linux-kernel - Re: [PATCH -tip] x86: atomic64: inline atomic64

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Date:	Fri, 3 Jul 2009 12:10:23 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Ingo Molnar <mingo@...e.hu>
cc:	Eric Dumazet <eric.dumazet@...il.com>, mingo@...hat.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH -tip] x86: atomic64: inline atomic64_read()

On Fri, 3 Jul 2009, Ingo Molnar wrote:
> > +
> > +	asm volatile(
> > +		"mov %%ebx, %%eax\n\t"
> > +		"mov %%ecx, %%edx\n\t"
> > +		LOCK_PREFIX "cmpxchg8b %1\n"
> > +			: "=&A" (res)
> > +			: "m" (*ptr) 
> > +		);
> 
> That clobbers or affects eax, ebx, ecx, edx. The only registers it 
> does not clobber is esi, edi and rbp.

No, it only clobbers eax/edx. Yes, it touches ebx/ecx, but that's kind of 
incidental, and read-only, and doesn't matter.

> Linus, what's your preference in this case?

I think it's kind of subtle to do that thing, but it _is_ very much a 
special case. It's odd to use a cmpxchg8b to just do a read in the first 
place, so the fact that we then play other games with it is kind of 
immaterial.

Adding a comment about the magic "we don't care about the initial values 
in eax/edx and ebx/ecx, but they have to match in case we end up doing a 
writeback" might be a good idea.

		Linus
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