Re: [PATCH] ARM: select HAVE_POSIX_CPU_TIMERS_TASK_WORK

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 <[email protected]>

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 <[email protected]>

> 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/[email protected]/