[PATCH] lib: bitmap: Mute some odd section mismatch warning in xtensa kernel build

* [PATCH] lib: bitmap: Mute some odd section mismatch warning in xtensa kernel build
@ 2021-08-15  3:21 Barry Song
  2021-08-15 10:30 ` Andy Shevchenko
  2021-08-15 15:55 ` Yury Norov
  0 siblings, 2 replies; 4+ messages in thread
From: Barry Song @ 2021-08-15  3:21 UTC (permalink / raw)
  To: gregkh, yury.norov, andriy.shevchenko, linux, linux-kernel
  Cc: linuxarm, Barry Song, kernel test robot, Max Filippov, Andrew Pinski

From: Barry Song <song.bao.hua@hisilicon.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

Another possible way to "fix" the warning is moving the involved
symboms to lib/bitmap.c:

  +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@intel.com>
Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Max Filippov <jcmvbkbc@gmail.com>
Cc: Andrew Pinski <pinskia@gmail.com>
Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92938
Signed-off-by: Barry Song <song.bao.hua@hisilicon.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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