Re: [Qemu-devel] [PATCH] char: kick main loop after adding a watch

[Stefan Hajnoczi]

On Fri, Mar 31, 2017 at 06:53:56PM +0200, Paolo Bonzini wrote:
> 
> 
> On 31/03/2017 18:43, Stefan Hajnoczi wrote:
> > The ISA serial port device's output can hang when the pipe on stdout
> > becomes full.  This is a race condition where the vcpu thread executing
> > serial emulation code adds a watch on stdout while the main loop thread
> > is blocked in ppoll(2).  If no timer or other event wakes up the main
> > loop, there will be no further output from the serial device even when
> > the pipe becomes writable.
> > 
> > Richard W. M. Jones was able to reproduce the hang on recent versions of
> > guestfs-tools-c and libglib2 on Fedora 26 hosts.
> > 
> > This patch kicks the main loop so the next iteration invokes ppoll(2)
> > with the watch fd.
> > 
> > Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=1435432
> > Reported-by: Richard W. M. Jones <address@hidden>
> > Tested-by: Richard W. M. Jones <address@hidden>
> > Signed-off-by: Stefan Hajnoczi <address@hidden>
> > ---
> >  chardev/char.c | 5 +++++
> >  1 file changed, 5 insertions(+)
> > 
> > diff --git a/chardev/char.c b/chardev/char.c
> > index 3df1163..6c99c34 100644
> > --- a/chardev/char.c
> > +++ b/chardev/char.c
> > @@ -1059,6 +1059,11 @@ guint qemu_chr_fe_add_watch(CharBackend *be, 
> > GIOCondition cond,
> >      tag = g_source_attach(src, NULL);
> >      g_source_unref(src);
> >  
> > +    /* The main loop may be in blocked waiting on events in another thread.
> > +     * Kick it so the new watch will be added.
> > +     */
> > +    qemu_notify_event();
> > +
> >      return tag;
> >  }
> >  
> > 
> 
> Thanks for looking at this, I was quite stuck and now I understand 
> what's going on.  However, I don't believe your patch is the right 
> solution.
> 
> According to Richard's bisection, the bug was introduced by the patch 
> at https://bug761102.bugzilla-attachments.gnome.org/attachment.cgi?id=319699.
> 
> The g_wakeup_signal that is removed (actually made conditional) in that 
> patch is doing exactly the same thing as qemu_notify_event, which is
> fishy...  It would still be a QEMU bug according to the theory below but,
> depending on how they handle backwards-compatibility, they might 
> consider undoing this change.
> 
> glib is expecting QEMU to use g_main_context_acquire around accesses to 
> GMainContext.  However QEMU is not doing that, instead it is taking its 
> own mutex.  So we should add g_main_context_acquire and
> g_main_context_release in the two implementations of 
> os_host_main_loop_wait; these should undo the effect of Frediano's 
> glib patch.
> 
> In all fairness, the docs do say "You must be the owner of a context 
> before you can call g_main_context_prepare(), g_main_context_query(), 
> g_main_context_check(), g_main_context_dispatch()".  However, it has
> worked until now and the documentation does not say exactly why that
> is necessary.

Thanks Paolo, very interesting.  I never realized that glib has its own
inter-thread signal.

NACK

Stefan

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