Re: [Qemu-devel] [PATCH v2] monitor: work around delayed CHR_EVENT_OPENED events

[mdroth]

On Wed, May 29, 2013 at 01:27:33PM -0400, Luiz Capitulino wrote:
> On Mon, 27 May 2013 12:59:25 -0500
> mdroth <address@hidden> wrote:
> 
> > On Mon, May 27, 2013 at 11:16:01AM -0500, Anthony Liguori wrote:
> > > Luiz Capitulino <address@hidden> writes:
> > > 
> > > > On Sun, 26 May 2013 10:33:39 -0500
> > > > Michael Roth <address@hidden> wrote:
> > > >
> > > >> In the past, CHR_EVENT_OPENED events were emitted via a pre-expired
> > > >> QEMUTimer. Due to timers being processing at the tail end of each main
> > > >> loop iteration, this generally meant that such events would be emitted
> > > >> within the same main loop iteration, prior any client data being read
> > > >> by tcp/unix socket server backends.
> > > >> 
> > > >> With 9f939df955a4152aad69a19a77e0898631bb2c18, this was switched to
> > > >> a "bottom-half" that is registered via g_idle_add(). This makes it
> > > >> likely that the event won't be sent until a subsequent iteration, and
> > > >> also adds the possibility that such events will race with the
> > > >> processing of client data.
> > > >> 
> > > >> In cases where we rely on the CHR_EVENT_OPENED event being delivered
> > > >> prior to any client data being read, this may cause unexpected 
> > > >> behavior.
> > > >> 
> > > >> In the case of a QMP monitor session using a socket backend, the 
> > > >> delayed
> > > >> processing of the CHR_EVENT_OPENED event can lead to a situation where
> > > >> a previous session, where 'qmp_capabilities' has already processed, can
> > > >> cause the rejection of 'qmp_capabilities' for a subsequent session,
> > > >> since we reset capabilities in response to CHR_EVENT_OPENED, which may
> > > >> not yet have been delivered. This can be reproduced with the following
> > > >> command, generally within 50 or so iterations:
> > > >> 
> > > >>   address@hidden:~$ cat cmds
> > > >>   {'execute':'qmp_capabilities'}
> > > >>   {'execute':'query-cpus'}
> > > >>   address@hidden:~$ while true; do if socat stdio 
> > > >> unix-connect:/tmp/qmp.sock
> > > >>   <cmds | grep CommandNotFound &>/dev/null; then echo failed; break; 
> > > >> else
> > > >>   echo ok; fi; done;
> > > >>   ok
> > > >>   ok
> > > >>   failed
> > > >>   address@hidden:~$
> > > >> 
> > > >> As a work-around, we reset capabilities as part of the CHR_EVENT_CLOSED
> > > >> hook, which gets called as part of the GIOChannel cb associated with 
> > > >> the
> > > >> client rather than a bottom-half, and is thus guaranteed to be 
> > > >> delivered
> > > >> prior to accepting any subsequent client sessions.
> > > >> 
> > > >> This does not address the more general problem of whether or not there
> > > >> are chardev users that rely on CHR_EVENT_OPENED being delivered prior 
> > > >> to
> > > >> client data, and whether or not we should consider moving 
> > > >> CHR_EVENT_OPENED
> > > >> "in-band" with connection establishment as a general solution, but 
> > > >> fixes
> > > >> QMP for the time being.
> > > >> 
> > > >> Reported-by: Stefan Priebe <address@hidden>
> > > >> Cc: address@hidden
> > > >> Signed-off-by: Michael Roth <address@hidden>
> > > >
> > > > Thanks for debugging this Michael. I'm going to apply your patch to the 
> > > > qmp
> > > > branch because we can't let this broken (esp. in -stable) but this is 
> > > > papering
> > > > over the real bug...
> > > 
> > > Do we really need OPENED to happen in a bottom half?  Shouldn't the open
> > > event handlers register the callback instead if they need it?
> > 
> > I think that's the more general fix, but wasn't sure if there were
> > historical reasons why we didn't do this in the first place.
> > 
> > Looking at it more closely it does seem like we need a general fix
> > sooner rather than later though, and I don't see any reason why we can't
> > move BH registration into the actual OPENED hooks as you suggest:
> > 
> > hw/usb/ccid-card-passthru.c
> >  - currently affected? no
> >     debug hook only
> >  - needs OPENED BH? no
> > 
> > hw/usb/redirect.c
> >  - currently affected? yes
> >     can_read handler checks for dev->parser != NULL, which may be
> >     true if CLOSED BH has been executed yet. In the past, OPENED
> >     quiesced outstanding CLOSED events prior to us reading client data.
> >  - need OPENED BH? possibly
> >     seems to only be needed by CLOSED events, which can be issued by
> >     USBDevice, so we do teardown/usb_detach in BH. For OPENED, this
> >     may not apply. if we do issue a BH, we'd need to modify can_read
> >     handler to avoid race.
> > 
> > hw/usb/dev-serial.c
> >  - currently affected? no
> >     can_read checks for dev.attached != NULL. set to NULL
> >     synchronously in CLOSED, will not accept reads until OPENED gets
> >     called and sets dev.attached
> >  - need opened BH?  no
> > 
> > hw/char/sclpconsole.c
> >  - currently affected? no
> >     guarded by can_read handler
> >  - need opened BH? no
> > 
> > hw/char/ipoctal232.c
> >  - currently affected? no
> >     debug hook only
> >  - need opened BH? no
> > 
> > hw/char/virtio-console.c
> >  - currently affected? no
> >     OPENED/CLOSED map to vq events handled asynchronously. can_read
> >     checks for guest_connected state rather than anything set by OPENED
> >  - need opened BH? no
> > 
> > qtest.c
> >  - currently affected? yes
> >     can_read handler doesn't check for qtest_opened == true, can read
> >     data before OPENED event is processed
> >  - need opened BH? no
> > 
> > monitor.c
> >  - current affected? yes
> >     negotiated session state can temporarilly leak into a new session
> >  - need opened BH? no
> > 
> > gdbstub.c
> >  - currently affected? yes
> >     can fail to pause machine prior to starting gdb session
> >  - need opened BH? no
> > 
> > So we have a number of cases that can be fixed by avoiding the use of
> > the generic BH, and only 1 possible cause where we might need a
> > device-specific BH.
> > 
> > At first glance anyway. So if this all seems reasonable I can send a
> > more general fix shortly.
> 
> Michael, I've dropped your first patch and am taking it that you're
> going to cook this more general fix.

