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 <rjo...@redhat.com> > Tested-by: Richard W. M. Jones <rjo...@redhat.com> > Signed-off-by: Stefan Hajnoczi <stefa...@redhat.com> > --- > 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. Paolo