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

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Date:	Thu, 29 May 2008 22:38:25 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Christoph Lameter <clameter@....com>
Cc:	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>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Mike Travis <travis@....com>
Subject: Re: [patch 04/41] cpu ops: Core piece for generic atomic per cpu
 operations

On Thu, 29 May 2008 22:17:55 -0700 (PDT) Christoph Lameter <clameter@....com> wrote:

> > > local_t is also very specific to the x86 processor.
> > 
> > And alpha, m32r, mips and powerpc, methinks.  Probably others, but
> > people just haven't got around to it.
> 
> No local_t does not do the relocation of the address to the correct percpu 
> 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.

> > I think I'll need to come back another time to understand all that ;)
> > 
> > Thanks for writing it up carefully.
> 
> Well this stuff is so large in scope that I have difficulties keeping 
> everything straight.
> 
> > I wonder if all this stuff should be in a new header file.
> > 
> > We could get lazy and include that header from percpu.h if needed.
> 
> 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.

otoh as you propose removing the old alloc_percpu() I guess the end
result is no worse than what we presently have.

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