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Date: Thu, 29 May 2008 23:12:59 -0700 (PDT)
From: Christoph Lameter <clameter@sgi.com>
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To: Andrew Morton <akpm@linux-foundation.org>
cc: linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org,
       David Miller <davem@davemloft.net>, Eric Dumazet <dada1@cosmosbay.com>,
       Peter Zijlstra <peterz@infradead.org>,
       Rusty Russell <rusty@rustcorp.com.au>, Mike Travis <travis@sgi.com>
Subject: Re: [patch 04/41] cpu ops: Core piece for generic atomic per cpu
 operations
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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. local_t 
cannot do that in an atomic (wrt interrupt/preempt fashion) fashion. cpu 
ops can use a segment prefix and thus the insructions can calculate the 
per cpu adress and perform the atomic inc without disabling preempt or 
interrupts.

local_t is only useful when you disable interrupt or premption otherwise. 
But then you could also use a regular increment.

> > But then its related to percpu operations and relies extensively on the 
> > various percpu.h files in asm-generic and asm-arch and include/linux
> 
> Well that should be fixed.  We should never have mixed the
> alloc_percpu() and DEFINE_PER_CPU things inthe same header.  They're
> different.

With cpu_alloc they are the same. They allocate from the same per cpu 
area.