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Message-Id: <20090115093007.4215b4b4.akpm@linux-foundation.org>
Date: Thu, 15 Jan 2009 09:30:07 -0800
From: Andrew Morton <akpm@...ux-foundation.org>
To: Tejun Heo <tj@...nel.org>
Cc: Ingo Molnar <mingo@...e.hu>, "H. Peter Anvin" <hpa@...or.com>,
Brian Gerst <brgerst@...il.com>, ebiederm@...ssion.com,
cl@...ux-foundation.org, rusty@...tcorp.com.au, travis@....com,
linux-kernel@...r.kernel.org, steiner@....com, hugh@...itas.com
Subject: Re: [PATCH] percpu: add optimized generic percpu accessors
On Thu, 15 Jan 2009 22:23:19 +0900 Tejun Heo <tj@...nel.org> wrote:
> --- a/include/asm-generic/percpu.h
> +++ b/include/asm-generic/percpu.h
> @@ -80,4 +80,56 @@ extern void setup_per_cpu_areas(void);
> #define DECLARE_PER_CPU(type, name) extern PER_CPU_ATTRIBUTES \
> __typeof__(type) per_cpu_var(name)
>
> +/*
> + * Optional methods for optimized non-lvalue per-cpu variable access.
> + *
> + * @var can be a percpu variable or a field of it and its size should
> + * equal char, int or long. percpu_read() evaluates to a lvalue and
> + * all others to void.
> + *
> + * These operations are guaranteed to be atomic w.r.t. preemption.
> + * The generic versions use plain get/put_cpu_var(). Archs are
> + * encouraged to implement single-instruction alternatives which don't
> + * require preemption protection.
> + */
> +#ifndef percpu_read
> +# define percpu_read(var) \
> + ({ \
> + typeof(per_cpu_var(var)) __tmp_var__; \
> + __tmp_var__ = get_cpu_var(var); \
> + put_cpu_var(var); \
> + __tmp_var__; \
> + })
> +#endif
I wonder if the preempt_disable()/preempt_enable() in here actually
does anything useful on any architecture.
> +#define __percpu_generic_to_op(var, val, op) \
> +do { \
> + get_cpu_var(var) op val; \
> + put_cpu_var(var); \
> +} while (0)
We'll need it here.
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