From: Rasmus Villemoes <linux@rasmusvillemoes.dk>
To: Yury Norov <yury.norov@gmail.com>, linux-kernel@vger.kernel.org
Cc: linux-m68k@lists.linux-m68k.org, linux-arch@vger.kernel.org,
linux-sh@vger.kernel.org, Alexey Klimov <aklimov@redhat.com>,
Andrew Morton <akpm@linux-foundation.org>,
Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
Arnd Bergmann <arnd@arndb.de>, David Sterba <dsterba@suse.com>,
Dennis Zhou <dennis@kernel.org>,
Geert Uytterhoeven <geert@linux-m68k.org>,
Jianpeng Ma <jianpeng.ma@intel.com>,
Joe Perches <joe@perches.com>,
John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>,
Josh Poimboeuf <jpoimboe@redhat.com>,
Rich Felker <dalias@libc.org>,
Stefano Brivio <sbrivio@redhat.com>,
Wei Yang <richard.weiyang@linux.alibaba.com>,
Wolfram Sang <wsa+renesas@sang-engineering.com>,
Yoshinori Sato <ysato@users.sourceforge.jp>
Subject: Re: [PATCH 06/14] bitsperlong.h: introduce SMALL_CONST() macro
Date: Fri, 19 Feb 2021 00:07:27 +0100 [thread overview]
Message-ID: <55f1e25a-3211-8247-9dd3-3777e29287db@rasmusvillemoes.dk> (raw)
In-Reply-To: <20210218040512.709186-7-yury.norov@gmail.com>
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?
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).
Rasmus
next prev parent reply other threads:[~2021-02-18 23:08 UTC|newest]
Thread overview: 28+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-02-18 4:04 [PATCH v3 00/14] lib/find_bit: fast path for small bitmaps Yury Norov
2021-02-18 4:04 ` [PATCH 01/14] tools: disable -Wno-type-limits Yury Norov
2021-02-18 4:05 ` [PATCH 02/14] tools: bitmap: sync function declarations with the kernel Yury Norov
2021-02-18 4:05 ` [PATCH 03/14] arch: rearrange headers inclusion order in asm/bitops for m68k and sh Yury Norov
2021-02-18 4:05 ` [PATCH 04/14] lib: introduce BITS_{FIRST,LAST} macro Yury Norov
2021-02-18 22:51 ` Rasmus Villemoes
2021-03-12 4:30 ` Yury Norov
2021-02-18 4:05 ` [PATCH 05/14] tools: sync BITS_MASK macros with the kernel Yury Norov
2021-02-18 4:05 ` [PATCH 06/14] bitsperlong.h: introduce SMALL_CONST() macro Yury Norov
2021-02-18 23:07 ` Rasmus Villemoes [this message]
2021-03-12 5:28 ` Yury Norov
2021-03-12 9:12 ` Rasmus Villemoes
2021-03-12 21:53 ` Yury Norov
2021-02-18 4:05 ` [PATCH 07/14] tools: " Yury Norov
2021-02-18 4:05 ` [PATCH 08/14] lib/Kconfig: introduce FAST_PATH option Yury Norov
2021-02-18 15:15 ` Andy Shevchenko
2021-02-18 19:24 ` Yury Norov
2021-02-19 10:52 ` Andy Shevchenko
2021-02-18 4:05 ` [PATCH 09/14] lib: inline _find_next_bit() wrappers Yury Norov
2021-02-18 4:05 ` [PATCH 10/14] tools: sync find_next_bit implementation Yury Norov
2021-02-18 4:05 ` [PATCH 11/14] lib: add fast path for find_next_*_bit() Yury Norov
2021-02-18 15:24 ` Andy Shevchenko
2021-02-18 4:05 ` [PATCH 12/14] lib: add fast path for find_first_*_bit() and find_last_bit() Yury Norov
2021-02-18 4:05 ` [PATCH 13/14] tools: sync lib/find_bit implementation Yury Norov
2021-02-18 4:05 ` [PATCH 14/14] MAINTAINERS: Add entry for the bitmap API Yury Norov
2021-02-18 15:28 ` Andy Shevchenko
2021-02-18 15:34 ` Yury Norov
2021-03-12 9:15 ` Rasmus Villemoes
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