linux-kernel - Re: [PATCH] percpu: add optimized generic percpu accessors

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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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