Re: [PATCH 1/3] block/nbd: allow drain during reconnect attempt

[Eric Blake <eblake@redhat.com>]

On 7/20/20 4:00 AM, Vladimir Sementsov-Ogievskiy wrote:
> It should be to reenter qio_channel_yield() on io/channel read/write
> path, so it's safe to reduce in_flight and allow attaching new aio
> context. And no problem to allow drain itself: connection attempt is
> not a guest request. Moreover, if remote server is down, we can hang
> in negotiation, blocking drain section and provoking a dead lock.
> 
> How to reproduce the dead lock:
> 

I tried to reproduce this; but in the several minutes it has taken me to 
write this email, it still has not hung.  Still, your stack trace is 
fairly good evidence of the problem, where adding a temporary sleep or 
running it under gdb with a breakpoint can probably make reproduction 
easier.

> 1. Create nbd-fault-injector.conf with the following contents:
> 
> [inject-error "mega1"]
> event=data
> io=readwrite
> when=before
> 
> 2. In one terminal run nbd-fault-injector in a loop, like this:
> 
> n=1; while true; do
>      echo $n; ((n++));

Bashism, but not a problem for the commit message.

>      ./nbd-fault-injector.py 127.0.0.1:10000 nbd-fault-injector.conf;
> done
> 
> 3. In another terminal run qemu-io in a loop, like this:
> 
> n=1; while true; do
>      echo $n; ((n++));
>      ./qemu-io -c 'read 0 512' nbd+tcp://127.0.0.1:10000;

I prefer the spelling nbd:// for TCP connections, but also inconsequential.

> Note, that the hang may be
> triggered by another bug, so the whole case is fixed only together with
> commit "block/nbd: on shutdown terminate connection attempt".
> 
> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
> ---
>   block/nbd.c | 11 +++++++++++
>   1 file changed, 11 insertions(+)
> 
> diff --git a/block/nbd.c b/block/nbd.c
> index 65a4f56924..49254f1c3c 100644
> --- a/block/nbd.c
> +++ b/block/nbd.c
> @@ -280,7 +280,18 @@ static coroutine_fn void nbd_reconnect_attempt(BDRVNBDState *s)
>           s->ioc = NULL;
>       }
>   
> +    bdrv_dec_in_flight(s->bs);
>       s->connect_status = nbd_client_connect(s->bs, &local_err);
> +    s->wait_drained_end = true;
> +    while (s->drained) {
> +        /*
> +         * We may be entered once from nbd_client_attach_aio_context_bh
> +         * and then from nbd_client_co_drain_end. So here is a loop.
> +         */
> +        qemu_coroutine_yield();
> +    }
> +    bdrv_inc_in_flight(s->bs);
> +

This is very similar to the code in nbd_co_reconnect_loop.  Does that 
function still need to wait on drained, since it calls 
nbd_reconnect_attempt which is now doing the same loop?  But off-hand, 
I'm not seeing a problem with keeping both places.

Reviewed-by: Eric Blake <eblake@redhat.com>

As a bug fix, I'll be including this in my NBD pull request for the next 
-rc build.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org