Anthony Williams wrote:
The mutex is simple, and you can see the code in boost/thread/win32/basic_mutex.hpp on the thread_rewrite branch.
The algorithm is:
init(): active_count=0; no semaphore
lock(): atomic increment active_count if new active_count ==1, that's us, so we've got the lock else get semaphore, and wait now we've got the lock
unlock(): atomic decrement active_count if new active_count >0, then other threads are waiting, so release semaphore.
This is what CRITICAL_SECTIONs do, and I recall being told by Alexander Terekhov that it is suboptimal (threads take the slow path when they could take the fast path). My tests with a contended queue seemed to prove him right. Unfortunately, I can no longer find his post where he explained that with Google.