Re: [PATCH] cpumask: alloc blank cpumask left over

Avi Kivity
2009-06-06T09:24:18Z

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@kernel.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.

--
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.