From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <pthreads-win32-return-940-listarch-pthreads-win32=sources.redhat.com@sources.redhat.com>
Received: (qmail 15639 invoked by alias); 4 Mar 2005 23:18:35 -0000
Mailing-List: contact pthreads-win32-help@sources.redhat.com; run by ezmlm
Precedence: bulk
List-Subscribe: <mailto:pthreads-win32-subscribe@sources.redhat.com>
List-Archive: <http://sources.redhat.com/ml/pthreads-win32/>
List-Post: <mailto:pthreads-win32@sources.redhat.com>
List-Help: <mailto:pthreads-win32-help@sources.redhat.com>, <http://sources.redhat.com/ml/#faqs>
Sender: pthreads-win32-owner@sources.redhat.com
Received: (qmail 15629 invoked from network); 4 Mar 2005 23:18:31 -0000
Received: from unknown (HELO canyonero.dot.net.au) (202.147.68.14)
  by sourceware.org with SMTP; 4 Mar 2005 23:18:31 -0000
Received: from [202.147.74.107] (helo=ip-74-170.dot.net.au)
	by canyonero.dot.net.au with esmtp (Exim 3.35 #1 (Debian))
	id 1D7M3y-0007Wn-00
	for <pthreads-win32@sources.redhat.com>; Sat, 05 Mar 2005 10:18:30 +1100
Subject: Re: starvation in pthread_once?
From: Ross Johnson <rpj@callisto.canberra.edu.au>
To: Pthreads-Win32 list <pthreads-win32@sources.redhat.com>
In-Reply-To: <97ffb3105030301323c1bae1@mail.gmail.com>
References: <97ffb3105030301323c1bae1@mail.gmail.com>
Content-Type: text/plain
Date: Fri, 04 Mar 2005 23:18:00 -0000
Message-Id: <1109978309.8332.70.camel@desk.home>
Mime-Version: 1.0
Content-Transfer-Encoding: 7bit
X-SW-Source: 2005/txt/msg00022.txt.bz2

On Thu, 2005-03-03 at 04:32 -0500, Gottlob Frege wrote:
> I'm concerned about the Sleep(0) in pthread_once:
> 

Thanks. It looks like this routine needs to be redesigned.

Regards.
Ross

>   if (!once_control->done)
>     {
>       if (InterlockedIncrement (&(once_control->started)) == 0)
>         {
>           /*
>            * First thread to increment the started variable
>            */
>           (*init_routine) ();
>           once_control->done = PTW32_TRUE;
> 
>         }
>       else
>         {
>           /*
>            * Block until other thread finishes executing the onceRoutine
>            */
>           while (!(once_control->done))
>             {
>               /*
>                * The following gives up CPU cycles without pausing
>                * unnecessarily
>                */
>               Sleep (0);
>             }
>         }
>     }
> 
> IIRC, Sleep(0) does not relinquish time slices to lower priority
> threads.  (Sleep(n) for n != 0 does, but 0 does not).  So, if a lower
> priority thread is first in, followed closely by a higher priority
> one, the higher priority thread will spin on Sleep(0) *forever*
> because the lower, first thread will never get a chance to set done.
> 
> So even Sleep(10) should be good enough.  In theory, there could be
> enough higher priority threads in the system that the first thread
> still doesn't get in (ever?!), but unlikely.  And that would probably
> mean a general design flaw of the calling code, not pthread_once.
> 
> ?
>