Re: [PATCH 06/14] bitsperlong.h: introduce SMALL_CONST() macro

Yury Norov
2021-03-12T05:29:11Z

On Fri, Feb 19, 2021 at 12:07:27AM +0100, Rasmus Villemoes wrote:
> On 18/02/2021 05.05, Yury Norov wrote:
> > Many algorithms become simpler if they are passed with relatively small
> > input values. One example is bitmap operations when the whole bitmap fits
> > into one word. To implement such simplifications, linux/bitmap.h declares
> > small_const_nbits() macro.
> >
> > Other subsystems may also benefit from optimizations of this sort, like
> > find_bit API in the following patches. So it looks helpful to generalize
> > the macro and extend it's visibility.
>
> Perhaps, but SMALL_CONST is too generic a name, it needs to keep "bits"
> somewhere in there. So why not just keep it at small_const_nbits?
>
> > Signed-off-by: Yury Norov <yury.norov@gmail.com>
> > ---
> > include/asm-generic/bitsperlong.h | 2 ++
> > include/linux/bitmap.h | 33 ++++++++++++++-----------------
> > 2 files changed, 17 insertions(+), 18 deletions(-)
> >
> > diff --git a/include/asm-generic/bitsperlong.h b/include/asm-generic/bitsperlong.h
> > index 3905c1c93dc2..0eeb77544f1d 100644
> > --- a/include/asm-generic/bitsperlong.h
> > +++ b/include/asm-generic/bitsperlong.h
> > @@ -23,4 +23,6 @@
> > #define BITS_PER_LONG_LONG 64
> > #endif
> >
> > +#define SMALL_CONST(n) (__builtin_constant_p(n) && (unsigned long)(n) < BITS_PER_LONG)
> > +
> > #endif /* __ASM_GENERIC_BITS_PER_LONG */
> > diff --git a/include/linux/bitmap.h b/include/linux/bitmap.h
> > index adf7bd9f0467..e89f1dace846 100644
> > --- a/include/linux/bitmap.h
> > +++ b/include/linux/bitmap.h
> > @@ -224,9 +224,6 @@ extern int bitmap_print_to_pagebuf(bool list, char *buf,
> > * so make such users (should any ever turn up) call the out-of-line
> > * versions.
> > */
> > -#define small_const_nbits(nbits) \
> > - (__builtin_constant_p(nbits) && (nbits) <= BITS_PER_LONG && (nbits) > 0)
> > -
> > static inline void bitmap_zero(unsigned long *dst, unsigned int nbits)
> > {
> > unsigned int len = BITS_TO_LONGS(nbits) * sizeof(unsigned long);
> > @@ -278,7 +275,7 @@ extern void bitmap_to_arr32(u32 *buf, const unsigned long *bitmap,
> > static inline int bitmap_and(unsigned long *dst, const unsigned long *src1,
> > const unsigned long *src2, unsigned int nbits)
> > {
> > - if (small_const_nbits(nbits))
> > + if (SMALL_CONST(nbits - 1))
>
> Please don't force most users to be changed to something less readable.
> What's wrong with just keeping small_const_nbits() the way it is,
> avoiding all this churn and keeping the readability?

The wrong thing is that it's defined in include/linux/bitmap.h, and I
cannot use it in include/asm-generic/bitops/find.h, so I have to either
move it to a separate header, or generalize and share with find.h and
other users this way. I prefer the latter option, thougt it's more
verbose.

> At a quick reading, one of the very few places where you end up not
> passing nbits-1 but just nbits is this
>
> unsigned long find_next_zero_bit_le(const void *addr, unsigned
> long size, unsigned long offset)
> {
> + if (SMALL_CONST(size)) {
> + unsigned long val = *(const unsigned long *)addr;
> +
> + if (unlikely(offset >= size))
> + return size;
>
> which is a regression, for much the same reason the nbits==0 case was
> excluded from small_const_nbits in the first place. If size is 0, we
> used to just return 0 early in _find_next_bit. But you've introduced a
> dereference of addr before that check is now done, which is
> theoretically an oops.
>
> If find_next_zero_bit_le cannot handle nbits==BITS_PER_LONG efficiently
> but requires one off-limits bit position, fine, so be it, add an extra
> "small_const_nbits() && nbits < BITS_PER_LONG" (and a comment).

Sure, it's my bad. I need to fix it.

What would you prefer, moving the small_const_nbits() to a separate
header, or introduce something like
#define small_const_size(n) SMALL_CONST((n) - 1)

to avoid mistakes like this? I think small_const_size() is better
because it might be potentially useful for someone else, but I can
go either way.