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

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Date:	Fri, 3 Jul 2009 21:17:09 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Eric Dumazet <eric.dumazet@...il.com>, mingo@...hat.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH -tip] x86: atomic64: inline atomic64_read()


* Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> 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.

Ok - i've applied it and added a comment as well about the reasons 
an subtleties.

	Ingo
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