fix sent:

http://article.gmane.org/gmane.comp.emulators.qemu/213863

> 
> > 
> > > 
> > > Regards,
> > > 
> > > Anthony Liguori
> > > 
> > > >
> > > >> ---
> > > >> v1->v2:
> > > >> * remove command_mode reset from CHR_EVENT_OPENED case, since this
> > > >>   might still cause a race
> > > >> 
> > > >>  monitor.c |    2 +-
> > > >>  1 file changed, 1 insertion(+), 1 deletion(-)
> > > >> 
> > > >> diff --git a/monitor.c b/monitor.c
> > > >> index 6ce2a4e..f1953a0 100644
> > > >> --- a/monitor.c
> > > >> +++ b/monitor.c
> > > >> @@ -4649,7 +4649,6 @@ static void monitor_control_event(void *opaque, 
> > > >> int event)
> > > >>  
> > > >>      switch (event) {
> > > >>      case CHR_EVENT_OPENED:
> > > >> -        mon->mc->command_mode = 0;
> > > >>          data = get_qmp_greeting();
> > > >>          monitor_json_emitter(mon, data);
> > > >>          qobject_decref(data);
> > > >> @@ -4660,6 +4659,7 @@ static void monitor_control_event(void *opaque, 
> > > >> int event)
> > > >>          json_message_parser_init(&mon->mc->parser, 
> > > >> handle_qmp_command);
> > > >>          mon_refcount--;
> > > >>          monitor_fdsets_cleanup();
> > > >> +        mon->mc->command_mode = 0;
> > > >>          break;
> > > >>      }
> > > >>  }
> > > 
> > 
>