[PATCH v6.18 0/3] ARM: PREEMPT_RT backports

[PATCH v6.18 0/3] ARM: PREEMPT_RT backports

[Sebastian Andrzej Siewior]

Hi,

ARM missed the PREEMPT_RT window for v6.18. The following three patches
have been merged as of v7.1-rc1 and are the missing pieces.

I've been asked by people if it would be possible to include them in the
stable tree as it would make their life easier.

Russell King (Oracle) (1):
  ARM: ensure interrupts are enabled in __do_user_fault()

Sebastian Andrzej Siewior (1):
  ARM: 9463/1: Allow to enable RT

Thomas Gleixner (1):
  ARM: 9459/1: Disable jump-label on PREEMPT_RT

 arch/arm/Kconfig    | 3 ++-
 arch/arm/mm/fault.c | 6 +++++-
 2 files changed, 7 insertions(+), 2 deletions(-)

Sebastian

[PATCH v6.18 1/3] ARM: 9459/1: Disable jump-label on PREEMPT_RT

[Sebastian Andrzej Siewior]

From: Thomas Gleixner <tglx@linutronix.de>

commit 256d97d3587b441630c345ab3d773a9e7dcd964b upstream.

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@linutronix.de>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
 arch/arm/Kconfig | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index e481254e36455..b7f3ad66ff15f 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -82,7 +82,7 @@ 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
-- 
2.53.0

[PATCH v6.18 2/3] ARM: ensure interrupts are enabled in __do_user_fault()

[Sebastian Andrzej Siewior]

From: "Russell King (Oracle)" <rmk+kernel@armlinux.org.uk>

commit 59e4f3b45b96a24fc9b7a89e5f8a2168b30f95af upstream.

__do_user_fault() may be called from fault handling paths where the
interrupts are enabled or disabled. E.g. do_page_fault() calls this
with interrupts enabled, whereas do_sect_fault()->do_bad_area()
will call this with interrupts disabled. Since this is a userspace
fault, we know that interrupts were enabled in the parent context,
so call local_irq_enable() here to give a consistent interrupt state.

This is necessary for force_sig_info() when PREEMPT_RT is enabled.

Reported-by: Yadi.hu <yadi.hu@windriver.com>
Reviewed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
 arch/arm/mm/fault.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/arch/arm/mm/fault.c b/arch/arm/mm/fault.c
index ed4330cc3f4e6..6c27ebd490938 100644
--- a/arch/arm/mm/fault.c
+++ b/arch/arm/mm/fault.c
@@ -190,7 +190,8 @@ __do_kernel_fault(struct mm_struct *mm, unsigned long addr, unsigned int fsr,
 
 /*
  * Something tried to access memory that isn't in our memory map..
- * User mode accesses just cause a SIGSEGV
+ * User mode accesses just cause a SIGSEGV. Ensure interrupts are enabled
+ * for preempt RT.
  */
 static void
 __do_user_fault(unsigned long addr, unsigned int fsr, unsigned int sig,
@@ -198,6 +199,8 @@ __do_user_fault(unsigned long addr, unsigned int fsr, unsigned int sig,
 {
 	struct task_struct *tsk = current;
 
+	local_irq_enable();
+
 #ifdef CONFIG_DEBUG_USER
 	if (((user_debug & UDBG_SEGV) && (sig == SIGSEGV)) ||
 	    ((user_debug & UDBG_BUS)  && (sig == SIGBUS))) {
@@ -268,6 +271,7 @@ do_kernel_address_page_fault(struct mm_struct *mm, unsigned long addr,
 		 * should not be faulting in kernel space, which includes the
 		 * vector/khelper page. Handle the branch predictor hardening
 		 * while interrupts are still disabled, then send a SIGSEGV.
+		 * Note that __do_user_fault() will enable interrupts.
 		 */
 		harden_branch_predictor();
 		__do_user_fault(addr, fsr, SIGSEGV, SEGV_MAPERR, regs);
-- 
2.53.0

[PATCH v6.18 3/3] ARM: 9463/1: Allow to enable RT

[Sebastian Andrzej Siewior]

commit c6e61c06d6061750597e79c598acb5dead44c35b upstream.

