The kernel already has non-atomic bitmap_complement(), but
no direct atomic implementation.

According to Intel and AMD manuals, the NOT instruction
supports a LOCK prefix.

Currently the only way to achieve this is arch_atomic_xor(-1, v),
which is non-obvious and hurts readability, so add arch_atomic_not()
to provide an explicit atomic complement operation.

Signed-off-by: Ziran Zhang <zhangcoder@yeah.net>
---
 arch/x86/include/asm/atomic.h | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/arch/x86/include/asm/atomic.h b/arch/x86/include/asm/atomic.h
index 75743f1df..d7e543194 100644
--- a/arch/x86/include/asm/atomic.h
+++ b/arch/x86/include/asm/atomic.h
@@ -168,6 +168,13 @@ static __always_inline int arch_atomic_fetch_xor(int i, atomic_t *v)
 }
 #define arch_atomic_fetch_xor arch_atomic_fetch_xor
 
+static __always_inline void arch_atomic_not(atomic_t *v)
+{
+	asm_inline volatile(LOCK_PREFIX "notl %0"
+			: "+m" (v->counter) :: "memory");
+}
+#define arch_atomic_not arch_atomic_not
+
 #ifdef CONFIG_X86_32
 # include <asm/atomic64_32.h>
 #else
-- 
2.51.0

On Tue Jul 14, 2026 at 6:42 AM BST, Ziran Zhang wrote:
> The kernel already has non-atomic bitmap_complement(), but
> no direct atomic implementation.
>
> According to Intel and AMD manuals, the NOT instruction
> supports a LOCK prefix.
>
> Currently the only way to achieve this is arch_atomic_xor(-1, v),
> which is non-obvious and hurts readability, so add arch_atomic_not()
> to provide an explicit atomic complement operation.

What do you need this for?

Best,
Gary

>
> Signed-off-by: Ziran Zhang <zhangcoder@yeah.net>
> ---
>  arch/x86/include/asm/atomic.h | 7 +++++++
>  1 file changed, 7 insertions(+)
>
> diff --git a/arch/x86/include/asm/atomic.h b/arch/x86/include/asm/atomic.h
> index 75743f1df..d7e543194 100644
> --- a/arch/x86/include/asm/atomic.h
> +++ b/arch/x86/include/asm/atomic.h
> @@ -168,6 +168,13 @@ static __always_inline int arch_atomic_fetch_xor(int i, atomic_t *v)
>  }
>  #define arch_atomic_fetch_xor arch_atomic_fetch_xor
>  
> +static __always_inline void arch_atomic_not(atomic_t *v)
> +{
> +	asm_inline volatile(LOCK_PREFIX "notl %0"
> +			: "+m" (v->counter) :: "memory");
> +}
> +#define arch_atomic_not arch_atomic_not
> +
>  #ifdef CONFIG_X86_32
>  # include <asm/atomic64_32.h>
>  #else

On Wed, 15 Jul 2026 12:49:20 +0100, Gary Guo wrote:
> > The kernel already has non-atomic bitmap_complement(), but
> > no direct atomic implementation.
> >
> > According to Intel and AMD manuals, the NOT instruction
> > supports a LOCK prefix.
> >
> > Currently the only way to achieve this is arch_atomic_xor(-1, v),
> > which is non-obvious and hurts readability, so add arch_atomic_not()
> > to provide an explicit atomic complement operation.
> 
> What do you need this for?
> 
> Best,
> Gary

Hi Gary,

At present there is no code in kernel that would call
arch_atomic_not() or arch_atomic_xor(~0,v).
I'll drop this patch and revisit if a real use case 
shows up in the future.

Thanks for the review!

Regards,
Ziran Zhang