Re: [Qemu-devel] [PATCH] nbd-client: avoid spurious qio_channel_yield() re-entry

[Eric Blake <eblake@redhat.com>]

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On 08/22/2017 07:51 AM, Stefan Hajnoczi wrote:
> The following scenario leads to an assertion failure in
> qio_channel_yield():
> 
> 1. Request coroutine calls qio_channel_yield() successfully when sending
>    would block on the socket.  It is now yielded.
> 2. nbd_read_reply_entry() calls nbd_recv_coroutines_enter_all() because
>    nbd_receive_reply() failed.
> 3. Request coroutine is entered and returns from qio_channel_yield().
>    Note that the socket fd handler has not fired yet so
>    ioc->write_coroutine is still set.
> 4. Request coroutine attempts to send the request body with nbd_rwv()
>    but the socket would still block.  qio_channel_yield() is called
>    again and assert(!ioc->write_coroutine) is hit.
> 
> The problem is that nbd_read_reply_entry() does not distinguish between
> request coroutines that are waiting to receive a reply and those that
> are not.
> 
> This patch adds a per-request bool receiving flag so
> nbd_read_reply_entry() can avoid spurious aio_wake() calls.
> 
> Reported-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

Using the steps in
https://lists.gnu.org/archive/html/qemu-devel/2017-08/msg03853.html,
I've verified that this avoids the hang that is otherwise present, so
I'm adding:

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

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org


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