linux-kernel - Re: [PATCH] cpumask: alloc blank cpumask left over

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Date:	Sat, 06 Jun 2009 02:36:57 -0700
From:	Yinghai Lu <yinghai@...nel.org>
To:	Avi Kivity <avi@...hat.com>
CC:	Rusty Russell <rusty@...tcorp.com.au>, Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] cpumask: alloc blank cpumask left over

Avi Kivity wrote:
> Rusty Russell wrote:
>> On Fri, 5 Jun 2009 03:26:57 pm Yinghai Lu wrote:
>>  
>>> Rusty Russell wrote:
>>>    
>>>> On Fri, 5 Jun 2009 06:31:31 am Yinghai Lu wrote:
>>>>      
>>>>> avoid suprise when MAXSMP is enabled
>>>>>
>>>>> Signed-off-by: Yinghai Lu <yinghai.lu@...nel.org>
>>>>>         
>>>> I understand the temptation, but two questions arise:
>>>> 1) Shouldn't we actually audit to see if any of these are currently
>>>> problems,
>>>>       
>>> those are defined as static cpumask_var_t, and if MAXSMP is not used,
>>> they
>>> are cleared already
>>>     
>>
>> OK, here's what I've got in my tree.  Ingo, I think this should go in the
>> current -rc to avoid nasty bugs.
>>
>> BTW, the original alloc_cpumask_var did zero; that was dropped after
>> arguments
>> over efficiency and fitting with other interfaces, but I clearly had
>> the old
>> semantics in my head for a while.
>>
>>   
> 
> Using __GFP_ZERO is equivalent to using memset() instead of
> cpumask_clear().  It's better to call cpumask_clear() or provide an API
> to alloc+clear.
> 
> Further, what about the non-MAXSMP case:
> 
> 
>    static inline bool alloc_cpumask_var(cpumask_var_t *mask, gfp_t flags)
>    {
>            return true;
>    }
> 
> 
> We explicity clear on MAXSMP and rely on static initialization for the
> non-MAXSMP, laying a neat trap for anyone who makes the variable
> non-static.  Let's be less subtle that that.

or have zalloc_cpumask_var() ?

YH
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