linux-kernel - Re: [PATCH] linux/bitmap.h: fix BITMAP_LAST_WORD

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Date:   Tue, 07 Aug 2018 15:15:18 +0800
From:   Wei Wang <wei.w.wang@...el.com>
To:     Rasmus Villemoes <linux@...musvillemoes.dk>,
        Yury Norov <ynorov@...iumnetworks.com>
CC:     linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
        corbet@....net, dgilbert@...hat.com,
        Andy Shevchenko <andy.shevchenko@...il.com>
Subject: Re: [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