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DVDRingBuffer: Unlock the ring buffer when waiting on state changes.

The DVD ringbuffer does not use the read ahead thread and hence holding
the writelock during safe_read is not strictly necessary (see
RingBuffer::ReadPriv). DVDRingBuffer::safe_read can also hold that lock
for tens of seconds when, for example, displaying a still frame (the dvd
device simply returns specific events, but not data, until it is told to
skip the still frame).

Under normal operation this is not a problem but events received from
external sources (such as network control) can trigger code that tries
to lock the ringbuffer from the main UI thread (e.g.
RingBuffer::GetFilename). The lock is not obtained, event processing in
the main thread is blocked and the wait state in the ringbuffer thread
is not cleared - and the frontend deadlocks as a result.

The solution is to release the lock while sleeping within
DVDRingBuffer::safe_read to allow other threads to obtain the lock
temporarily. I've also added a warning message should the read ahead
thread be running in the DVD ringbuffer.

Refs #9780
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1 parent 17876e6 commit fd5e33a69b630aa9c0b94a8c2dd7693f02726316 Mark Kendall committed May 18, 2011
Showing with 12 additions and 0 deletions.
  1. +12 −0 mythtv/libs/libmythtv/dvdringbuffer.cpp
12 mythtv/libs/libmythtv/dvdringbuffer.cpp
@@ -502,7 +502,12 @@ void DVDRingBuffer::WaitForPlayer(void)
m_playerWait = true;
int count = 0;
while (m_playerWait && count++ < 200)
+ {
+ rwlock.unlock();
+ rwlock.lockForWrite();
+ }
if (m_playerWait)
@@ -530,6 +535,9 @@ int DVDRingBuffer::safe_read(void *data, uint sz)
return -1;
+ if (readaheadrunning)
+ VERBOSE(VB_IMPORTANT, LOC_ERR + "read ahead thread running.");
while (needed)
blockBuf = m_dvdBlockWriteBuf;
@@ -886,7 +894,9 @@ int DVDRingBuffer::safe_read(void *data, uint sz)
// pause a little as the dvdnav VM will continue to return
// this event until it has been skipped
+ rwlock.unlock();
+ rwlock.lockForWrite();
// when scanning the file or exiting playback, skip immediately
// otherwise update the timeout in the player
@@ -919,7 +929,9 @@ int DVDRingBuffer::safe_read(void *data, uint sz)
m_dvdWaiting = true;
+ rwlock.unlock();
+ rwlock.lockForWrite();
// release buffer

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