[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

 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
 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
 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
 #1  0x00007f1102647864 in _L_lock_1024 () from /lib/libpthread.so.0
 #2  0x00007f11026476c7 in __pthread_mutex_lock (mutex=0x7f11023b7120) at
 #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
 #7  0x00007f10feefe11d in QByteArray::resize(int) () from
 #8  0x00007f10ff025fee in ?? () from /usr/lib/libQtCore.so.4
 #9  0x00007f10fef4017e in QString::toUtf8() const () from
 #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/>
MythTV Media Center

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