PATCH v5 2/6] nbd: make sure request completion won't concurrent

[Ming Lei]

On Thu, Sep 09, 2021 at 10:12:52PM +0800, Yu Kuai wrote:
> commit cddce0116058 ("nbd: Aovid double completion of a request")
> try to fix that nbd_clear_que() and recv_work() can complete a
> request concurrently. However, the problem still exists:
> 
> t1                    t2                     t3
> 
> nbd_disconnect_and_put
>  flush_workqueue
>                       recv_work
>                        blk_mq_complete_request
>                         blk_mq_complete_request_remote -> this is true
>                          WRITE_ONCE(rq->state, MQ_RQ_COMPLETE)
>                           blk_mq_raise_softirq
>                                              blk_done_softirq
>                                               blk_complete_reqs
>                                                nbd_complete_rq
>                                                 blk_mq_end_request
>                                                  blk_mq_free_request
>                                                   WRITE_ONCE(rq->state, MQ_RQ_IDLE)
>   nbd_clear_que
>    blk_mq_tagset_busy_iter
>     nbd_clear_req
>                                                    __blk_mq_free_request
>                                                     blk_mq_put_tag
>      blk_mq_complete_request -> complete again
> 
> There are three places where request can be completed in nbd:
> recv_work(), nbd_clear_que() and nbd_xmit_timeout(). Since they
> all hold cmd->lock before completing the request, it's easy to
> avoid the problem by setting and checking a cmd flag.
> 
> Signed-off-by: Yu Kuai <yukuai3@xxxxxxxxxx>
> ---
>  drivers/block/nbd.c | 11 +++++++++--
>  1 file changed, 9 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
> index 04861b585b62..550c8dc438ac 100644
> --- a/drivers/block/nbd.c
> +++ b/drivers/block/nbd.c
> @@ -406,7 +406,11 @@ static enum blk_eh_timer_return nbd_xmit_timeout(struct request *req,
>  	if (!mutex_trylock(&cmd->lock))
>  		return BLK_EH_RESET_TIMER;
>  
> -	__clear_bit(NBD_CMD_INFLIGHT, &cmd->flags);
> +	if (!__test_and_clear_bit(NBD_CMD_INFLIGHT, &cmd->flags)) {
> +		mutex_unlock(&cmd->lock);
> +		return BLK_EH_DONE;
> +	}
> +
>  	if (!refcount_inc_not_zero(&nbd->config_refs)) {
>  		cmd->status = BLK_STS_TIMEOUT;
>  		mutex_unlock(&cmd->lock);
> @@ -842,7 +846,10 @@ static bool nbd_clear_req(struct request *req, void *data, bool reserved)
>  
>  	mutex_lock(&cmd->lock);
>  	cmd->status = BLK_STS_IOERR;
> -	__clear_bit(NBD_CMD_INFLIGHT, &cmd->flags);
> +	if (!__test_and_clear_bit(NBD_CMD_INFLIGHT, &cmd->flags)) {
> +		mutex_unlock(&cmd->lock);
> +		return true;
> +	}
>  	mutex_unlock(&cmd->lock);

If this request has completed from other code paths, ->status shouldn't be
updated here, maybe it is done successfully.

-- 
Ming