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

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Date:   Fri, 27 Jul 2018 10:13:45 +0800
From:   Wei Wang <wei.w.wang@...el.com>
To:     Yury Norov <ynorov@...iumnetworks.com>
CC:     linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
        corbet@....net, linux@...musvillemoes.dk, dgilbert@...hat.com,
        Andy Shevchenko <andy.shevchenko@...il.com>
Subject: Re: [PATCH] linux/bitmap.h: fix BITMAP_LAST_WORD_MASK

On 07/26/2018 08:10 PM, Yury Norov wrote:
> On Thu, Jul 26, 2018 at 06:15:59PM +0800, Wei Wang wrote:
>> External Email
>>
>> 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.
> But your patch doesn't remove external conditional, and it fact
> introduces overhead, right? Also, in some cases it's not so trivial to
> remove it. Consider __bitmap_intersects() for example.
>
> Anyway, this patch changes the very basic API. In that case you should
> check every user of the macro to be safe against the change, including
> possible performance downsides.
>
> If you find this corner case behavior of macro confusing, I think that
> the better option would be introducing detailed comment to the
> BITMAP_LAST_WORD_MASK(), or writing wrapper around it that handles
> nbits == 0 as you expect.
>

OK. Thanks Yury and Andy for the discussion. It seems the more preferred 
way is just to add comments as a note. Agree with that.

Best,
Wei