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Re: [Qemu-devel] [Nbd] NBD_CMD_DISC

From: Alex Bligh
Subject: Re: [Qemu-devel] [Nbd] NBD_CMD_DISC
Date: Tue, 12 Apr 2016 12:14:41 +0100

On 12 Apr 2016, at 11:34, Daniel P. Berrange <address@hidden> wrote:

> On Tue, Apr 12, 2016 at 10:48:20AM +0100, Daniel P. Berrange wrote:
>> On Sun, Apr 10, 2016 at 10:49:00AM +0100, Alex Bligh wrote:
>>> (Daniel: if you want to replicate the issue, just run qemu-img info
>>> against my gonbdserver with TLS. Every fifth NBD_CMD_DISC doesn't
>>> get through before the TCP session closes).
>> Hmm, I'll have a look at this - I'm not sure its caused by
>> the lack of gnutls_bye, as opposed to incorrect handling
>> of EAGAIN when the block layer sends CMD_DISC
> I have tried to reproduce with your server yet, but I did look at the
> code and identified one potential flaw that might cause the behaviour
> you mention. Can you try testing with the following change applied
> to QEMU:
> diff --git a/nbd/common.c b/nbd/common.c
> index 8ddb2dd..47757b6 100644
> --- a/nbd/common.c
> +++ b/nbd/common.c
> @@ -50,7 +50,7 @@ ssize_t nbd_wr_syncv(QIOChannel *ioc,
>                  * qio_channel_yield() that works with AIO contexts
>                  * and consider using that in this branch */
>                 qemu_coroutine_yield();
> -            } else if (done) {
> +            } else if (done || !do_read) {
>                 /* XXX this is needed by nbd_reply_ready.  */
>                 qio_channel_wait(ioc,
>                                  do_read ? G_IO_IN : G_IO_OUT);
> The current code will cause it to silently not send CMD_DISC if the socket
> returns EAGAIN on send(). This change will cause it to do a poll to wait
> and retry the send(). That said I'd be pretty surprised if we do actually
> get EAGAIN in this scenario, as I'd expect the outgoing TCP buffers to be
> empty at the point in which we close the client connection, but this fix
> is worth a try.

Well, what with pulling to the latest qemu master I now can't reproduce
the original problem against my server :-/ However, I have verified that
against my server it works with your patch (as well as without). I have
verified each time that I am receiving NBD_CMD_DISC.

It's also now working (with and without your patch) against nbd-server.c
(reference implementation) with my GnuTLS patches. This should be exactly
the same as before.

I suspect the issue may be timing related. Unless there's something else
fixed in master, the main change I've made is rebooted my MBP, which
was running very slowly.

I think you should probably still do a gnutls_bye() though.

Alex Bligh

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