Re: [Qemu-stable] [Qemu-devel] [PATCH] qemu-char: don't issue CHR_EVENT_OPEN in a BH

[Anthony Liguori]

mdroth <address@hidden> writes:

> On Thu, May 30, 2013 at 11:55:56AM -0500, Anthony Liguori wrote:
>> Michael Roth <address@hidden> writes:
>> 
>> > When CHR_EVENT_OPEN was initially added, it was CHR_EVENT_RESET, and
>> > it was issued as a bottom-half:
>> >
>> > 86e94dea5b740dad65446c857f6959eae43e0ba6
>> >
>> > AFAICT the only reason this was ever done in a BH was because it was
>> > initially used to to issue a CHR_EVENT_RESET when we initialized the
>> > monitor,
>> 
>> It was specifically added so that we could redisplay the monitor
>> banner.  Because the event was emitted in open(), if we tried to
>> redisplay the monitor banner in the callback, the device would be in a
>> partially initialized state and badness would ensue.  The BH was there
>> to ensure the CharDriverState was fully initialized before firing the
>> callback.
>> 
>> > and we would in some cases modify the chr_write handler for
>> > a new chardev backend *after* the site where we issued the reset
>> > (see: 86e94d:qemu_chr_open_stdio())
>> >
>> > So we executed the reset in a BH to ensure the chardev was fully
>> > initialized before we executed the CHR_EVENT_RESET handler (which
>> > generally involved printing out a greeting/prompt for the monitor).
>> >
>> > At some point this event was renamed to CHR_EVENT_OPEN, and we've
>> > maintained the use of this BH ever since.
>> >
>> > However, due to 9f939df955a4152aad69a19a77e0898631bb2c18, we schedule
>> > the BH via g_idle_add(), which is causing events to sometimes be
>> > delivered after we've already begun processing data from backends,
>> > leading to:
>> >
>> >  known bugs:
>> >
>> >   QMP:
>> >     session negotation resets with OPEN event, in some cases this
>> >     is causing new sessions to get sporadically reset
>> >
>> >  potential bugs:
>> >
>> >   hw/usb/redirect.c:
>> >     can_read handler checks for dev->parser != NULL, which may be
>> >     true if CLOSED BH has not been executed yet. In the past, OPENED
>> >     quiesced outstanding CLOSED events prior to us reading client
>> >     data. If it's delayed, our check may allow reads to occur even
>> >     though we haven't processed the OPENED event yet, and when we
>> >     do finally get the OPENED event, our state may get reset.
>> >
>> >   qtest.c:
>> >     can begin session before OPENED event is processed, leading to
>> >     a spurious reset of the system and irq_levels
>> >
>> >   gdbstub.c:
>> >     may start a gdb session prior to the machine being paused
>> >
>> > To fix these, let's just drop the BH.
>> >
>> > Since the initial reasoning for using it still applies to an extent,
>> > work around that be deferring the delivery of CHR_EVENT_OPENED until
>> > after the chardevs have been fully initialized by setting a
>> > 'be_open_on_init' flag that gets checked toward the end of
>> > qmp_chardev_add(). This defers delivery long enough that we can
>> > be assured a CharDriverState is fully initialized before
>> > CHR_EVENT_OPENED is sent.
>> >
>> > Reported-by: Stefan Priebe <address@hidden>
>> > Cc: address@hidden
>> > Signed-off-by: Michael Roth <address@hidden>
>> > ---
>> >  backends/baum.c       |    2 +-
>> >  include/sysemu/char.h |    2 +-
>> >  qemu-char.c           |   29 ++++++++++-------------------
>> >  ui/console.c          |    2 +-
>> >  ui/gtk.c              |    2 +-
>> >  5 files changed, 14 insertions(+), 23 deletions(-)
>> >
>> > diff --git a/backends/baum.c b/backends/baum.c
>> > index 4cba79f..8384ef2 100644
>> 
>> <snip>
>> 
>> > @@ -3803,6 +3791,9 @@ ChardevReturn *qmp_chardev_add(const char *id, 
>> > ChardevBackend *backend,
>> >          chr->label = g_strdup(id);
>> >          chr->avail_connections =
>> >              (backend->kind == CHARDEV_BACKEND_KIND_MUX) ? MAX_MUX : 1;
>> > +        if (chr->be_open_on_init) {
>> > +            qemu_chr_be_event(chr, CHR_EVENT_OPENED);
>> > +        }
>> 
>> Why does this need to be called conditionally?  Could we drop
>> be_open_on_init and just call this unconditionally here?
>
> We have a couple instances where we don't immediately set the backend to
> an open state on init.

Would it make more sense to mark those since they are less common?

Regards,

Anthony Liguori

> The main one is socket backends, where where we
> don't send the OPENED event until a client connects.
>
>> 
>> Regards,
>> 
>> Anthony Liguori
>>