[mythtv-commits] Ticket #8854: Locking problem in videobuffers WaitForAvailable()

[MythTV]

#8854: Locking problem in videobuffers WaitForAvailable()
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 Reporter:  tomi.orava@…                               |            Type:  defect          
   Status:  new                                        |        Priority:  minor           
Milestone:  unknown                                    |       Component:  MythTV - General
  Version:  Trunk Head                                 |        Severity:  medium          
 Keywords:  Timed out waiting for free video buffers.  |   Ticket locked:  0               
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 The following code block is flawed by design in videobuffers.h:

     bool WaitForAvailable(uint w)
      {
         // Qt4 requires a QMutex as a parameter...
         // not sure if this is the best solution.  Mutex Must be locked
 before wait.
         QMutex mutex;
         mutex.lock();
         return available_wait.wait(&mutex, w);
      }

 The problem is of course that the mutex is completely bogus when defined
 inside the method. Also none of the calls to the available_wait.WakeAll()
 have the same mutex locked and therefore it is very easy to have a race
 condition between threads where the available_wait.WakeAll() signal is
 being missed by this method completely thus allowing the mythfrontend to
 hang completely with the following error message repeating itself over and
 over again:

 010-09-02 21:06:52.733 Player(1): Timed out waiting for free video
 buffers.
 2010-09-02 21:06:54.763 Player(1): Timed out waiting for free video
 buffers.
 2010-09-02 21:06:56.800 Player(1): Timed out waiting for free video
 buffers.
 2010-09-02 21:06:58.824 Player(1): Timed out waiting for free video
 buffers.
 2010-09-02 21:07:00.848 Player(1): Timed out waiting for free video
 buffers.
 2010-09-02 21:07:02.870 Player(1): Timed out waiting for free video
 buffers.
 2010-09-02 21:07:04.894 Player(1): Timed out waiting for free video
 buffers.
 2010-09-02 21:07:06.917 Player(1): Timed out waiting for free video
 buffers.

 A pretty easy way to reproduce this problem is to rewind quickly and then
 press enter to continue playback. If the rewinding is short enough the
 frontend has a high chance of hanging at least on all the machines I have
 here.

 The problematic code block (and problem) has existed for ages even in
 trunk and the current version I have is: r26022

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Ticket URL: <http://svn.mythtv.org/trac/ticket/8854>
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