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" <[email protected]> (hyperv) CC: Haiyang Zhang <[email protected]> (hyperv) CC: Jason Gunthorpe <[email protected]> (infiniband) CC: Leon Romanovsky <[email protected]> (infiniband) CC: Mark Brown <[email protected]> (spi) CC: Steve French <[email protected]> (smb) CC: Alexander Graf <[email protected]> (kexec) CC: Mike Rapoport <[email protected]> (kexec) CC: Pasha Tatashin <[email protected]> (kexec) Signed-off-by: Yury Norov <[email protected]> --- 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
