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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 16:12:24 +0800
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.2.1

On 2019/1/25 下午3:12, Markus Armbruster wrote:
Jason Wang <address@hidden> writes:

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.
So the *socket* is connected (in the TCP sense),

UNIX domain socket actually in the case of this test.

but something else
(whatever owns is_connected) is not.  Can you point me to where
is_connected is set to true?

Sorry, should be "connected". It was set in tcp_chr_connect(). So if filter want to send a packet to socket chardev before tcp_chr_connect() is called, the packet will be dropped silently by tcp_chr_write(). This will fail this unit-test.

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.
We should really, really, really improve the test harness to run each
test program in its very own temporary directory.  Then tests can simply
create files with fixed names, and leave cleanup to the test harness.

Agree, but for this test, since passing fd works now. I tend to using socketpair().

   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().
Now I neglected to provide enough context for you :)

I read

     recv_sock = unix_connect(sock_path, NULL);

and immediately went "why are errors ignored".  If I had read on (as I
should've), I would've seen the are not:

     g_assert_cmpint(recv_sock, !=, -1);

Sorry for the noise.

I'd replace both lines by

     recv_sock = unix_connect(sock_path, &error_abort);

Reports the actual error, which is an obvious improvement, with the
location pointing to the failing spot within unix_connect().  To find
where unix_connect() was called, you need to examine the stack
backtrace.  Strictly more information, but your actual mileage may vary.

I see.


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