Re: [PATCH 01 of 12] Core of mmu notifiers

Eric Dumazet
2008-04-22T15:37:38Z

Andrea Arcangeli a ecrit :
> On Tue, Apr 22, 2008 at 04:56:10PM +0200, Eric Dumazet wrote:
>
>> Andrea Arcangeli a ecrit :
>>
>>> +
>>> +static int mm_lock_cmp(const void *a, const void *b)
>>> +{
>>> + cond_resched();
>>> + if ((unsigned long)*(spinlock_t **)a <
>>> + (unsigned long)*(spinlock_t **)b)
>>> + return -1;
>>> + else if (a == b)
>>> + return 0;
>>> + else
>>> + return 1;
>>> +}
>>> +
>>>
>> This compare function looks unusual...
>> It should work, but sort() could be faster if the
>> if (a == b) test had a chance to be true eventually...
>>
>
> Hmm, are you saying my mm_lock_cmp won't return 0 if a==b?
>
I am saying your intent was probably to test

else if ((unsigned long)*(spinlock_t **)a ==
(unsigned long)*(spinlock_t **)b)
return 0;

Because a and b are pointers to the data you want to compare. You need
to dereference them.

>> static int mm_lock_cmp(const void *a, const void *b)
>> {
>> unsigned long la = (unsigned long)*(spinlock_t **)a;
>> unsigned long lb = (unsigned long)*(spinlock_t **)b;
>>
>> cond_resched();
>> if (la < lb)
>> return -1;
>> if (la > lb)
>> return 1;
>> return 0;
>> }
>>
>
> If your intent is to use the assumption that there are going to be few
> equal entries, you should have used likely(la > lb) to signal it's
> rarely going to return zero or gcc is likely free to do whatever it
> wants with the above. Overall that function is such a slow path that
> this is going to be lost in the noise. My suggestion would be to defer
> microoptimizations like this after 1/12 will be applied to mainline.
>
> Thanks!
>
>
Hum, it's not a micro-optimization, but a bug fix. :)

Sorry if it was not clear