Messages in this thread | | | Date | Tue, 07 Aug 2018 15:15:18 +0800 | From | Wei Wang <> | Subject | Re: [PATCH] linux/bitmap.h: fix BITMAP_LAST_WORD_MASK |
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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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