From: Yury Norov [NVIDIA] <yury.norov@gmail.com>

The function helps to implement cpumask_andnot() APIs.

Signed-off-by: Yury Norov [NVIDIA] <yury.norov@gmail.com>
---
 include/linux/find.h | 25 +++++++++++++++++++++++++
 lib/find_bit.c       | 11 +++++++++++
 2 files changed, 36 insertions(+)

diff --git a/include/linux/find.h b/include/linux/find.h
index 68685714bc18..d1578cfb667c 100644
--- a/include/linux/find.h
+++ b/include/linux/find.h
@@ -29,6 +29,8 @@ unsigned long __find_nth_and_andnot_bit(const unsigned long *addr1, const unsign
 					unsigned long n);
 extern unsigned long _find_first_and_bit(const unsigned long *addr1,
 					 const unsigned long *addr2, unsigned long size);
+unsigned long _find_first_andnot_bit(const unsigned long *addr1, const unsigned long *addr2,
+				 unsigned long size);
 unsigned long _find_first_and_and_bit(const unsigned long *addr1, const unsigned long *addr2,
 				      const unsigned long *addr3, unsigned long size);
 extern unsigned long _find_first_zero_bit(const unsigned long *addr, unsigned long size);
@@ -347,6 +349,29 @@ unsigned long find_first_and_bit(const unsigned long *addr1,
 }
 #endif
 
+/**
+ * find_first_andnot_bit - find the first bit set in 1st memory region and unset in 2nd
+ * @addr1: The first address to base the search on
+ * @addr2: The second address to base the search on
+ * @size: The bitmap size in bits
+ *
+ * Returns the bit number for the matched bit
+ * If no bits are set, returns >= @size.
+ */
+static __always_inline
+unsigned long find_first_andnot_bit(const unsigned long *addr1,
+				 const unsigned long *addr2,
+				 unsigned long size)
+{
+	if (small_const_nbits(size)) {
+		unsigned long val = *addr1 & (~*addr2) & GENMASK(size - 1, 0);
+
+		return val ? __ffs(val) : size;
+	}
+
+	return _find_first_andnot_bit(addr1, addr2, size);
+}
+
 /**
  * find_first_and_and_bit - find the first set bit in 3 memory regions
  * @addr1: The first address to base the search on
diff --git a/lib/find_bit.c b/lib/find_bit.c
index 0836bb3d76c5..06b6342aa3ae 100644
--- a/lib/find_bit.c
+++ b/lib/find_bit.c
@@ -116,6 +116,17 @@ unsigned long _find_first_and_bit(const unsigned long *addr1,
 EXPORT_SYMBOL(_find_first_and_bit);
 #endif
 
+/*
+ * Find the first bit set in 1st memory region and unset in 2nd.
+ */
+unsigned long _find_first_andnot_bit(const unsigned long *addr1,
+				  const unsigned long *addr2,
+				  unsigned long size)
+{
+	return FIND_FIRST_BIT(addr1[idx] & ~addr2[idx], /* nop */, size);
+}
+EXPORT_SYMBOL(_find_first_andnot_bit);
+
 /*
  * Find the first set bit in three memory regions.
  */
-- 
2.43.0

Hi Yury,

On 4/7/25 8:38 AM, Yury Norov wrote:
> From: Yury Norov [NVIDIA] <yury.norov@gmail.com>
> 
> The function helps to implement cpumask_andnot() APIs.
> 
> Signed-off-by: Yury Norov [NVIDIA] <yury.norov@gmail.com>
> ---
>  include/linux/find.h | 25 +++++++++++++++++++++++++
>  lib/find_bit.c       | 11 +++++++++++
>  2 files changed, 36 insertions(+)
> 
> diff --git a/include/linux/find.h b/include/linux/find.h
> index 68685714bc18..d1578cfb667c 100644
> --- a/include/linux/find.h
> +++ b/include/linux/find.h
> @@ -29,6 +29,8 @@ unsigned long __find_nth_and_andnot_bit(const unsigned long *addr1, const unsign
>  					unsigned long n);
>  extern unsigned long _find_first_and_bit(const unsigned long *addr1,
>  					 const unsigned long *addr2, unsigned long size);
> +unsigned long _find_first_andnot_bit(const unsigned long *addr1, const unsigned long *addr2,
> +				 unsigned long size);
>  unsigned long _find_first_and_and_bit(const unsigned long *addr1, const unsigned long *addr2,
>  				      const unsigned long *addr3, unsigned long size);
>  extern unsigned long _find_first_zero_bit(const unsigned long *addr, unsigned long size);
> @@ -347,6 +349,29 @@ unsigned long find_first_and_bit(const unsigned long *addr1,
>  }
>  #endif
>  
> +/**
> + * find_first_andnot_bit - find the first bit set in 1st memory region and unset in 2nd
> + * @addr1: The first address to base the search on
> + * @addr2: The second address to base the search on
> + * @size: The bitmap size in bits
> + *
> + * Returns the bit number for the matched bit
> + * If no bits are set, returns >= @size.

