[Spice-devel] help debugging: Assertion `!xcb_xlib_threads_sequence_lost' failed.

[Daniel P. Berrangé]

On Wed, Jan 29, 2020 at 04:19:05AM -0500, Frediano Ziglio wrote:
> > 
> > On Thu, Jan 23, 2020 at 11:54:50AM -0500, Cole Robinson wrote:
> > > Hi all,
> > > 
> > > For the past year+ we have had a steady trickle of virt-manager X11
> > > threading crashes. This is the f31 bug I've duped all the other ones
> > > too, but there were a handful that were auto closed by Fedora
> > > end-of-life bug cleanup:
> > > https://bugzilla.redhat.com/show_bug.cgi?id=1756065
> > > 
> > > The message that goes along with this is:
> > > 
> > > [xcb] Unknown sequence number while processing queue
> > > [xcb] Most likely this is a multi-threaded client and XInitThreads has
> > > not been called
> > > [xcb] Aborting, sorry about that.
> > > remote-viewer: xcb_io.c:263: poll_for_event: Assertion
> > > `!xcb_xlib_threads_sequence_lost' failed.
> > > Aborted (core dumped)
> > > 
> > > That may not apply to all the poll_for_error() bugs but it's most of the
> > > ones I have dug into.
> > > 
> > > I figured there was some threading issue in virt-manager and I was
> > > mostly ignoring the bugs until I found time to try and diagnose. I'm not
> > > really finding anything in virt-manager though, and some similar issues
> > > make me think maybe this is spice-gtk or something lower.
> > > 
> > > This user can reproduce the same error with remote-viewer + spice + X11
> > > ssh forwarding: https://bugzilla.redhat.com/show_bug.cgi?id=1758384
> > > 
> > > This user can reproduce the virt-manager crash multiple times a day, and
> > > it's always when interacting with a spice-gtk console:
> > > https://bugzilla.redhat.com/show_bug.cgi?id=1792576
> > > 
> > > Anyone have any ideas how to debug this further? What spice-gtk
> > > debugging info would be helpful?
> > 
> > FWIW if you're trying to reproduce this yourself, the key factor here
> > is that they are using X11, not Wayland. Given that Fedora defaults
> > to Wayland this likely explains why only certain people are seeing
> > it.
> > 
> > The error message strongly suggests that there are multiple threads
> > making X11 calls at the same time. This in turn means multiple threads
> > are making GTK calls at the same time, and this is expressly forbidden
> > by GTK - GTK APIs are only permitted to be invoked from the main thread
> > which runs the event loop.
> > 
> > It does look like a probable spice issue given the common crash in
> > remote-viewer, as aside from GTK itself and GTK-VNC, that's the only
> > UI library both apps use.
> > 
> > Regards,
> > Daniel
> 
> Hi,
>   found the root cause, a bug in libX11, see updates to
> https://bugzilla.redhat.com/show_bug.cgi?id=1758384.
> As it's not specifically related to virt-manager/virt-viewer which
> Product/Component should the bug moved to?

This is an upstream reported bug, so moving to Fedora/libX11 is a good
first choice.  I'd probably *also* clone it to RHEL-8/libX11 as that
will increase the likelihood of someone caring enough to fix it.

Perhaps also link both BZ to a upstream libX11 bug tracker ?

Regards,
Daniel
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