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

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Date:	Fri, 16 Jan 2009 06:51:03 +0900
From:	Tejun Heo <tj@...nel.org>
To:	roel kluin <roel.kluin@...il.com>
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, akpm@...ux-foundation.org,
	steiner@....com, hugh@...itas.com
Subject: Re: [patch] add optimized generic percpu accessors

Hello, Roel.

roel kluin wrote:
>> +#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'm sorry for your eyes, but since var occurs twice, isn't it better to do:
> 
> # define percpu_read(var)                                              \
>   ({                                                                   \
>        typeof(var) __pcpu_read_var__ = var;
>                \
>        typeof(per_cpu_var(__pcpu_read_var__)) __tmp_var__;                    \
>        __tmp_var__ = get_cpu_var(__pcpu_read_var__);                           \
>        put_cpu_var(__pcpu_read_var__);                                         \
>        __tmp_var__;                                                    \
>   })
> 
>> +
>> +#define __percpu_generic_to_op(var, val, op)                           \
>> +do {                                                                   \
>> +       get_cpu_var(var) op val;                                        \
>> +       put_cpu_var(var);                                               \
>> +} while (0)
> 
> and:
> 
> #define __percpu_generic_to_op(var, val, op)                           \
> do {                                                                   \
>        typeof(var) __pcpu_gto_var__ = var;
>               \
>        get_cpu_var(__pcpu_gto_var__) op val;
>              \
>        put_cpu_var(__pcpu_gto_var__);
>              \
> } while (0)

@var has to be simple identifier as it ends up getting concatenated to
a string.  There's even a check for it in get_cpu_var() macro.  Please
also note that lack of any protecting ()'s around @var for the same
reason.  So, basically, typeof(var) just doesn't exist.

Thanks.

-- 
tejun
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