Subj : Re: Insane "Metered Section" example on MSDN
To   : comp.programming.threads
From : Neill Clift [MSFT]
Date : Fri Apr 22 2005 08:43 pm

"Agresao" <agresao@yahoo.com> wrote in message 
news:1113900515.926413.282240@o13g2000cwo.googlegroups.com...
>I was trying to implement a semaphore that won't always do a kernel
> transition to a Win32 object and came up with some code that seemed to
> work. Then I found this article on MSDN about this thing called a
> "metered section":
>
> http://msdn.microsoft.com/library/default.asp?url=/library/en-us/dndllpro/html/msdn_metrsect.asp
>
> The code sample looked very similar to my own efforts which sort of
> made me think that my solution was correct. However by thinking a
> little deeper one realizes that their design has a terrible race
> condition, in which threads are blocked waiting for an auto-reset event
> to be signaled when there's nobody to signal it. Testing showed that
> this actually happens.
>
> I'm sure some of you have seen this article before and talked about it,
> but what puzzles me is that it has been in MSDN since 1998 and nobody
> has bothered to revise it...
>

I just looked through this and I agree. Any algorithm that calls SetEvent
in a loop expecting to wake n threads is broken. I'll notify the appropriate
people.
Thanks.
Neill.

.