linux-kernel - Re: [patch 04/41] cpu ops: Core piece for generic atomic per cpu operations

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Date:	Fri, 30 May 2008 17:08:50 +1000
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Christoph Lameter <clameter@....com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
	David Miller <davem@...emloft.net>,
	Eric Dumazet <dada1@...mosbay.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Mike Travis <travis@....com>
Subject: Re: [patch 04/41] cpu ops: Core piece for generic atomic per cpu operations

On Friday 30 May 2008 16:12:59 Christoph Lameter wrote:
> On Thu, 29 May 2008, Andrew Morton wrote:
> > > area. It requies disabling of interrupts etc.
> >
> > No it doesn't.  Look:
> >
> > static inline void local_inc(local_t *l)
> > {
> > 	asm volatile(_ASM_INC "%0"
> >
> > 		     : "+m" (l->a.counter));
> >
> > }
> >
> > > Its not atomic (wrt
> > > interrupts) because of that.
> >
> > Yes it is.
>
> No its not! In order to increment a per cpu value you need to calculate
> the per cpu pointer address in the current per cpu segment.

Christoph, you just missed it, that's all.  Look at cpu_local_read et al in 
include/asm-i386/local.h (ie. before the x86 mergers chose the lowest common 
denominator one).

Cheers,
Rusty.
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