[mythtv-commits] Ticket #9168: use of toUtf8() in myth_system_fork can *still* wedge the child: multiple mythfrontend.real processes; no sound
MythTV
mythtv at cvs.mythtv.org
Mon Nov 1 14:38:41 UTC 2010
#9168: use of toUtf8() in myth_system_fork can *still* wedge the child: multiple
mythfrontend.real processes; no sound
-------------------------------------------+--------------------------------
Reporter: Kevin Buhr <buhr@…> | Type: defect
Status: new | Priority: minor
Milestone: unknown | Component: MythTV - General
Version: Trunk Head | Severity: medium
Keywords: | Ticket locked: 0
-------------------------------------------+--------------------------------
Short story: the patch in bug #9016 isn't a complete fix for locking
issues in myth_system_fork(). toUtf8() can cause a deadlock, and
operator+(QString,...) probably remains an issue, too.
After an upgrade to Mythbuntu's trunk autobuild
(0.24.0~trunk26882-0ubuntu0~mythbuntu2), I noticed that sometimes after
switching programs or stopping and restarting Live TV, sound would stop
working. I tracked it down to lingering extra "mythfrontend.real"
processes that were launched to poke the screen savers but never got
there.
In the front-end logs, I'd see:
{{{
2010-10-31 21:38:49.049 ScreenSaverX11Private: Calling xscreensaver-
command -deactivate
2010-10-31 21:38:49.049 Launching: xscreensaver-command -deactivate >&-
2>&- &
2010-10-31 21:38:49.056 PID 12775: launched in the background, not waiting
2010-10-31 21:38:49.056 ScreenSaverX11Private: Calling gnome-screensaver-
command --poke
2010-10-31 21:38:49.056 Launching: gnome-screensaver-command --poke >&-
2>&- &
2010-10-31 21:38:49.064 PID 12776: launched in the background, not waiting
2010-10-31 21:38:49.271 PID 12775: exited: status=0, result=0
}}}
with PID 12776 forked but not execed and never exiting, and sound would
fail with:
{{{
2010-10-31 21:57:29.125 Opening ALSA audio device
'iec958:CARD=Intel,DEV=0'.
2010-10-31 21:57:29.125 ALSA, Error:
snd_pcm_open(iec958:CARD=Intel,DEV=0): Device or resource busy
2010-10-31 21:57:29.125 AudioOutput Error: Aborting reconfigure
}}}
If the extra process was killed, sound would start working again.
A backtrace of one of the processes showed it getting stuck in the
toUtf8() call after the fork and just before the execl:
{{{
buhr at medi:~$ sudo gdb /usr/bin/mythfrontend.real 12776
[ . . . ]
__lll_lock_wait () at
../nptl/sysdeps/unix/sysv/linux/x86_64/lowlevellock.S:136
136 ../nptl/sysdeps/unix/sysv/linux/x86_64/lowlevellock.S: No such
file or directory.
in ../nptl/sysdeps/unix/sysv/linux/x86_64/lowlevellock.S
(gdb) info stack
#0 __lll_lock_wait () at
../nptl/sysdeps/unix/sysv/linux/x86_64/lowlevellock.S:136
#1 0x00007f1102647864 in _L_lock_1024 () from /lib/libpthread.so.0
#2 0x00007f11026476c7 in __pthread_mutex_lock (mutex=0x7f11023b7120) at
pthread_mutex_lock.c:82
#3 0x00007f110215e7a9 in ?? () from /usr/lib/nvidia-current/libGL.so.1
#4 0x00007f1102161709 in ?? () from /usr/lib/nvidia-current/libGL.so.1
#5 0x00007f1102161b3a in ?? () from /usr/lib/nvidia-current/libGL.so.1
#6 0x00007f10fceba98d in ?? () from /usr/lib/nvidia-current/tls
/libnvidia-tls.so.260.19.06
#7 0x00007f10feefe11d in QByteArray::resize(int) () from
/usr/lib/libQtCore.so.4
#8 0x00007f10ff025fee in ?? () from /usr/lib/libQtCore.so.4
#9 0x00007f10fef4017e in QString::toUtf8() const () from
/usr/lib/libQtCore.so.4
#10 0x00007f1106d4518a in myth_system_fork (command=...,
result=@0x7fff64f6ed0c) at mythsystem.cpp:411
#11 0x00007f1106d44360 in myth_system (command=..., flags=23, timeout=0)
at mythsystem.cpp:256
#12 0x00007f11069e6032 in ScreenSaverX11Private::ResetScreenSaver
(this=0x7f10f002ffa0) at screensaver-x11.cpp:218
#13 0x00007f11069e34fe in ScreenSaverX11::resetSlot (this=0x7f10f0033aa0)
at screensaver-x11.cpp:346
#14 0x00007f1106a17554 in ScreenSaverX11::qt_metacall
(this=0x7f10f0033aa0, _c=QMetaObject::InvokeMetaMethod, _id=0,
_a=0x7fff64f6ef50) at moc_screensaver-x11.cpp:74
[ . . . ]
}}}
Note that there are probably additional potential deadlocks with
operator+() on QStrings, since I assume that function can call
QByteArray::resize() too.
--
Ticket URL: <http://svn.mythtv.org/trac/ticket/9168>
MythTV <http://www.mythtv.org/>
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