Support for atomic Compare-And-Swap instructions has been in the RISC-V
port of the Linux kernel for a long time. That being said, we apparently
never bothered to set HAVE_CMPXCHG_DOUBLE and HAVE_CMPXCHG_LOCAL in the
Kconfig, despite having all the framework to support them.

Signed-off-by: Miquel Sabaté Solà <mssola@mssola.com>
---
I have built this patch with multiple configurations and ran it with KVM
(the VisionFive2 board that I have lacks the needed extensions). All seems
to work, but I do wonder if we did not enable these for a reason or this
just slipped through. So far in the code I believe everything is in place,
and I haven't seen any commit in the git log stating otherwise.

 Documentation/features/locking/cmpxchg-local/arch-support.txt | 2 +-
 arch/riscv/Kconfig                                            | 2 ++
 2 files changed, 3 insertions(+), 1 deletion(-)

diff --git a/Documentation/features/locking/cmpxchg-local/arch-support.txt b/Documentation/features/locking/cmpxchg-local/arch-support.txt
index 2c3a4b91f16d..28d5fa8c3b4f 100644
--- a/Documentation/features/locking/cmpxchg-local/arch-support.txt
+++ b/Documentation/features/locking/cmpxchg-local/arch-support.txt
@@ -20,7 +20,7 @@
     |    openrisc: | TODO |
     |      parisc: | TODO |
     |     powerpc: | TODO |
-    |       riscv: | TODO |
+    |       riscv: |  ok  |
     |        s390: |  ok  |
     |          sh: | TODO |
     |       sparc: | TODO |
diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
index 7e76b6316425..7c6726a8d738 100644
--- a/arch/riscv/Kconfig
+++ b/arch/riscv/Kconfig
@@ -151,6 +151,8 @@ config RISCV
 	select HAVE_ARCH_USERFAULTFD_WP if 64BIT && MMU && USERFAULTFD && RISCV_ISA_SVRSW60T59B
 	select HAVE_ARCH_VMAP_STACK if MMU && 64BIT
 	select HAVE_ASM_MODVERSIONS
+	select HAVE_CMPXCHG_DOUBLE if RISCV_ISA_ZACAS && RISCV_ISA_ZABHA
+	select HAVE_CMPXCHG_LOCAL if RISCV_ISA_ZACAS && RISCV_ISA_ZABHA
 	select HAVE_CONTEXT_TRACKING_USER
 	select HAVE_DEBUG_KMEMLEAK
 	select HAVE_DMA_CONTIGUOUS if MMU
--
2.53.0

Miquel Sabaté Solà @ 2026-02-20 08:44 +01:

> Support for atomic Compare-And-Swap instructions has been in the RISC-V
> port of the Linux kernel for a long time. That being said, we apparently
> never bothered to set HAVE_CMPXCHG_DOUBLE and HAVE_CMPXCHG_LOCAL in the
> Kconfig, despite having all the framework to support them.
>
> Signed-off-by: Miquel Sabaté Solà <mssola@mssola.com>
> ---
> I have built this patch with multiple configurations and ran it with KVM
> (the VisionFive2 board that I have lacks the needed extensions). All seems
> to work, but I do wonder if we did not enable these for a reason or this
> just slipped through. So far in the code I believe everything is in place,
> and I haven't seen any commit in the git log stating otherwise.
>
>  Documentation/features/locking/cmpxchg-local/arch-support.txt | 2 +-
>  arch/riscv/Kconfig                                            | 2 ++
>  2 files changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/Documentation/features/locking/cmpxchg-local/arch-support.txt b/Documentation/features/locking/cmpxchg-local/arch-support.txt
> index 2c3a4b91f16d..28d5fa8c3b4f 100644
> --- a/Documentation/features/locking/cmpxchg-local/arch-support.txt
> +++ b/Documentation/features/locking/cmpxchg-local/arch-support.txt
> @@ -20,7 +20,7 @@
>      |    openrisc: | TODO |
>      |      parisc: | TODO |
>      |     powerpc: | TODO |
> -    |       riscv: | TODO |
> +    |       riscv: |  ok  |
>      |        s390: |  ok  |
>      |          sh: | TODO |
>      |       sparc: | TODO |
> diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> index 7e76b6316425..7c6726a8d738 100644
> --- a/arch/riscv/Kconfig
> +++ b/arch/riscv/Kconfig
> @@ -151,6 +151,8 @@ config RISCV
>  	select HAVE_ARCH_USERFAULTFD_WP if 64BIT && MMU && USERFAULTFD && RISCV_ISA_SVRSW60T59B
>  	select HAVE_ARCH_VMAP_STACK if MMU && 64BIT
>  	select HAVE_ASM_MODVERSIONS
> +	select HAVE_CMPXCHG_DOUBLE if RISCV_ISA_ZACAS && RISCV_ISA_ZABHA
> +	select HAVE_CMPXCHG_LOCAL if RISCV_ISA_ZACAS && RISCV_ISA_ZABHA
>  	select HAVE_CONTEXT_TRACKING_USER
>  	select HAVE_DEBUG_KMEMLEAK
>  	select HAVE_DMA_CONTIGUOUS if MMU

Gentle ping :)