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

Yury Norov
2026-03-18T16:47:39Z

On Tue, Mar 17, 2026 at 11:14:11AM +0200, Leon Romanovsky wrote:
> On Fri, Mar 13, 2026 at 02:14:49PM -0400, Yury Norov wrote:
> > On Fri, Mar 13, 2026 at 02:18:55PM -0300, Jason Gunthorpe wrote:
> > > On Thu, Mar 12, 2026 at 07:08:16PM -0400, Yury Norov wrote:
> > > > 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;
> > >
> > > So long as 32 bit works the same as 64 bit it is correct for ib
> >
> > This is what the patch does, except that it doesn't account for the
> > word length. In you case, 'mask' is dma_addr_t, which is u32 or u64
> > depending ARCH_DMA_ADDR_T_64BIT.
> >
> > This config is:
> >
> > config ARCH_DMA_ADDR_T_64BIT
> > def_bool 64BIT || PHYS_ADDR_T_64BIT
> >
> > And PHYS_ADDR_T_64BIT is simply def_bool 64BIT. So, at least now
> > dma_addr_t simply follows unsigned long, and thus, the patch is
> > correct. But IDK what's the history behind this configurations.
> >
> > Anyways, the patch aligns 32-bit count_trailing_zeros() with the
> > 64-bit one. If you OK with that, as you said, can you please send
> > an explicit ack?
>
> I can do that, 32 bits architectures are rarely used in the IB world.
>
> Thanks,
> Acked-by: Leon Romanovsky <leon@kernel.org>

Thanks, Leon. Seemingly no headwinds for the patch. Taking in in -next
for testing.

Thanks,
Yury