linux-kernel - Re: [PATCH 1/2] FRV: Implement atomic64

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Message-ID: <20090701211117.GC631@elte.hu>
Date:	Wed, 1 Jul 2009 23:11:17 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	David Howells <dhowells@...hat.com>, akpm@...ux-foundation.org,
	paulus@...ba.org, arnd@...db.de, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] FRV: Implement atomic64_t


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

> On Wed, 1 Jul 2009, David Howells wrote:
> > +
> > +#define ATOMIC64_INIT(i)	{ (i) }
> > +#define atomic64_read(v)	((v)->counter)
> > +#define atomic64_set(v, i)	(((v)->counter) = (i))
> 
> These seem to be buggy.
> 
> At least "atomic64_read()" needs to make sure to actually read it 
> atomically - otherwise you'll do two 32-bit reads, and that just 
> gets crap. Imagine if somebody is adding 1 to 0x00000000ffffffff, 
> and then "atomic64_read()" reads it as two accesses in the wrong 
> place, and gets either 0, or 0x00000001ffffffff, both of which are 
> totally incorrect.
> 
> The case of 'atomic64_set()' is _slightly_ less clear, because I 
> think we use it mostly for initializers, so atomicity is often not 
> strictly required. But at least on x86, we do guarantee that it 
> sets it atomically too.
> 
> Btw, Ingo: I looked at the x86-32 versions to be sure, and noticed 
> a couple of buglets:

thanks - we'll fix these!

	Ingo
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