In a multi-threaded C++ app, do I need a mutex to protect a simple boolean?

Question

I have a multi-threaded C++ app which does 3D rendering with the OpenSceneGraph library. I'm planning to kick off OSG's render loop as a separate thread using boost::threads, passing a data structure containing shared state in to the thread. I'm trying to avoid anything too heavyweight (like mutexes) for synchronization, as the render loop needs to be pretty tight, and OSG itself tries to avoid having to ever lock. Most of the shared state is set before the thread is started, and never changed. I do have some data that does need to be changed, which I am planning to double-buffer. However, I have a simple boolean for signaling the thread to suspend rendering, and later resume rendering, and another to kill it. In both cases the app thread sets the bool, and the render thread only reads it. Do I need to synchronize access to these bools? As far as I can tell, the worse thing that could happen is the the render loop continues on for an extra frame before suspending or quitting.

Answer 1

In C++11 and later, which has standards-defined concurrency, use std::atomic<bool> for this purpose. From http://en.cppreference.com/w/cpp/atomic/atomic:

If one thread writes to an atomic object while another thread reads from it, the behavior is well-defined (see memory model for details on data races).

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The following old answer may have been true at some time in the past with some compilers and some operating environments, but it should not be relied upon today:

You're right, in this case you won't need to synchronise the bools. You should declare them volatile though, to ensure that the compiler actually reads them from memory each time, instead of caching the previous read in a thread (that's a simplified explanation, but it should do for this purpose).

The following question has more information about this: C++ Thread, shared data

Answer 2

Why not simply use an interlocked variable?

Answer 3

As for C++11 and later it is finally threads-aware and clearly states that modifying a bool (or other non-atomic variable) in one thread and accessing it at the same time in another one is undefined behavior. In you case using std::atomic<bool> should be enough to make your program correct, saving you from using locks.
Do not use volatile. It has nothing to do with threads. For more discussion look at Can I read a bool variable in a thread without mutex?

Answer 4

I don't think you need a fully fledged mutex here -- though the render thread will need to busy wait in the 'suspended' state if you aren't using a synchronization object that supports a wait primitive.

You should look into using the various interlocked exchange primitives though (InterlockedExchange under Windows). Not because read/writes from the bool are non-atomic, but to ensure that there are no weird behaviours the compiler reordering memory accesses on a single thread.

Answer 5

This thread has a little more info and discussion on thread-safety, especially for simple data types:

How can I create a thread-safe singleton pattern in Windows?