From: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
To: Roman Kagan <rvkagan@yandex-team.ru>, qemu-devel@nongnu.org
Cc: yc-core@yandex-team.ru, Eric Blake <eblake@redhat.com>,
Max Reitz <mreitz@redhat.com>, Kevin Wolf <kwolf@redhat.com>,
qemu-block@nongnu.org
Subject: Re: [PATCH 6/7] block/nbd: decouple reconnect from drain
Date: Mon, 15 Mar 2021 23:10:14 +0300 [thread overview]
Message-ID: <995a9667-b7ae-3e00-caa5-6c80fc0962f3@virtuozzo.com> (raw)
In-Reply-To: <20210315060611.2989049-7-rvkagan@yandex-team.ru>
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
next prev parent reply other threads:[~2021-03-15 20:11 UTC|newest]
Thread overview: 30+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-03-15 6:06 [PATCH 0/7] block/nbd: decouple reconnect from drain Roman Kagan
2021-03-15 6:06 ` [PATCH 1/7] block/nbd: avoid touching freed connect_thread Roman Kagan
2021-03-15 15:40 ` Vladimir Sementsov-Ogievskiy
2021-03-16 15:29 ` Roman Kagan
2021-04-06 16:21 ` Vladimir Sementsov-Ogievskiy
2021-03-15 6:06 ` [PATCH 2/7] block/nbd: use uniformly nbd_client_connecting_wait Roman Kagan
2021-03-15 15:48 ` Vladimir Sementsov-Ogievskiy
2021-03-15 6:06 ` [PATCH 3/7] block/nbd: assert attach/detach runs in the proper context Roman Kagan
2021-03-15 16:41 ` Vladimir Sementsov-Ogievskiy
2021-03-15 19:57 ` Roman Kagan
2021-03-15 6:06 ` [PATCH 4/7] block/nbd: transfer reconnection stuff across aio_context switch Roman Kagan
2021-03-15 6:06 ` [PATCH 5/7] block/nbd: better document a case in nbd_co_establish_connection Roman Kagan
2021-03-15 6:06 ` [PATCH 6/7] block/nbd: decouple reconnect from drain Roman Kagan
2021-03-15 20:10 ` Vladimir Sementsov-Ogievskiy [this message]
2021-03-16 16:03 ` Roman Kagan
2021-03-16 18:09 ` Vladimir Sementsov-Ogievskiy
2021-03-26 6:16 ` Roman Kagan
2021-03-15 6:06 ` [PATCH 7/7] block/nbd: stop manipulating in_flight counter Roman Kagan
2021-03-15 20:15 ` Vladimir Sementsov-Ogievskiy
2021-03-16 16:08 ` Roman Kagan
2021-03-16 18:37 ` Vladimir Sementsov-Ogievskiy
2021-03-26 7:41 ` Roman Kagan
2021-03-15 19:45 ` [PATCH 0/7] block/nbd: decouple reconnect from drain Vladimir Sementsov-Ogievskiy
2021-03-16 15:52 ` Roman Kagan
2021-03-16 14:41 ` Eric Blake
2021-03-16 16:10 ` Roman Kagan
2021-03-17 8:35 ` Vladimir Sementsov-Ogievskiy
2021-03-26 8:07 ` Roman Kagan
2021-04-07 7:45 ` Roman Kagan
2021-04-07 10:13 ` Vladimir Sementsov-Ogievskiy
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