linux-kernel - Re: [PATCH 3/3] 64-bit futexes: x86 support

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Message-ID: <4840C394.2080807@redhat.com>
Date:	Fri, 30 May 2008 20:18:44 -0700
From:	Ulrich Drepper <drepper@...hat.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
	mtk.manpages@...il.com
Subject: Re: [PATCH 3/3] 64-bit futexes: x86 support

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Linus Torvalds wrote:
> Also, doesn't this mean that effectively you have given up on any CPU 
> before the Pentium entirely? 

Yes, but only for the speed-up.  Not for the functionality.  All
programs will work just fine.

> Not that I want to take the patches _anyway_, but it seems doubly stupid. 
> Those things are going to be UP machines, so you could have just emulated 
> it with no correctness issues by disabling preemption.

First, you're thinking too much in kernel terms.  At userlevel I cannot
disable interrupts etc.  I cannot read 64 bits atomically on those
processors.  Second, the old implementation is not significantly slower
in normal programs on UP machines.  It's really not a big issue.

In any case, robbing 64-bit architectures of the possible performance
gain just because there are ancient versions which cannot do it is just
plain stupid.  I know you cannot suggest that.

- --
➧ Ulrich Drepper ➧ Red Hat, Inc. ➧ 444 Castro St ➧ Mountain View, CA ❖
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