[mythtv-commits] Ticket #9714: mythwelcome hanging (MythSystemUnix::Fork child QString use leads to possible deadlock)

[MythTV]

#9714: mythwelcome hanging (MythSystemUnix::Fork child QString use leads to
possible deadlock)
-------------------------------------+------------------------
     Reporter:  naf@…                |      Owner:  beirdo
         Type:  Patch - Bug Fix      |     Status:  new
     Priority:  minor                |  Milestone:  unknown
    Component:  MythTV - MythSystem  |    Version:  Trunk Head
     Severity:  medium               |   Keywords:
Ticket locked:  0                    |
-------------------------------------+------------------------
 I noticed mythwelcome sporadically hanging when calling mythstatus. Logs
 ended like:
 {{{
 2011-04-07 20:08:19.377 Locking input devices
 2011-04-07 20:08:19.386 Launching: /usr/local/bin/mythshutdown --status 0
 2011-04-07 20:08:19.388 Managed child (PID: 27773) has started! *
 command=/usr/local/bin/mythshutdown --status 0, timeout=0
 }}}

 Backtrace of the hung process looked like:
 {{{
 #0  0x00007fe8f25d81ac in __lll_lock_wait () from /lib64/libpthread.so.0
 #1  0x00007fe8f25d34bf in _L_lock_1058 () from /lib64/libpthread.so.0
 #2  0x00007fe8f25d331b in pthread_mutex_lock () from
 /lib64/libpthread.so.0
 #3  0x0000003c2d2978a9 in ?? () from /usr/lib64/libGL.so.1
 #4  0x0000003c2d29a8b9 in ?? () from /usr/lib64/libGL.so.1
 #5  0x0000003c2d29ace8 in ?? () from /usr/lib64/libGL.so.1
 #6  0x0000003c2da0098d in ?? () from /usr/lib64/tls/libnvidia-
 tls.so.260.19.29
 #7  0x00000031338b6623 in QString::fromLatin1_helper(char const*, int) ()
 from /usr/lib64/libQtCore.so.4
 #8  0x00000031338b92bd in QString::fromAscii_helper(char const*, int) ()
 from /usr/lib64/libQtCore.so.4
 #9  0x00007fe8f445902d in QString (this=0x1ffb5b0, timeout=0) at
 /usr/include/QtCore/qstring.h:426
 #10 GetSetting (this=0x1ffb5b0, timeout=0) at mythsystem.h:86
 #11 GetSetting (this=0x1ffb5b0, timeout=0) at mythsystem.h:147
 #12 MythSystemUnix::Fork (this=0x1ffb5b0, timeout=0) at system-
 unix.cpp:766
 #13 0x00007fe8f43d1732 in MythSystem::Run (this=0x201e6b0, timeout=0) at
 mythsystem.cpp:152
 #14 0x00007fe8f43d58c6 in myth_system (command=..., flags=<value optimized
 out>, timeout=0) at mythsystem.cpp:368
 #15 0x0000000000404ba1 in isRunning (program=0x40f770 "mythtranscode") at
 main.cpp:196
 #16 0x00000000004052a6 in getStatus (bWantRecStatus=false) at main.cpp:231
 #17 0x000000000040e8f1 in main (argc=3, argv=0x7fffeb559068) at
 main.cpp:932
 }}}

 So the deadlock was happening in MythSystemUnix::Fork() doing QString
 stuff during a call to GetSetting() between fork() and execl().

 It appears that bac42c09 fixed possible child deadlocks by removing all
 QString use between fork() and execl() [and added comments referencing
 their danger!], but another QString was reintroduced in 7bad51a1.

 Attached patch moves the GetSetting() call (and thus the use of QString)
 to before the fork(), which solved my intermittent mythwelcome freezes.

-- 
Ticket URL: <http://code.mythtv.org/trac/ticket/9714>
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