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From: Martin Bonner (martin.bonner_at_[hidden])
Date: 2006-09-19 11:33:43

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

> "Peter Dimov" <pdimov_at_[hidden]> 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.


        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
Pi Technology, Milton Hall, Ely Road, Milton, Cambridge, CB4 6WZ,
ENGLAND Tel: +44 (0)1223 203894

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