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[216.228.112.22]) by smtp.gmail.com with ESMTPSA id e24sm22761599pfn.8.2021.10.26.12.28.51 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 26 Oct 2021 12:28:51 -0700 (PDT) Date: Tue, 26 Oct 2021 12:28:48 -0700 From: Yury Norov To: Andy Shevchenko Cc: Qian Cai , Rasmus Villemoes , linux-kernel@vger.kernel.org Subject: Re: [PATCH] bitmap: simplify GENMASK(size - 1, 0) lines Message-ID: References: <20211026144108.35373-1-quic_qiancai@quicinc.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Oct 26, 2021 at 10:21:58PM +0300, Andy Shevchenko wrote: > On Tue, Oct 26, 2021 at 11:54:16AM -0700, Yury Norov wrote: > > On Tue, Oct 26, 2021 at 10:41:08AM -0400, Qian Cai wrote: > > > Since "size" is an "unsigned int", the rvalue "size - 1" will still be > > > "unsigned int" according to the C standard (3.2.1.5 Usual arithmetic > > > conversions). Therefore, GENMASK(size - 1, 0) will always return 0UL. Those > > > are also caught by GCC (W=2): > > > > > > ./include/linux/find.h: In function 'find_first_bit': > > > ./include/linux/bits.h:25:22: warning: comparison of unsigned expression in '< 0' is always false [-Wtype-limits] > > > 25 | __is_constexpr((l) > (h)), (l) > (h), 0))) > > > | ^ > > > ./include/linux/build_bug.h:16:62: note: in definition of macro 'BUILD_BUG_ON_ZERO' > > > 16 | #define BUILD_BUG_ON_ZERO(e) ((int)(sizeof(struct { int:(-!!(e)); }))) > > > | ^ > > > ./include/linux/bits.h:25:3: note: in expansion of macro '__is_constexpr' > > > 25 | __is_constexpr((l) > (h)), (l) > (h), 0))) > > > | ^~~~~~~~~~~~~~ > > > ./include/linux/bits.h:38:3: note: in expansion of macro 'GENMASK_INPUT_CHECK' > > > 38 | (GENMASK_INPUT_CHECK(h, l) + __GENMASK(h, l)) > > > | ^~~~~~~~~~~~~~~~~~~ > > > ./include/linux/find.h:119:31: note: in expansion of macro 'GENMASK' > > > 119 | unsigned long val = *addr & GENMASK(size - 1, 0); > > > | ^~~~~~~ > > > ./include/linux/bits.h:25:34: warning: comparison of unsigned expression in '< 0' is always false [-Wtype-limits] > > > 25 | __is_constexpr((l) > (h)), (l) > (h), 0))) > > > | ^ > > > ./include/linux/build_bug.h:16:62: note: in definition of macro 'BUILD_BUG_ON_ZERO' > > > 16 | #define BUILD_BUG_ON_ZERO(e) ((int)(sizeof(struct { int:(-!!(e)); }))) > > > | ^ > > > ./include/linux/bits.h:38:3: note: in expansion of macro 'GENMASK_INPUT_CHECK' > > > 38 | (GENMASK_INPUT_CHECK(h, l) + __GENMASK(h, l)) > > > | ^~~~~~~~~~~~~~~~~~~ > > > ./include/linux/find.h:119:31: note: in expansion of macro 'GENMASK' > > > 119 | unsigned long val = *addr & GENMASK(size - 1, 0); > > > | ^~~~~~~ > > > > > > Signed-off-by: Qian Cai > > > --- > > > include/linux/find.h | 28 ++++++++-------------------- > > > 1 file changed, 8 insertions(+), 20 deletions(-) > > > > > > diff --git a/include/linux/find.h b/include/linux/find.h > > > index 5bb6db213bcb..5ce2b17aea42 100644 > > > --- a/include/linux/find.h > > > +++ b/include/linux/find.h > > > @@ -115,11 +115,8 @@ unsigned long find_next_zero_bit(const unsigned long *addr, unsigned long size, > > > static inline > > > unsigned long find_first_bit(const unsigned long *addr, unsigned long size) > > > { > > > - if (small_const_nbits(size)) { > > > - unsigned long val = *addr & GENMASK(size - 1, 0); > > > - > > > - return val ? __ffs(val) : size; > > > - } > > > + if (small_const_nbits(size)) > > > + return size; > > > > > > return _find_first_bit(addr, size); > > > } > > > > [...] > > > > Nice catch! I'm a bit concerned that small_const_nbits() will never > > allow GENMASK() to be passed with size == 0, but the patch looks > > good to me overall. > > Can you explain to me how it is supposed to work? > > For example, > > x = 0xaa55; > size = 5; > > printf("%lu\n", find_first_bit(&x, size)); > > In the resulting code we will always have 5 as the result, > but is it correct one? I think it would work really bad and fail to load the kernel for many systems, especially those with NR_CPUS == 64 or less. That's why I think Apr 1 branch is a good place for it.