[PATCH] ARM: select HAVE_POSIX_CPU_TIMERS_TASK_WORK

[PATCH] ARM: select HAVE_POSIX_CPU_TIMERS_TASK_WORK

[Karl Mehltretter]

Commit c6e61c06d606 ("ARM: 9463/1: Allow to enable RT") enabled PREEMPT_RT
on ARM but did not select HAVE_POSIX_CPU_TIMERS_TASK_WORK. This leaves
CONFIG_POSIX_CPU_TIMERS_TASK_WORK disabled, so CPU timers expire in hard
IRQ context.

On PREEMPT_RT this makes run_posix_cpu_timers() take the sleeping
sighand->siglock:

  BUG: sleeping function called from invalid context at spinlock_rt.c:48
    rt_spin_lock from lock_task_sighand
    lock_task_sighand from run_posix_cpu_timers
    run_posix_cpu_timers from update_process_times

ARM handles TIF_NOTIFY_RESUME on all return-to-user paths, including v7-M.
ARM32 KVM host support was removed by commit 541ad0150ca4 ("arm: Remove
32bit KVM host support"), so the select need not be conditional on KVM.

Select it to defer POSIX CPU timer expiry to task context.

Reproduced with setrlimit(RLIMIT_CPU, ...) and a busy loop. The same path
is used by setitimer(ITIMER_PROF or ITIMER_VIRTUAL) and POSIX CPU timers
created with timer_create().

Fixes: c6e61c06d606 ("ARM: 9463/1: Allow to enable RT")
Assisted-by: Claude:claude-fable-5
Signed-off-by: Karl Mehltretter <kmehltretter@gmail.com>
---
Found with a self-hosted syzkaller instance patched to fuzz kernels on
QEMU's versatilepb machine (ARM926EJ-S, ARMv5TE). Runtime-tested before
and after the change on that setup, and compile-tested mps2_defconfig
with PREEMPT_RT (NOMMU, CPU_V7M).

 arch/arm/Kconfig | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index 9187240a02db..f073b5102d4e 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -133,6 +133,7 @@ config ARM
 	select HAVE_PERF_EVENTS
 	select HAVE_PERF_REGS
 	select HAVE_PERF_USER_STACK_DUMP
+	select HAVE_POSIX_CPU_TIMERS_TASK_WORK
 	select MMU_GATHER_RCU_TABLE_FREE if SMP && ARM_LPAE
 	select HAVE_REGS_AND_STACK_ACCESS_API
 	select HAVE_RSEQ
-- 
2.53.0

Re: [PATCH] ARM: select HAVE_POSIX_CPU_TIMERS_TASK_WORK

[Arnd Bergmann]

On Sat, Jul 18, 2026, at 23:45, Karl Mehltretter wrote:
> Commit c6e61c06d606 ("ARM: 9463/1: Allow to enable RT") enabled PREEMPT_RT
> on ARM but did not select HAVE_POSIX_CPU_TIMERS_TASK_WORK. This leaves
> CONFIG_POSIX_CPU_TIMERS_TASK_WORK disabled, so CPU timers expire in hard
> IRQ context.
>
> On PREEMPT_RT this makes run_posix_cpu_timers() take the sleeping
> sighand->siglock:
>
>   BUG: sleeping function called from invalid context at spinlock_rt.c:48
>     rt_spin_lock from lock_task_sighand
>     lock_task_sighand from run_posix_cpu_timers
>     run_posix_cpu_timers from update_process_times
>
> ARM handles TIF_NOTIFY_RESUME on all return-to-user paths, including v7-M.
> ARM32 KVM host support was removed by commit 541ad0150ca4 ("arm: Remove
> 32bit KVM host support"), so the select need not be conditional on KVM.
>
> Select it to defer POSIX CPU timer expiry to task context.
>
> Reproduced with setrlimit(RLIMIT_CPU, ...) and a busy loop. The same path
> is used by setitimer(ITIMER_PROF or ITIMER_VIRTUAL) and POSIX CPU timers
> created with timer_create().
>
> Fixes: c6e61c06d606 ("ARM: 9463/1: Allow to enable RT")
> Assisted-by: Claude:claude-fable-5
> Signed-off-by: Karl Mehltretter <kmehltretter@gmail.com>

Indeed selecting HAVE_POSIX_CPU_TIMERS_TASK_WORK was previously
part of the RT patch [1] set but got lost during one of the
updates before the patch got merged.

Reviewed-by: Arnd Bergmann <arnd@arndb.de>

> Found with a self-hosted syzkaller instance patched to fuzz kernels on
> QEMU's versatilepb machine (ARM926EJ-S, ARMv5TE). Runtime-tested before
> and after the change on that setup

Good to hear that this configuration works with your patch,
I don't think many people have tested ARMv5 RT recently.

    Arnd

[1] https://lore.kernel.org/all/20251030152253.tK-rY89Y@linutronix.de/

Re: [PATCH] ARM: select HAVE_POSIX_CPU_TIMERS_TASK_WORK

[Sebastian Andrzej Siewior]

On 2026-07-18 23:45:15 [+0200], Karl Mehltretter wrote:
> Commit c6e61c06d606 ("ARM: 9463/1: Allow to enable RT") enabled PREEMPT_RT
> on ARM but did not select HAVE_POSIX_CPU_TIMERS_TASK_WORK. This leaves
> CONFIG_POSIX_CPU_TIMERS_TASK_WORK disabled, so CPU timers expire in hard
> IRQ context.
…
> Fixes: c6e61c06d606 ("ARM: 9463/1: Allow to enable RT")
> Assisted-by: Claude:claude-fable-5
> Signed-off-by: Karl Mehltretter <kmehltretter@gmail.com>

Reviewed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>

> Found with a self-hosted syzkaller instance patched to fuzz kernels on
> QEMU's versatilepb machine (ARM926EJ-S, ARMv5TE). Runtime-tested before
> and after the change on that setup, and compile-tested mps2_defconfig
> with PREEMPT_RT (NOMMU, CPU_V7M).

I obviously got the rules confused while dropping the KVM patches.

Sebastian