All known issues have been adressed.
Allow to select RT.

Acked-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
 arch/arm/Kconfig | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index b7f3ad66ff15f..bfb1734e50bec 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -41,6 +41,7 @@ config ARM
 	select ARCH_SUPPORTS_CFI
 	select ARCH_SUPPORTS_HUGETLBFS if ARM_LPAE
 	select ARCH_SUPPORTS_PER_VMA_LOCK
+	select ARCH_SUPPORTS_RT
 	select ARCH_USE_BUILTIN_BSWAP
 	select ARCH_USE_CMPXCHG_LOCKREF
 	select ARCH_USE_MEMTEST
-- 
2.53.0

Re: [PATCH v6.18 0/3] ARM: PREEMPT_RT backports

[Greg KH]

On Mon, Jun 29, 2026 at 04:41:28PM +0200, Sebastian Andrzej Siewior wrote:
> Hi,
> 
> ARM missed the PREEMPT_RT window for v6.18. The following three patches
> have been merged as of v7.1-rc1 and are the missing pieces.
> 
> I've been asked by people if it would be possible to include them in the
> stable tree as it would make their life easier.

Why can't the -rt patchset just include these?  Why put the burden on
us?

thanks,

greg k-h

Re: [PATCH v6.18 0/3] ARM: PREEMPT_RT backports

[Sebastian Andrzej Siewior]

On 2026-07-02 16:06:48 [+0200], Greg KH wrote:
> On Mon, Jun 29, 2026 at 04:41:28PM +0200, Sebastian Andrzej Siewior wrote:
> > Hi,
> > 
> > ARM missed the PREEMPT_RT window for v6.18. The following three patches
> > have been merged as of v7.1-rc1 and are the missing pieces.
> > 
> > I've been asked by people if it would be possible to include them in the
> > stable tree as it would make their life easier.
> 
> Why can't the -rt patchset just include these?  Why put the burden on
> us?

It is part of the -rt patchset. I've been asked if it would be possible
to include it (ARM support) as part of -stable tree. I've been looking
at what is missing and it included two Kconfig changes and one code
change. This looked small so I thought maybe, why not let's ask. I'm
sorry if it looked other than asking.

If this puts burden on you and it does not qualify for a stable change
then it can't be included.

> thanks,
> 
> greg k-h

Sebastian

Re: [PATCH v6.18 0/3] ARM: PREEMPT_RT backports

[Greg KH]

On Mon, Jul 06, 2026 at 10:59:40AM +0200, Sebastian Andrzej Siewior wrote:
> On 2026-07-02 16:06:48 [+0200], Greg KH wrote:
> > On Mon, Jun 29, 2026 at 04:41:28PM +0200, Sebastian Andrzej Siewior wrote:
> > > Hi,
> > > 
> > > ARM missed the PREEMPT_RT window for v6.18. The following three patches
> > > have been merged as of v7.1-rc1 and are the missing pieces.
> > > 
> > > I've been asked by people if it would be possible to include them in the
> > > stable tree as it would make their life easier.
> > 
> > Why can't the -rt patchset just include these?  Why put the burden on
> > us?
> 
> It is part of the -rt patchset. I've been asked if it would be possible
> to include it (ARM support) as part of -stable tree. I've been looking
> at what is missing and it included two Kconfig changes and one code
> change. This looked small so I thought maybe, why not let's ask. I'm
> sorry if it looked other than asking.
> 
> If this puts burden on you and it does not qualify for a stable change
> then it can't be included.

Please let's have new features stick to feature-patchsets like the -rt
kernel is, and not put that burden on us stable maintainers for
something that we don't use :)

thanks,

greg k-h

Re: [PATCH v6.18 0/3] ARM: PREEMPT_RT backports

[Sebastian Andrzej Siewior]

On 2026-07-06 11:27:33 [+0200], Greg KH wrote:
> Please let's have new features stick to feature-patchsets like the -rt
> kernel is, and not put that burden on us stable maintainers for
> something that we don't use :)

Understood.

> thanks,
> 
> greg k-h

Sebastian