What do you think of the attached implementation? I am still analyzing it,
but it passes the tests and appears to be free of that problem. It does have
one minor glitch though:
If two threads come in, the semaphore is created. If both are cancelled and
no new calls a made to finish the job, the semaphore is never destroyed.
I am not sure how big a deal this is.

Re. optimizations: Great, I will try to do something.

Thnx,
--vlad 

> -----Original Message-----
> From: pthreads-win32-owner@sources.redhat.com [mailto:pthreads-win32-
> owner@sources.redhat.com] On Behalf Of Ross Johnson
> Sent: Saturday, May 28, 2005 9:55 AM
> To: Vladimir Kliatchko
> Cc: 'Gottlob Frege'; Pthreads-Win32 list
> Subject: RE: New pthread_once implementation
> 
> On Sat, 2005-05-28 at 06:51 -0400, Vladimir Kliatchko wrote:
> >
> > > -----Original Message-----
> > > From: pthreads-win32-owner@sources.redhat.com [mailto:pthreads-win32-
> > > owner@sources.redhat.com] On Behalf Of Ross Johnson
> > > Sent: Friday, May 27, 2005 11:48 PM
> > > To: Vladimir Kliatchko
> > > Cc: 'Gottlob Frege'; Pthreads-Win32 list
> > > Subject: RE: New pthread_once implementation
> > >
> > > On Fri, 2005-05-27 at 21:30 -0400, Vladimir Kliatchko wrote:
> > > > Nice catch. Let me see if I can fix it.
> > > >
> > > > Note that the same problem exists in the currently released event-
> based
> > > > implementation (cvs version 1.16):
> > > >
> > > > thread1 comes in, start initing
> > > > thread2 creates event, starts waiting
> > > > thread3 comes in starts waiting
> > > > thread1 is cancelled, signals event
> > > > thread2 wakes up, proceeds to the point right before the resetEvent
> > > > thread3 wakes up, closes event handle
> > > > thread2 resets closed handle
> > >
> > > Relies on HANDLE uniqueness and assumes that an error will result.
> This
> > > is why the 2.6.0 version (and earlier) checks the return code and
> > > restores Win32 LastError if necessary - for GetLastError transparency.
> >
> > Does Windows guarantee that the handles are not reused? What happens if
> a
> > thread closes a handle while another thread is blocked on it? Is any of
> this
> > in Microsoft documentation? Consider the following scenario for the
> > event-based implementation:
> 
> Well, apparently they're not unique when recycled, so there is a bug
> here to fix in both versions:
> http://msdn.microsoft.com/library/default.asp?url=/library/en-
> us/dngenlib/html/msdn_handles1.asp
> [Under "Native Windows NT Objects"]
> "Unlike the handles that are maintained by the Win32 USER and GDI
> subsystem components, handles to native objects under Windows NT are not
> unique; that is, upon destruction of an object, the corresponding handle
> may be recycled and will look exactly like the handle to the destroyed
> object."
> 
> But they are local to the process, rather than system wide if that
> helps.
> 
> > > > Also, regarding my previous comment to Ross about very high cost of
> > > using
> > > > InterlockedExchangeAdd for MBR:
> > > > I did some simple benchmarking. Running pthread_once 50,000,000 on
> my
> > > pretty
> > > > slow single CPU machine takes about 2.1 seconds. Replacing
> > > > InterlockedExchangeAdd with simple read brings it down to 0.6
> seconds.
> > > This
> > > > looks significant.
> > >
> > > Using the PTW32_INTERLOCKED_COMPARE_EXCHANGE macro as in your latest
> (in
> > > CVS) version and building the library for inlined functions (nmake VC-
> > > inlined) and x86 architecture causes customised versions of
> > > InterlockedCompareExchange to be used, and this results in inlined
> asm.
> > > Same for PTW32_INTERLOCKED_EXCHANGE.
> > >
> > > Also, on single-CPU x86, the library dynamically switches to using
> > > 'cmpxchg' rather than 'lock cmpxchg' to avoid locking the bus. This
> > > appears to match what the kernel32.dll versions do. On non-x86
> > > architectures the kernel32.dll versions are called, with call
> overhead.
> > >
> > > PTW32_INTERLOCKED_EXCHANGE_ADD could be added, as could other
> > > architectures. See ptw32_InterlockedCompareExchange.c
> >
> > I have rerun my benchmark with VC-inline. The difference is now less
> > significant 0.9 vs 0.6 but still noticeable. I guess cmpxchg even
> without
> > locking is quite expensive. On multi-CPU systems the difference should
> be
> > much higher due to the time it takes to lock the bus and to the
> contention
> > it may cause. It sounded as if you did not care much to try to optimize
> it.
> > I did not mean to suggest that we have to do it right now either. I just
> > wanted to get your opinion on whether we want to deal with this in the
> > future.
> 
> By all means include any optimisation you think is worthwhile. I was
> just pointing out that the difference isn't necessarily 2.1 v 0.6.
>