Re: [PATCH 1/3] graph-lock: Disable locking for now

[Eric Blake]

On Wed, May 17, 2023 at 05:28:32PM +0200, Kevin Wolf wrote:
> In QEMU 8.0, we've been seeing deadlocks in bdrv_graph_wrlock(). They
> come from callers that hold an AioContext lock, which is not allowed
> during polling. In theory, we could temporarily release the lock, but
> callers are inconsistent about whether they hold a lock, and if they do,
> some are also confused about which one they hold. While all of this is
> fixable, it's not trivial, and the best course of action for 8.0.1 is
> probably just disabling the graph locking code temporarily.
> 
> We don't currently rely on graph locking yet. It is supposed to replace
> the AioContext lock eventually to enable multiqueue support, but as long
> as we still have the AioContext lock, it is sufficient without the graph
> lock. Once the AioContext lock goes away, the deadlock doesn't exist any
> more either and this commit can be reverted. (Of course, it can also be
> reverted while the AioContext lock still exists if the callers have been
> fixed.)

Makes sense - certainly easier than finding all the lurking bugs,
while we still have the AioContext lock protection.

> 
> Cc: qemu-stable@nongnu.org
> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
> ---
>  block/graph-lock.c | 21 +++++++++++++++++++++
>  1 file changed, 21 insertions(+)

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

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
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