Subject: [PATCH 1/2] arm: Disable jump-label on PREEMPT_RT.
From: Thomas Gleixner <[email-masked]>
Date: Wed, 8 Jul 2015 17:14:48 +0200

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.

Disable jump-label support on a PREEMPT_RT system.

[bigeasy: Patch description.]

Signed-off-by: Thomas Gleixner <[email-masked]>
Signed-off-by: Sebastian Andrzej Siewior <[email-masked]>
Link: https://lkml.kernel.org/r/[email-masked]
---
 arch/arm/Kconfig |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -79,7 +79,7 @@ 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
 	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