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Re: [Qemu-devel] test-filter-mirror hangs

From: Jason Wang
Subject: Re: [Qemu-devel] test-filter-mirror hangs
Date: Fri, 25 Jan 2019 11:56:18 +0800
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.2.1

On 2019/1/24 下午5:47, Markus Armbruster wrote:
Please cc: me on QMP issues.


Jason Wang <address@hidden> writes:

On 2019/1/24 上午3:53, Dr. David Alan Gilbert wrote:
* Jason Wang (address@hidden) wrote:
On 2019/1/22 上午2:56, Peter Maydell wrote:
On Thu, 17 Jan 2019 at 09:46, Jason Wang<address@hidden>  wrote:
On 2019/1/15 上午12:33, Zhang Chen wrote:
On Sat, Jan 12, 2019 at 12:15 AM Dr. David Alan Gilbert
<address@hidden  <mailto:address@hidden>> wrote:

       * Peter Maydell (address@hidden
       <mailto:address@hidden>) wrote:
       > Recently I've noticed that test-filter-mirror has been hanging
       > intermittently, typically when run on some other TCG architecture.
       > In the instance I've just looked at, this was with s390x guest on
       > x86-64 host, though I've also seen it on other host archs and
       > perhaps with other guests.

       Watch out to see if you really do see it for other guests;
       it carefully avoids using virtio-net to avoid vhost; but on s390x it
       uses virtio-net-ccw - could that hit the vhost it was trying to avoid?

       > Below is a backtrace, though it seems to be pretty unhelpful.
       > Anybody got any theories ? Does the mirror test rely on dirty
       > memory bitmaps like the migration test (which also hangs
       > occasionally with TCG due to some bug I'm sure we've investigated
       > in the past) ?

       I don't think it relies on the CPU at all.
    I have no idea about this currently, but Jason and I designed the
test case.
Add Jason: Have any comments about this ?
I can't reproduce this locally with s390x-softmmu. It looks to me the
test should be independent to any kinds of emulation. It should pass
when mainloop work.
I've just seen a hang with ppc64 guest on s390x host, so it is
indeed not specific to s390x guest (and so not specific to
virtio-net either, since the ppc64 guest setup uses e1000).

-- PMM
Finally reproduced locally after hundreds (sometimes thousands) times of

Bisection points to OOB monitor[1].

It looks to me after OOB is used unconditionally we lose a barrier to make
sure socket is connected before sending packets in test-filter-mirror.c. Is
there any other similar and simple thing that we could do to kick the
Do you mean the:

      /* send a qmp command to guarantee that 'connected' is setting to true. */
      qmp_discard_response(qts, "{ 'execute' : 'query-status'}");


why was that ever sufficient to know the socket was ready?

It was suggested by Fam, I don't remember the details. Can we make
sure all pending events has been processed (UNIX socket was set to
connected) after query-status is returned with an non OOB monitor?
I'm afraid I lack context.  Which socket are you talking about?  The
test has at least the QMP socket, the send_sock[], and recv_sock.  What
exactly are you trying to accomplish?

I mean recv_sock. If mirror tries to send a packet to it before its is_connected is set to true, packet will be dropped.

By the way, mkstemp(sock_path) followed by unix_connect(sock_path, NULL)
looks rather fishy.  Why create a temporary file only to create a Unix
domain socket right over it?

I vaguely remember passing fd created by unix domain socket doesn't work when the test is introduced. So my understanding is the author needs a way to create a unique file name which will be used b Unix domain socket at that time.

  Why is ignoring errors a good idea?

I don't get, which error is missed, it checks the return value of both mkstemp() and unix_connect().


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