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POStuck in a circular wait condition. Need help to get out
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copied!<p>I am currently developing an multi-threaded application that does raw video data playback using Qt. There are three worker threads. One is Reader Thread, one is Processor Thread and the other is a Video Output Thread. The reader thread will read raw data into a buffer, then the processor will process the raw data and put displayable data into a frame FIFO. </p> <p>My raw data buffer is implemented as a Hash table which means every frame index will have its corresponding raw data. Once the processor finish processing the raw data one a certain frame it deletes the raw data from the raw data buffer so that the reader can read data into the buffer once it comes back to a frame it read before.</p> <p>I now have problem when implementing pause functionality. What I wanna do is when the video is pause, the processor and reader should quit and video output thread will just display one same frame in the fifo. I can now exit from processor thread properly. The reader seems to get into a circular wait condition situation.</p> <p>Here are some code for my reader thread.</p> <pre><code> void ReaderThread::run() { forever { //inputState_ is internal state of the reader if (inputState_ == stop) // return from run() if state is stop return; //dataBuffer is a pointer to raw data buffer object //This grab the mutex lock of the raw data buffer dataBuffer->lock() //frameIndex is counter for frame if (dataBuffer->contains(frameIndex_)) { //mutex_ is a member variable of type QMutex mutex_.lock(); //Wait for the data to be removed. dataBuffer->waitForWritable(); mutex_.unlock(); } dataBuffer->unlock(); // Read data to raw frame buffer uchar rawDataPointer = readRawData(); dataBuffer->lock(); //insert the frame number and corresponding pointer into the hash table dataBuffer->insert(frameIndex, rawDataPointer); //wake up processor. dataBuffer->wakeForReadable() dataBuffer->unlock(); mutex_.lock(); if (inputState_ == play) frameIndex++; mutex_.unlock(); } } void ReaderThread::stop() { mutex_.lock(); inputState_ = stop; //Wake up the reader if the reader is waiting dataBuffer->wakeAllWritable(); mutex_.unlock(); // Wait for run() to return. QThread::wait(); } </code></pre> <p>Processor thread is implemented similarly. I stop the processor at first and it succeeded. I call ReaderThread::stop() from main thread when I want to return from run(). But I got into a circular wait condition, where if the reader thread is waiting on the buffer to be writable, it never will unlock mutex_ of this reader thread. As a result, the stop will always get block and the program just hangs. I tried not to lock the mutex_ before the dataBuffer->waitForWritable(), however, I got into a situation where the reader thread will wait on the writable right after the dataBuffer->wakeAllWritable() is called. </p> <p>Can anyone suggest a way to resolve this issue? I have been trying hard to figure out how to get out of this circular wait condition situation but didn't quite work out as I am still new to threaded programming. </p>

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