Re: [PATCH 3/4] cpuset: Fix unchecked calls to NODEMASK_ALLOC()

[Li Zefan]

Paul Menage wrote:
> On Thu, Feb 17, 2011 at 2:46 PM, Andrew Morton
> <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
>> On Thu, 17 Feb 2011 09:50:09 +0800
>> Li Zefan <lizf@xxxxxxxxxxxxxx> wrote:
>>
>>> +/*
>>> + * In functions that can't propogate errno to users, to avoid declaring a
>>> + * nodemask_t variable, and avoid using NODEMASK_ALLOC that can return
>>> + * -ENOMEM, we use this global cpuset_mems.
>>> + *
>>> + * It should be used with cgroup_lock held.
>>
>> I'll do s/should/must/ - that would be a nasty bug.
>>
>> I'd be more comfortable about the maintainability of this optimisation
>> if we had
>>
>>        WARN_ON(!cgroup_is_locked());
>>
>> at each site.
>>
> 
> Agreed - that was my first thought on reading the patch. How about:
> 
> static nodemask_t *cpuset_static_nodemask() {

Then this should be 'noinline', otherwise we'll have one copy for each
function that calls it.

>   static nodemask_t nodemask;
>   WARN_ON(!cgroup_is_locked());
>   return &nodemask;
> }
> 
> and then just call cpuset_static_nodemask() in the various locations
> being patched?
> 

I think a defect of this is people might call it twice in one function
but don't know it returns the same variable?

For example in cpuset_attach():

void cpuset_attach(...)
{
	nodemask_t *from = cpuset_static_nodemask();
	nodemask_t *to = cpuset_static_nodemask();
	...
}
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