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

Yury Norov
2026-03-12T23:17:33Z

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;

None of them explicitly tweak their code for a word length, or x == 0.

Requesting comments from the corresponding maintainers on how to proceed
with this.

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> (hyperv)
CC: Haiyang Zhang <haiyangz@microsoft.com> (hyperv)
CC: Jason Gunthorpe <jgg@ziepe.ca> (infiniband)
CC: Leon Romanovsky <leon@kernel.org> (infiniband)
CC: Mark Brown <broonie@kernel.org> (spi)
CC: Steve French <sfrench@samba.org> (smb)
CC: Alexander Graf <graf@amazon.com> (kexec)
CC: Mike Rapoport <rppt@kernel.org> (kexec)
CC: Pasha Tatashin <pasha.tatashin@soleen.com> (kexec)
Signed-off-by: Yury Norov <ynorov@nvidia.com>
---
include/linux/count_zeros.h | 9 +++------
1 file changed, 3 insertions(+), 6 deletions(-)

diff --git a/include/linux/count_zeros.h b/include/linux/count_zeros.h
index 4e5680327ece..5034a30b5c7c 100644
--- a/include/linux/count_zeros.h
+++ b/include/linux/count_zeros.h
@@ -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_ */
--
2.43.0