RE: New pthread_once implementation

[Ross Johnson <ross.johnson@homemail.com.au>]

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.

Remember, these are very rare situations following a cancellation. It
might delay things a little but shouldn't break anything.

> Re: you previous message:
> >"If only one thread ever comes in, and is canceled in the init_routine, 
> >then the semaphore is never cleaned up."
> 
> If only one thread ever comes in, and is canceled in the init_routine, then
> the semaphore is never created to begin with, right?
> 
> 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

> -----Original Message-----
> From: Gottlob Frege [mailto:gottlobfrege@gmail.com] 
> Sent: Friday, May 27, 2005 12:42 PM
> To: Ross Johnson
> Cc: Vladimir Kliatchko
> Subject: Re: New pthread_once implementation
> 
> thread1 comes in, start initing
> thread2 creates sema and waits
> thread1 starts to cancel - resets control->state
> thread3 comes in, goes into init
> thread4 comes in, goes into else block
> thread1 finishes cancel - releases semaphore
> thread2 wakes up
> thread2 decrements numSemaUsers to 0
> thread4 increments numSemaUsers
> thread4 does NOT set new semaphore
> thread2 closes semaphore
> thread4 tries to wait on closed semaphore...
> 
> 
> On 5/27/05, Ross Johnson <ross.johnson@homemail.com.au> wrote:
> > Guys,
> > 
> > Is there anything you want to change before I cast a new release?
> > 
> > http://sources.redhat.com/cgi-bin/cvsweb.cgi/pthreads/pthread_once.c?
> > rev=1.18&content-type=text/x-cvsweb-markup&cvsroot=pthreads-win32
> > 
> > Thanks.
> > Ross
> > 
> > On Fri, 2005-05-27 at 08:39 -0500, Tim Theisen wrote:
> > > I picked up the latest and compiled with VC 7.1.  All tests passed.
> > > Then, I ran 100 iterations of the once [1-4] tests.  These tests passed
> > > as well.  So, it has my stamp of approval.
> > >
> > > ...Tim
> > > --
> > >        Tim Theisen                     Lead Research Software Engineer
> > > Phone: +1 608 824 2848                 TomoTherapy Incorporated
> > >   Fax: +1 608 824 2996                 1240 Deming Way
> > >   Web: http://www.tomotherapy.com      Madison, WI 53717-1954
> > >
> > >
> > >
> > > -----Original Message-----
> > > From: Ross Johnson [mailto:ross.johnson@homemail.com.au]
> > > Sent: Friday, May 27, 2005 02:44
> > > To: Tim Theisen
> > > Cc: Vladimir Kliatchko; Gottlob Frege
> > > Subject: New pthread_once implementation
> > >
> > >
> > > Hi Tim,
> > >
> > > The current CVS head contains the latest and much simpler implementation
> > > of pthread_once just presented on the mailing list. It passes on a UP
> > > machine as usual but no-one has run it through an MP system yet. Could
> > > you when you get time?
> > >
> > > Thanks.
> > > Ross
> > >
> > >
> > >
> > 
> >
> 
>