Re: [PATCH] nbd: fail requests stuck on a dead socket when timeout is 0

[Bart Van Assche]

On 10/10/26 8:37 PM, Nguyen Ngoc Thang wrote:
> With a zero command timeout, nbd_xmit_timeout() only logs a "possible
> stuck request" message and resets the timer. A request already sent on
> a socket that has since died is never completed this way, because
> nothing will ever answer it.
>
> That blocks nbd_set_size(), which freezes the queue while holding
> nbd->config_lock and genl_mutex. The freeze waits for the stuck request
> forever, and reconnecting a socket (which would change the cookie and
> get the request requeued) can't happen as it is handled later by the
> same reconfigure call. Everything touching the device then hangs,
> including partition scanning in udev:
>
>    INFO: task udevd blocked for more than 143 seconds.
>     __bio_queue_enter+0x369/0x7f0 block/blk-core.c:396
>     blk_mq_submit_bio+0x776/0x2a70 block/blk-mq.c:3127
>     ...
>    INFO: task syz.0.17 blocked for more than 143 seconds.
>     blk_mq_freeze_queue_wait+0x101/0x180 block/blk-mq.c:191
>     queue_limits_commit_update_frozen+0x55/0xd0 block/blk-settings.c:590
>     nbd_set_size+0x4c5/0x7d0 drivers/block/nbd.c:383
>     nbd_genl_reconfigure+0x7e1/0x1040 drivers/block/nbd.c:2444
>
> Requeue the request when its socket is dead, as is already done when
> the socket was replaced. The submit path then waits for a reconnect up
> to dead_conn_timeout and otherwise fails the request with an I/O error.
>
> Reported-by: syzbot+90895f5bfaa7243c2d20@xxxxxxxxxxxxxxxxxxxxxxxxx
> Closes: https://syzkaller.appspot.com/bug?extid=90895f5bfaa7243c2d20
> Fixes: 2da22da57348 ("nbd: fix zero cmd timeout handling v2")
> Signed-off-by: Nguyen Ngoc Thang <ngocthang2710.1999@xxxxxxxxx>
> ---
>   drivers/block/nbd.c | 3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
> index ffce519bf008..218b99f9e1ef 100644
> --- a/drivers/block/nbd.c
> +++ b/drivers/block/nbd.c
> @@ -532,7 +532,8 @@ static enum blk_eh_timer_return nbd_xmit_timeout(struct request *req)
>   			blk_rq_bytes(req), (req->timeout / HZ) * cmd->retries);
>   
>   		mutex_lock(&nsock->tx_lock);
> -		if (cmd->cookie != nsock->cookie) {
> +		/* A dead socket will never complete this request. */
> +		if (cmd->cookie != nsock->cookie || nsock->dead) {
>   			nbd_requeue_cmd(cmd);
>   			mutex_unlock(&nsock->tx_lock);
>   			mutex_unlock(&cmd->lock);

If AI was used to create this patch an Assisted-by tag should be added.
See also Documentation/process/coding-assistants.rst. Anyway, since this
patch looks good to me:

Reviewed-by: Bart Van Assche <bvanassche@xxxxxxx>