----Original Message---- From: boost-bounces@lists.boost.org [mailto:boost-bounces@lists.boost.org] On Behalf Of Anthony Williams Sent: 19 September 2006 15:39 To: boost@lists.boost.org Subject: [boost] win32 mutex and condition variable algorithms (was:[threads] Permission given to change to Boost.License)

"Peter Dimov" <pdimov@mmltd.net> writes:

...

Inventing mutex and condition variable algorithms is a slight cause of concern, I have to admit. The easiest way to get a free review of the algorithms is to post the code on comp.programming.threads and hope that someone of the experts there has enough time to look at them. CVs in particular have proven to be quite hard to implement correctly, and Alexander Terekhov's "8a" algorithm has emerged as a de-facto standard.

The mutex is simple, and you can see the code in boost/thread/win32/basic_mutex.hpp on the thread_rewrite branch.

I'm missing something here (I hope).

The algorithm is:

init(): active_count=0; no semaphore

lock(): atomic increment active_count

At this point, thread2 atomically increments active_count as well, so it is now 2.

if new active_count ==1, that's us, so we've got the lock else get semaphore, and wait Both threads will go into this branch. Why don't they wait forever? now we've got the lock

unlock(): atomic decrement active_count if new active_count >0, then other threads are waiting, so release semaphore.

Alternatively, Thread A Thread B locks mutex atomic increment active_count (in lock) atomic decrements active_count (in unlock) new active_count > 0, so other threads are waiting release semaphore ... oops. What semaphore? Please tell me I have missed something! -- Martin Bonner Martin.Bonner@Pitechnology.com Pi Technology, Milton Hall, Ely Road, Milton, Cambridge, CB4 6WZ, ENGLAND Tel: +44 (0)1223 203894