[PATCH v2 3/5] ARM: Disable jump-label on PREEMPT_RT

[Ard Biesheuvel]

On Mon, 3 Nov 2025 at 11:15, Sebastian Andrzej Siewior
<bigeasy at linutronix.de> wrote:
>
> From: Thomas Gleixner <tglx at linutronix.de>
>
> jump-labels are used to efficiently switch between two possible code
> paths. To achieve this, stop_machine() is used to keep the CPU in a
> known state while the opcode is modified. The usage of stop_machine()
> here leads to large latency spikes which can be observed on PREEMPT_RT.
>
> Jump labels may change the target during runtime and are not restricted
> to debug or "configuration/ setup" part of a PREEMPT_RT system where
> high latencies could be defined as acceptable.
>
> On 64-bit Arm, it is possible to use jump labels without the
> stop_machine() call, which architecturally provides a way to atomically
> change one 32-bit instruction word while keeping maintaining consistency,
> but this is not generally the case on 32-bit, in particular in thumb2
> mode.
>
> Disable jump-label support on a PREEMPT_RT system when SMP is enabled.
>
> [bigeasy: Patch description.]
> [arnd: add !SMP case, extend changelog]
>
> Signed-off-by: Thomas Gleixner <tglx at linutronix.de>
> Cc: Mark Rutland <mark.rutland at arm.com>
> Cc: Ard Biesheuvel <ardb at kernel.org>
> Signed-off-by: Arnd Bergmann <arnd at arndb.de>
> Reviewed-by: Linus Walleij <linus.walleij at linaro.org>
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy at linutronix.de>

Acked-by: Ard Biesheuvel <ardb at kernel.org>

> ---
>  arch/arm/Kconfig | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
> index 2e3f93b690f47..92ca24336ec4b 100644
> --- a/arch/arm/Kconfig
> +++ b/arch/arm/Kconfig
> @@ -80,7 +80,7 @@ config ARM
>         select HAS_IOPORT
>         select HAVE_ARCH_AUDITSYSCALL if AEABI && !OABI_COMPAT
>         select HAVE_ARCH_BITREVERSE if (CPU_32v7M || CPU_32v7) && !CPU_32v6
> -       select HAVE_ARCH_JUMP_LABEL if !XIP_KERNEL && !CPU_ENDIAN_BE32 && MMU
> +       select HAVE_ARCH_JUMP_LABEL if !XIP_KERNEL && !CPU_ENDIAN_BE32 && MMU && (!PREEMPT_RT || !SMP)
>         select HAVE_ARCH_KFENCE if MMU && !XIP_KERNEL
>         select HAVE_ARCH_KGDB if !CPU_ENDIAN_BE32 && MMU
>         select HAVE_ARCH_KASAN if MMU && !XIP_KERNEL
> --
> 2.51.0
>