Xorg hangs use 99% of CPU on systemd reboot

[Peter Hutterer]

On Mon, Jul 13, 2015 at 05:18:54PM +0100, Barry Scott wrote:
> On Fri 03 Jul 2015 16:14:13 Peter Hutterer wrote:
> > On Thu, Jul 02, 2015 at 04:26:19PM +0100, Barry Scott wrote:
> > > The attached patch works around an issue we hit after we
> > > ported from Fedora 20 to Fedora 21.
> > > 
> > > When systemd reboot starts something happens to dbus that
> > > causes Xorg to loop hard using 99% CPU in WaitForSomething.
> > > 
> > > The reason for the loop is that the bus_info.fd in dbus-core.c
> > > has been closed. But the expected Disconnected signal is not
> > > received by the messages filter.
> > 
> > wouldn't it be better to set up the filter so we get the disconnect signal?
> 
> The dbus ping example does not add a filter to see Disconnected.
> https://github.com/sgh/dbus-examples/blob/master/dbus-ping-listen.c

looking at config/dbus-core.c, there's message_filter which has the handling
for the disconnect signal. and the dbus doc says it's always sent. so the
question is now: did you put printfs into message_filter() to verify that
the signal never arrives?

I also notice that message_filter partially shuts down stuff, but it leaves
bus_info.fd in the select set even though as I understand this is closed
now. so it seem the RemoveGeneralSocket() and info->fd = -1; call should go
into message_filter.

Cheers,
   Peter

> > > The patch works around this issue by removing the fd from the
> > > fd_set that is feed into the Select in WaitForSomething by
> > > calling RemoveGeneralSocket from within wakeup_handler in
> > > dbus-core.c.
> > > 
> > > When systemctl is used to stop and restart Xorg outside of the
> > > reboot it works without any problems.
> > > 
> > > Barry
> > > 
> > > 
> > > From ca770dd6e8ebb9a780ce68a07535f976f6120247 Mon Sep 17 00:00:00 2001
> > > From: Barry Scott <barry.scott at onelan.co.uk>
> > > Date: Tue, 30 Jun 2015 15:58:22 +0100
> > > Subject: [PATCH] server loops in WaitForSomething when dbus closed by
> > > systemd> 
> > >  shutdown
> > > 
> > > Work around issue with dbus fd becoming closed by WaitForSomething
> > > still using it in Select. The Select return EBADF but becuase the dbus fd
> > > is not taken out of the fs_set the code loop forever.
> > > 
> > > This state has been seen when systemd starts to shutdown on Fedora 21.
> > > 
> > > Signed-off-by: Barry Scott <barry.scott at onelan.co.uk>
> > > ---
> > > 
> > >  config/dbus-core.c | 13 +++++++++++++
> > >  1 file changed, 13 insertions(+)
> > > 
> > > diff --git a/config/dbus-core.c b/config/dbus-core.c
> > > index 8351ea4..fac8116 100644
> > > --- a/config/dbus-core.c
> > > +++ b/config/dbus-core.c
> > > @@ -52,6 +52,19 @@ wakeup_handler(void *data, int num_fds, void
> > > *read_mask)
> > > 
> > >  {
> > >  
> > >      struct dbus_core_info *info = data;
> > > 
> > > +    if (info->fd < 0) {
> > > +        return;
> > > +    }
> > > +
> > > +    if (!dbus_connection_get_is_connected(info->connection)) {
> > > +        /* must remove the FD here as WaitForSomething's Select will now
> > > fail +           and WaitForSomething will loop hard calling Select.
> > > +           The "Disconnected" message has been seen to not arrive. */
> > > +        RemoveGeneralSocket(info->fd);
> > > +        info->fd = -1;
> > > +        return;
> > > +    }
> > > +
> > > 
> > >      if (info->connection && num_fds > 0 && FD_ISSET(info->fd, (fd_set *)
> > >      read_mask)) {>      
> > >          do {
> > >          
> > >              dbus_connection_read_write_dispatch(info->connection, 0);
> > > 
> > > _______________________________________________
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>