Re: [PATCH 2/3 v3] genirq: Fix interrupt threads affinity vs. cpuset isolated partitions

[Chris Mason <clm@meta.com>]

On Fri, 21 Nov 2025 15:34:59 +0100 Frederic Weisbecker <frederic@kernel.org> wrote:

Hi everyone,

I ran some recent commits through AI patch review and it flagged this one.
The comments below looks right to me, but I might be missing something:

> diff --git a/kernel/irq/manage.c b/kernel/irq/manage.c
> index c1ce30c9c3ab6..61da1c68ff82a 100644
> --- a/kernel/irq/manage.c
> +++ b/kernel/irq/manage.c
> @@ -1408,16 +1408,23 @@ setup_irq_thread(struct irqaction *new, unsigned int irq, bool secondary)
>  	 * references an already freed task_struct.
>  	 */
>  	new->thread = get_task_struct(t);

[ ... ]

> -	set_bit(IRQTF_AFFINITY, &new->thread_flags);
> +	kthread_bind_mask(t, cpu_possible_mask);
> +
> +	/*
> +	 * Ensure the thread adjusts the affinity once it reaches the
> +	 * thread function.
> +	 */
> +	new->thread_flags = BIT(IRQTF_AFFINITY);
                          ^^^

Can this clobber IRQTF_FORCED_THREAD?

In __setup_irq(), irq_setup_forced_threading() is called before
setup_irq_thread(). When forced threading is enabled,
irq_setup_forced_threading() sets IRQTF_FORCED_THREAD via set_bit():

    set_bit(IRQTF_FORCED_THREAD, &new->thread_flags);

Then setup_irq_thread() overwrites thread_flags with a direct assignment:

    new->thread_flags = BIT(IRQTF_AFFINITY);

This clears IRQTF_FORCED_THREAD. Later in irq_thread():

    if (force_irqthreads() && test_bit(IRQTF_FORCED_THREAD,
                                       &action->thread_flags))
        handler_fn = irq_forced_thread_fn;
    else
        handler_fn = irq_thread_fn;

Without IRQTF_FORCED_THREAD, the handler uses irq_thread_fn() instead of
irq_forced_thread_fn(). The forced-threaded handler then runs without the
local_bh_disable() and local_irq_disable() protection that non-threaded
interrupt handlers expect.

Should this be:

    new->thread_flags |= BIT(IRQTF_AFFINITY);

or:

    set_bit(IRQTF_AFFINITY, &new->thread_flags);

> +
>  	return 0;
>  }


-chris