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SubjectRe: [PATCH] linux/bitmap.h: fix BITMAP_LAST_WORD_MASK
On 08/07/2018 03:03 PM, Wei Wang wrote:
> On 08/07/2018 07:30 AM, Rasmus Villemoes wrote:
>> On 2018-07-26 12:15, Wei Wang wrote:
>>> On 07/26/2018 05:37 PM, Yury Norov wrote:
>>>> On Thu, Jul 26, 2018 at 04:07:51PM +0800, Wei Wang wrote:
>>>>> The existing BITMAP_LAST_WORD_MASK macro returns 0xffffffff if
>>>>> nbits is
>>>>> 0. This patch changes the macro to return 0 when there is no bit
>>>>> needs to
>>>>> be masked.
>>>> I think this is intentional behavour. Previous version did return ~0UL
>>>> explicitly in this case. See patch 89c1e79eb3023 (linux/bitmap.h:
>>>> improve
>>>> BITMAP_{LAST,FIRST}_WORD_MASK) from Rasmus.
>>> Yes, I saw that. But it seems confusing for the corner case that
>>> nbits=0
>>> (no bits to mask), the macro returns with all the bits set.
>>>
>>>
>>>> Introducing conditional branch would affect performance. All existing
>>>> code checks nbits for 0 before handling last word where needed
>>>> explicitly. So I think we'd better change nothing here.
>>> I think that didn't save the conditional branch essentially, because
>>> it's just moved from inside this macro to the caller as you mentioned.
>>> If callers missed the check for some reason and passed 0 to the macro,
>>> they will get something unexpected.
>>>
>>> Current callers like __bitmap_weight, __bitmap_equal, and others,
>>> they have
>>>
>>> if (bits % BITS_PER_LONG)
>>> w += hweight_long(bitmap[k] & BITMAP_LAST_WORD_MASK(bits));
>>>
>>> we could remove the "if" check by "w += hweight_long(bitmap[k] &
>>> BITMAP_LAST_WORD_MASK(bits % BITS_PER_LONG));" the branch is the same.
>> Absolutely not! That would access bitmap[lim] (the final value of the k
>> variable) despite that word not being part of the bitmap.
>
> Probably it's more clear to post the entire function here for a
> discussion:
>
> int __bitmap_weight(const unsigned long *bitmap, unsigned int bits)
> {
> unsigned int k, lim = bits/BITS_PER_LONG;
> int w = 0;
>
> for (k = 0; k < lim; k++)
> w += hweight_long(bitmap[k]);
>
> if (bits % BITS_PER_LONG)
> ==> w += hweight_long(bitmap[k] &
> BITMAP_LAST_WORD_MASK(bits));
>
> return w;
> }
>
> When the execution reaches "==>", isn't "k=lim"?

And accessing to bitmap[lim] which does not exist should be a case
considered by the caller rather than the macro. For example, with
"BITMAP_LAST_WORD_MASK(bits) & bitmap[k]", making
BITMAP_LAST_WORD_MASK(0) be 0 will not be a problem.
Anyway, my point is that we could make the macro itself robust.

Best,
Wei

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