Subject: Re: [PATCH 2.6.13] lockless pagecache 2/7
From: Alan Cox <alan@lxorguk.ukuu.org.uk>
To: Nick Piggin <nickpiggin@yahoo.com.au>
Cc: Andi Kleen <ak@suse.de>, Linux Memory Management <linux-mm@kvack.org>,
       linux-kernel <linux-kernel@vger.kernel.org>
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Date: Sat, 03 Sep 2005 00:57:51 +0100
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On Sad, 2005-09-03 at 07:22 +1000, Nick Piggin wrote:
> > Actually we have cmpxchg on i386 these days - we don't support
> > any SMP i386s so it's just done non atomically.
> 
> Yes, I guess that's what Alan must have meant.

Well I was thinking about things like pre-empt. Also the x86 cmpxchg()
is defined for non i386 processors to allow certain things to use it
(ACPI, DRM etc) which know they won't be on a 386. The implementation in
this case will blow up on a 386 and the __HAVE_ARCH_CMPXCHG remains
false.

> but I suspect that SMP isn't supported on those CPUs without ll/sc,
> and thus an atomic_cmpxchg could be emulated by disabling interrupts.

It's obviously emulatable on any platform - the question is at what
cost. For x86 it probably isn't a big problem as there are very very few
people who need to build for 386 any more and there is already a big
penalty for such chips.

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