linux-kernel - Re: [PATCH] local

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Message-ID: <20070109220616.GA30535@Krystal>
Date:	Tue, 9 Jan 2007 17:06:16 -0500
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Andrew Morton <akpm@...l.org>
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

* Andrew Morton (akpm@...l.org) 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.
> 
> Consequently, as atomic-wrt-interrupts means atomic-wrt-preemption, there
> is no need to do a preempt_disable() around local_inc() and local_dec().
> 

Hi Andrew,

Not exactly : the increment operation is atomic, but not the selection of the
local variable. local_inc(&__get_cpu_var()) implies the following sequence 
of operations :

1 - Get the variable copy corresponding to the currently running CPU.
2 - atomically increment the variable.

It would be wrong to be scheduled on another CPU between 1 and 2, because the
atomic increment should only be done by the CPU "owner" of the local variable,
as the local atomic increment is not atomic wrt other CPUs.

Mathieu

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