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Message-ID: <YRk47aybKnJIMeV0@yury-ThinkPad>
Date: Sun, 15 Aug 2021 08:55:25 -0700
From: Yury Norov <yury.norov@...il.com>
To: Barry Song <21cnbao@...il.com>
Cc: gregkh@...uxfoundation.org, andriy.shevchenko@...ux.intel.com,
linux@...musvillemoes.dk, linux-kernel@...r.kernel.org,
linuxarm@...wei.com, Barry Song <song.bao.hua@...ilicon.com>,
kernel test robot <lkp@...el.com>,
Max Filippov <jcmvbkbc@...il.com>,
Andrew Pinski <pinskia@...il.com>,
linux-xtensa@...ux-xtensa.org, gcc@....gnu.org,
gcc-bugs@....gnu.org
Subject: Re: [PATCH] lib: bitmap: Mute some odd section mismatch warning in
xtensa kernel build
On Sun, Aug 15, 2021 at 03:21:32PM +1200, Barry Song wrote:
> From: Barry Song <song.bao.hua@...ilicon.com>
>
> Constanly there are some section mismatch issues reported in test_bitmap
> for xtensa platform such as:
>
> Section mismatch in reference from the function bitmap_equal() to the
> variable .init.data:initcall_level_names
> The function bitmap_equal() references the variable __initconst
> __setup_str_initcall_blacklist. This is often because bitmap_equal
> lacks a __initconst annotation or the annotation of
> __setup_str_initcall_blacklist is wrong.
>
> Section mismatch in reference from the function bitmap_copy_clear_tail()
> to the variable .init.rodata:__setup_str_initcall_blacklist
> The function bitmap_copy_clear_tail() references the variable __initconst
> __setup_str_initcall_blacklist.
> This is often because bitmap_copy_clear_tail lacks a __initconst
> annotation or the annotation of __setup_str_initcall_blacklist is wrong.
>
> To be honest, hardly to believe kernel code is wrong since bitmap_equal is
> always called in __init function in test_bitmap.c just like __bitmap_equal.
> But gcc doesn't report any issue for __bitmap_equal even when bitmap_equal
> and __bitmap_equal show in the same function such as:
>
> static void noinline __init test_mem_optimisations(void)
> {
> ...
> for (start = 0; start < 1024; start += 8) {
> for (nbits = 0; nbits < 1024 - start; nbits += 8) {
> if (!bitmap_equal(bmap1, bmap2, 1024)) {
> failed_tests++;
> }
> if (!__bitmap_equal(bmap1, bmap2, 1024)) {
> failed_tests++;
> }
> ...
> }
> }
> }
>
> The different between __bitmap_equal() and bitmap_equal() is that the
> former is extern and a EXPORT_SYMBOL. So noinline, and probably in fact
> noclone. But the later is static and unfortunately not inlined at this
> time though it has a "inline" flag.
>
> bitmap_copy_clear_tail(), on the other hand, seems more innocent as it is
> accessing stack only by its wrapper bitmap_from_arr32() in function
> test_bitmap_arr32():
> static void __init test_bitmap_arr32(void)
> {
> unsigned int nbits, next_bit;
> u32 arr[EXP1_IN_BITS / 32];
> DECLARE_BITMAP(bmap2, EXP1_IN_BITS);
>
> memset(arr, 0xa5, sizeof(arr));
>
> for (nbits = 0; nbits < EXP1_IN_BITS; ++nbits) {
> bitmap_to_arr32(arr, exp1, nbits);
> bitmap_from_arr32(bmap2, arr, nbits);
> expect_eq_bitmap(bmap2, exp1, nbits);
> ...
> }
> }
> Looks like gcc optimized arr, bmap2 things to .init.data but it seems
> nothing is wrong in kernel since test_bitmap_arr32() is __init.
>
> Max Filippov reported a bug to gcc but gcc people don't ack. So here
> this patch removes the involved symbols by forcing inline. It might
> not be that elegant but I don't see any harm as bitmap_equal() and
> bitmap_copy_clear_tail() are both quite small. In addition, kernel
> doc also backs this modification "We don't use the 'inline' keyword
> because it's broken": www.kernel.org/doc/local/inline.html
This is a 2006 article. Are you sure nothing has been changed over the
last 15 years?
> Another possible way to "fix" the warning is moving the involved
> symboms to lib/bitmap.c:
So, it's a GCC issue already reported to GCC? For me it sounds like
nothing to fix in kernel. If I was a GCC developer, I'd prefer to have
all bugs clearly reproducible.
Let's wait for GCC and xtensa people comments. (CC xtensa and GCC
lists)
Yury
> +int bitmap_equal(const unsigned long *src1,
> + const unsigned long *src2, unsigned int nbits)
> +{
> + if (small_const_nbits(nbits))
> + return !((*src1 ^ *src2) & BITMAP_LAST_WORD_MASK(nbits));
> + if (__builtin_constant_p(nbits & BITMAP_MEM_MASK) &&
> + IS_ALIGNED(nbits, BITMAP_MEM_ALIGNMENT))
> + return !memcmp(src1, src2, nbits / 8);
> + return __bitmap_equal(src1, src2, nbits);
> +}
> +EXPORT_SYMBOL(bitmap_equal);
>
> This is harmful to the performance.
>
> Reported-by: kernel test robot <lkp@...el.com>
> Cc: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
> Cc: Max Filippov <jcmvbkbc@...il.com>
> Cc: Andrew Pinski <pinskia@...il.com>
> Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92938
> Signed-off-by: Barry Song <song.bao.hua@...ilicon.com>
> ---
> include/linux/bitmap.h | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/include/linux/bitmap.h b/include/linux/bitmap.h
> index 37f36dad18bd..3eec9f68a0b6 100644
> --- a/include/linux/bitmap.h
> +++ b/include/linux/bitmap.h
> @@ -258,7 +258,7 @@ static inline void bitmap_copy(unsigned long *dst, const unsigned long *src,
> /*
> * Copy bitmap and clear tail bits in last word.
> */
> -static inline void bitmap_copy_clear_tail(unsigned long *dst,
> +static __always_inline void bitmap_copy_clear_tail(unsigned long *dst,
> const unsigned long *src, unsigned int nbits)
> {
> bitmap_copy(dst, src, nbits);
> @@ -334,7 +334,7 @@ static inline void bitmap_complement(unsigned long *dst, const unsigned long *sr
> #endif
> #define BITMAP_MEM_MASK (BITMAP_MEM_ALIGNMENT - 1)
>
> -static inline int bitmap_equal(const unsigned long *src1,
> +static __always_inline int bitmap_equal(const unsigned long *src1,
> const unsigned long *src2, unsigned int nbits)
> {
> if (small_const_nbits(nbits))
> --
> 2.25.1
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