linux-kernel - Re: Why Semaphore Hardware-Dependent?

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Message-ID: <Pine.LNX.4.64.0608272220470.24098@schroedinger.engr.sgi.com>
Date:	Sun, 27 Aug 2006 22:21:17 -0700 (PDT)
From:	Christoph Lameter <clameter@....com>
To:	Chris Wedgwood <cw@...f.org>
cc:	Paul Mackerras <paulus@...ba.org>,
	Dong Feng <middle.fengdong@...il.com>, ak@...e.de,
	linux-kernel@...r.kernel.org
Subject: Re: Why Semaphore Hardware-Dependent?

On Sun, 27 Aug 2006, Chris Wedgwood wrote:

> On Mon, Aug 28, 2006 at 10:18:35AM +1000, Paul Mackerras wrote:
> 
> > I believe the reason for not doing something like this on x86 was
> > the fact that we still support i386 processors, which don't have the
> > cmpxchg instruction.  That's fair enough, but I would be opposed to
> > making semaphores bigger and slower on PowerPC because of that.
> > Hence the two different styles of implementation.
> 
> The i386 is older than some of the kernel hackers, and given that a
> modern kernel is pretty painful with less than say 16MB or RAM in
> practice, I don't see that it would be all that terrible to drop
> support for ancient CPUs at some point (yes, I know some newer
> embedded (and similar) CPUs might be affected here too, but surely not
> that many that people really use --- and they could just use 2.4.x).

Also note that i386 has a cmpxchg emulation for those machines that do not 
support cmpxchg.

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