linux-kernel - Re: [PATCH 4/6] cpuset: don't allocate trial cpuset on stack

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Date:	Mon, 05 Jan 2009 17:13:01 +0800
From:	Li Zefan <lizf@...fujitsu.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	Ingo Molnar <mingo@...e.hu>, Rusty Russell <rusty@...tcorp.com.au>,
	Mike Travis <travis@....com>, Paul Menage <menage@...gle.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 4/6] cpuset: don't allocate trial cpuset on stack

>> -static int update_cpumask(struct cpuset *cs, const char *buf)
>> +static int update_cpumask(struct cpuset *cs, struct cpuset *trialcs,
>> +			  const char *buf)
>>  {
>>  	struct ptr_heap heap;
>> -	struct cpuset trialcs;
>>  	int retval;
>>  	int is_load_balanced;
>>  
>> @@ -891,8 +909,6 @@ static int update_cpumask(struct cpuset *cs, const char *buf)
>>  	if (cs == &top_cpuset)
>>  		return -EACCES;
>>  
>> -	trialcs = *cs;
> 
> Yes, we already do.
> 
> That thing contains spinlocks and list_heads (at least), which cannot
> be copied in this way.
> 
> Seems that we're doing this gross thing because it just so happens that
> we only use the cpus_allowed and mems_allowed fields, and because

and also cpuset->flags.

> several of the called functions require a cpuset*, but only needed a
> cpumask_t.
> 
> How perfectly beastly.
> 

trial cpuset is used to ease the validation of changes to a cpuset. It
can be get rid of, but I'm afraid that the resulting code will be more
complex and less readable.
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