Should this be "If no bits are set, returns @size." to match similar document
snippets as well as what the code does?

I am not familiar with the customs of this area but I did notice that
this patch triggers some checkpatch.pl warnings about alignment not
matching open parenthesis ... but looking at the existing content of this
file this custom does not seem to apply here.

Reinette

On Wed, Apr 23, 2025 at 02:28:26PM -0700, Reinette Chatre wrote:
> Hi Yury,
> 
> On 4/7/25 8:38 AM, Yury Norov wrote:
> > From: Yury Norov [NVIDIA] <yury.norov@gmail.com>
> > 
> > The function helps to implement cpumask_andnot() APIs.
> > 
> > Signed-off-by: Yury Norov [NVIDIA] <yury.norov@gmail.com>
> > ---
> >  include/linux/find.h | 25 +++++++++++++++++++++++++
> >  lib/find_bit.c       | 11 +++++++++++
> >  2 files changed, 36 insertions(+)
> > 
> > diff --git a/include/linux/find.h b/include/linux/find.h
> > index 68685714bc18..d1578cfb667c 100644
> > --- a/include/linux/find.h
> > +++ b/include/linux/find.h
> > @@ -29,6 +29,8 @@ unsigned long __find_nth_and_andnot_bit(const unsigned long *addr1, const unsign
> >  					unsigned long n);
> >  extern unsigned long _find_first_and_bit(const unsigned long *addr1,
> >  					 const unsigned long *addr2, unsigned long size);
> > +unsigned long _find_first_andnot_bit(const unsigned long *addr1, const unsigned long *addr2,
> > +				 unsigned long size);
> >  unsigned long _find_first_and_and_bit(const unsigned long *addr1, const unsigned long *addr2,
> >  				      const unsigned long *addr3, unsigned long size);
> >  extern unsigned long _find_first_zero_bit(const unsigned long *addr, unsigned long size);
> > @@ -347,6 +349,29 @@ unsigned long find_first_and_bit(const unsigned long *addr1,
> >  }
> >  #endif
> >  
> > +/**
> > + * find_first_andnot_bit - find the first bit set in 1st memory region and unset in 2nd
> > + * @addr1: The first address to base the search on
> > + * @addr2: The second address to base the search on
> > + * @size: The bitmap size in bits
> > + *
> > + * Returns the bit number for the matched bit
> > + * If no bits are set, returns >= @size.
> 
> Should this be "If no bits are set, returns @size." to match similar document
> snippets as well as what the code does?
> 
> I am not familiar with the customs of this area but I did notice that
> this patch triggers some checkpatch.pl warnings about alignment not
> matching open parenthesis ... but looking at the existing content of this
> file this custom does not seem to apply here.

It was a really unclever decision to make find_bit() API returning
the size exactly, because this ends up with a few extra instructions
that we can avoid. For example, in FIND_FIRST_BIT() we have to return

        min(idx * BITS_PER_LONG + __ffs(MUNGE(val)), sz);

instead of just

        idx * BITS_PER_LONG + __ffs(MUNGE(val)

Unfortunately, the existing codebase relies on this overly strict
guarantee. For the new API, I encourage people to guarantee '>= size',
not '== size'. There's no difference from practical perspective. But
if one day somebody will decide to clean this up, there will be less 
burden to rework.