C++ Boost :Variable sync between 2 threads - c++

Question

I have the following code:
Main thread notifies worker thread to start/stop some job.In the main thread the trigger is some UI button(Qt SDK in this case):
void PlaySlot(bool checked){
boost::unique_lock<boost::mutex> lock(m_mutex);
if(checked == true){
m_isPlayMode = true;
m_event.notify_one(); //tell thread to start playing.
}else{
m_isPlayMode = false;
}
}
Now,in the worker thread,once m_isPlayMode becomes true, some loop starts running for a limited period of time and it will exit when the time is finished or m_isPlayMode becomes false.
Inside the thread operator:
while(true)
{
boost::unique_lock<boost::mutex> lock(m_mutex);
m_event.wait(lock); //wait for next event
if(m_isPlayMode == true){
while(m_frameIndex< totalFrames && m_isPlayMode){
m_frameIndex++;
///do some work
}
m_isPlayMode = false;
emit playEnded(false);
}
}
Now,what is happening that after the loop starts playing,when PlaySlot() gets triggered with 'checked' = false it doesn't update m_isPlayMode and the program becomes unresponsive.I suspect that's condition race issue as I am trying to lock mutex which is already locked in the thread loop.
I solved it by removing unique_lock from PlaySlot method and converting m_isPlayMode to atomic variable.It works.
But I want to know 2 things:
Are there any perils in such a solution.
Can it be solved in another way?

Answer 1

Note that m_isPlayMode is protected by the same mutex, hence can't be updated when the worker is running. Use two separate mutexes for these, or atomics.
Edit: Fast fix would probably to add a second mutex:
void PlaySlot(bool checked){
boost::unique_lock<boost::mutex> lock(m_isPlayModeMutex); // <--
// ...
}
worker thread:
for (;;) {
boost::unique_lock<boost::mutex> lock(m_mutex);
m_event.wait(lock); // wait for next event
boost::unique_lock<boost::mutex> playModeLock(m_isPlayModeLock);
if(m_isPlayMode == true){
while(m_frameIndex< totalFrames && m_isPlayMode){
playModeLock.unlock();
/// ... (not locked here)
playModeLock.lock();
}
m_isPlayMode = false;
emit playEnded(false);
}
}