linux-kernel - Re: [PATCH] local

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Message-Id: <20070109130110.8934c29f.akpm@osdl.org>
Date:	Tue, 9 Jan 2007 13:01:10 -0800
From:	Andrew Morton <akpm@...l.org>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Cc:	Pavel Machek <pavel@...e.cz>, 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 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.

Consequently, as atomic-wrt-interrupts means atomic-wrt-preemption, there
is no need to do a preempt_disable() around local_inc() and local_dec().

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