Re: [PATCH v3 10/13] sched: Guard sched_tick_start/stop against uninitialized tick_work_cpu

[Thomas Gleixner <tglx@kernel.org>]

On Thu, Jun 18 2026 at 11:11, Jing Wu wrote:
> sched_tick_start() and sched_tick_stop() are called during CPU hotplug
> for CPUs not in the HK_TYPE_KERNEL_NOISE set.  They dereference
> tick_work_cpu, which is allocated by sched_tick_offload_init() and only
> called from housekeeping_init() when nohz_full= is present at boot.
>
> When the DHM subsystem first-enables HK_TYPE_KERNEL_NOISE at runtime via
> housekeeping_update_types(), tick_work_cpu remains NULL because
> sched_tick_offload_init() is __init-only and cannot be re-invoked.  A
> subsequent CPU offline/online cycle for an isolated CPU triggers
> WARN_ON_ONCE(!tick_work_cpu) followed by a NULL-pointer dereference in
> per_cpu_ptr(tick_work_cpu, cpu), crashing the kernel.
>
> Since nohz_full= was not active at boot, tick_nohz_full_running remains
> false and the tick-offload infrastructure is never activated; isolated
> CPUs continue to receive their own ticks.  Guard both helpers with an
> additional !tick_work_cpu check so they become no-ops in this case.

This is the same fake functionality as with the tick itself. Seriously?

> -	if (housekeeping_cpu(cpu, HK_TYPE_KERNEL_NOISE))
> +	if (housekeeping_cpu(cpu, HK_TYPE_KERNEL_NOISE) || !tick_work_cpu)
>  		return;
>  
>  	WARN_ON_ONCE(!tick_work_cpu);
> @@ -5799,7 +5799,7 @@ static void sched_tick_stop(int cpu)
>  	struct tick_work *twork;
>  	int os;
>  
> -	if (housekeeping_cpu(cpu, HK_TYPE_KERNEL_NOISE))
> +	if (housekeeping_cpu(cpu, HK_TYPE_KERNEL_NOISE) || !tick_work_cpu)
>  		return;
>  
>  	WARN_ON_ONCE(!tick_work_cpu);

Brilliant stuff that. Guard against tick_work_cpu == NULL and then keep
the WARN_ON() there, which became completely pointless.

But that's all just mindless tinkering and fixing the symptoms.

If all of this is runtime managed, then all the initialization needs to
be made unconditional. Yes, that wastes a few bytes of memory per CPU if
it's not used, but avoids these completely inconsistent hacks all over
the place and provides a coherent user interface.

Stop trying to duct tape this in. This needs more thoughts than just
sprinkling works a few works for me hacks all over the place.

Thanks,

        tglx