linux-kernel - Re: [PATCH] local

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Message-ID: <20070109223826.GA6555@elf.ucw.cz>
Date:	Tue, 9 Jan 2007 23:38:26 +0100
From:	Pavel Machek <pavel@....cz>
To:	Andrew Morton <akpm@...l.org>
Cc:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
	Greg Kroah-Hartman <gregkh@...e.de>,
	Christoph Hellwig <hch@...radead.org>, ltt-dev@...fik.org,
	systemtap@...rces.redhat.com,
	Douglas Niehaus <niehaus@...s.ku.edu>,
	"Martin J. Bligh" <mbligh@...igh.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] local_t : Documentation

On Tue 2007-01-09 13:01:10, Andrew Morton wrote:
> On Mon, 8 Jan 2007 22:14:46 -0500
> Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca> wrote:
> 
> > +* How to use local atomic operations
> > +
> > +#include <linux/percpu.h>
> > +#include <asm/local.h>
> > +
> > +static DEFINE_PER_CPU(local_t, counters) = LOCAL_INIT(0);
> > +
> > +
> > +* Counting
> > +
> > +In preemptible context, use get_cpu_var() and put_cpu_var() around local atomic
> > +operations : it makes sure that preemption is disabled around write access to
> > +the per cpu variable. For instance :
> > +
> > +	local_inc(&get_cpu_var(counters));
> > +	put_cpu_var(counters);
> 
> Confused.  The whole point behind local_t is that we can do
> atomic-wrt-interrupts inc and dec on them.

Could we get this short of two line description into the Doc/ file? It
talks about how to implement them, mentions LOCK prefixes unlikely to
be present on non-i386, but does not tell me what they guarantee...
								Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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