----Original Message---- From: boost-bounces@lists.boost.org [mailto:boost-bounces@lists.boost.org] On Behalf Of Anthony Williams Sent: 19 September 2006 17:02 To: boost@lists.boost.org Subject: Re: [boost] win32 mutex and condition variable algorithms

"Martin Bonner" <martin.bonner@pitechnology.com> writes:

...

From: boost-bounces@lists.boost.org [mailto:boost-bounces@lists.boost.org] On Behalf Of Anthony Williams

...

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?

No. The atomic increments are atomic, and return the new value: ^^^^^^^^^^^^^^^^^^^^^^^^ That's the bit I had missed. Thanks.

we don't do an extra read. One thread will see "1", and the other "2".

...

...

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?

Sorry I didn't make it clear. If we go into this branch of unlock, we have to call get_semaphore() to get the semaphore to release. This ensures that there is one and only semaphore associated with this mutex, which is returned to all callers, and is therefore released by the unlock().

Hope that makes things clearer.

It does. I am now reassured that there are no trivially obvious errors. ... which doesn't mean there aren't any errors :-( -- Martin Bonner Martin.Bonner@Pitechnology.com Pi Technology, Milton Hall, Ely Road, Milton, Cambridge, CB4 6WZ, ENGLAND Tel: +44 (0)1223 203894