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lib: count_zeros: fix 32/64-bit inconsistency in count_trailing_zeros()

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Based on 'sizeof(x) == 4' condition, in 32-bit case the function is wired
to ffs(), while in 64-bit case to __ffs(). The difference is substantial:
ffs(x) == __ffs(x) + 1. Also, ffs(0) == 0, while __ffs(0) is undefined.

The 32-bit behaviour is inconsistent with the function description, so it
needs to get fixed.

There are 9 individual users for the function in 6 different subsystems.
Some arches and drivers are 64-bit only:
 - arch/loongarch/kvm/intc/eiointc.c;
 - drivers/hv/mshv_vtl_main.c;
 - kernel/liveupdate/kexec_handover.c;

The others are:
 - ib_umem_find_best_pgsz(): as per comment, __ffs() should be correct;
 - rzv2m_csi_reg_write_bit(): ARCH_RENESAS only, unclear;
 - lz77_match_len(): CIFS_COMPRESSION only, unclear, experimental;

IB and CIFS are explicitly OK with the change.

The attached patch gets rid of 32-bit explicit support, so that both
32- and 64-bit versions rely on __ffs().

CC: K. Y. Srinivasan <kys@microsoft.com>
CC: Haiyang Zhang <haiyangz@microsoft.com>
CC: Mark Brown <broonie@kernel.org>
CC: Steve French <sfrench@samba.org>
CC: Alexander Graf <graf@amazon.com>
CC: Mike Rapoport <rppt@kernel.org>
CC: Pasha Tatashin <pasha.tatashin@soleen.com>
Acked-by: Leon Romanovsky <leon@kernel.org>
Signed-off-by: Yury Norov <ynorov@nvidia.com>
This commit is contained in:
Yury Norov
2026-03-12 19:08:16 -04:00
parent 18c4889965
commit 4e64c91b81
1 changed files with 3 additions and 6 deletions
Download Patch File Download Diff File Expand all files Collapse all files

9
include/linux/count_zeros.h
View File

@@ -10,6 +10,8 @@
#include <asm/bitops.h>
#define COUNT_TRAILING_ZEROS_0 (-1)
/**
* count_leading_zeros - Count the number of zeros from the MSB back
* @x: The value
@@ -40,12 +42,7 @@ static inline int count_leading_zeros(unsigned long x)
*/
static inline int count_trailing_zeros(unsigned long x)
{
#define COUNT_TRAILING_ZEROS_0 (-1)
if (sizeof(x) == 4)
return ffs(x);
else
return (x != 0) ? __ffs(x) : COUNT_TRAILING_ZEROS_0;
return (x != 0) ? __ffs(x) : COUNT_TRAILING_ZEROS_0;
}
#endif /* _LINUX_BITOPS_COUNT_ZEROS_H_ */