Re: [PATCH 6/7] block/nbd: decouple reconnect from drain

[Vladimir Sementsov-Ogievskiy]

15.03.2021 09:06, Roman Kagan wrote:
> The reconnection logic doesn't need to stop while in a drained section.
> Moreover it has to be active during the drained section, as the requests
> that were caught in-flight with the connection to the server broken can
> only usefully get drained if the connection is restored.  Otherwise such
> requests can only either stall resulting in a deadlock (before
> 8c517de24a), or be aborted defeating the purpose of the reconnection
> machinery (after 8c517de24a).
>
> Since the pieces of the reconnection logic are now properly migrated
> from one aio_context to another, it appears safe to just stop messing
> with the drained section in the reconnection code.
>
> Fixes: 5ad81b4946 ("nbd: Restrict connection_co reentrance")

I'd not think that it "fixes" it. Behavior changes.. But 5ad81b4946 didn't 
introduce any bugs.

> Fixes: 8c517de24a ("block/nbd: fix drain dead-lock because of nbd 
> reconnect-delay")

And here..

1. There is an existing problem (unrelated to nbd) in Qemu that long io request 
which we have to wait for at drained_begin may trigger a dead lock 
(https://lists.gnu.org/archive/html/qemu-devel/2020-09/msg01339.html)

2. So, when we have nbd reconnect (and therefore long io requests) we simply 
trigger this deadlock.. That's why I decided to cancel the requests (assuming 
they will most probably fail anyway).

I agree that nbd driver is wrong place for fixing the problem described in 
(https://lists.gnu.org/archive/html/qemu-devel/2020-09/msg01339.html), but if 
you just revert 8c517de24a, you'll see the deadlock again..



--
Best regards,
Vladimir