Re: [PATCH v2 2/6] genirq: Allow an interrupt to be marked as 'raw'

Valentin Schneider
2020-11-26T18:19:08Z

Hi Marc,

On 24/11/20 14:14, Marc Zyngier wrote:
> @@ -680,14 +679,22 @@ int __handle_domain_irq(struct irq_domain *domain, unsigned int hwirq,
> * Some hardware gives randomly wrong interrupts. Rather
> * than crashing, do something sensible.
> */
> - if (unlikely(!irq || irq >= nr_irqs)) {
> + if (unlikely(!irq || irq >= nr_irqs || !(desc = irq_to_desc(irq)))) {
> ack_bad_irq(irq);
> ret = -EINVAL;
> + goto out;
> + }
> +
> + if (IS_ENABLED(CONFIG_ARCH_WANTS_IRQ_RAW) &&
> + unlikely(irq_settings_is_raw(desc))) {
> + generic_handle_irq_desc(desc);

If I got the RCU bits right from what Thomas mentioned in

https://lore.kernel.org/r/87ft5q18qs.fsf@nanos.tec.linutronix.de
https://lore.kernel.org/r/87lfewnmdz.fsf@nanos.tec.linutronix.de

then we're still missing something to notify RCU in the case the IRQ hits
the idle task. All I see on our entry path is

trace_hardirqs_off();
...
irq_handler()
handle_domain_irq();
...
trace_hardirqs_on();

so we do currently rely on handle_domain_irq()'s irq_enter() + irq_exit()
for that. rcu_irq_enter() says CONFIG_RCU_EQS_DEBUG=y can detect missing
bits, but I don't get any warnings with your series on my Juno.

Now, irq_enter() gives us:

rcu_irq_enter();
irq_enter_rcu()
raise_softirq faffery;
__irq_enter()
irqtime accounting;
preempt count + lockdep; } __irq_enter_raw()

Looking at irqentry_enter() + DEFINE_IDTENTRY_SYSVEC_SIMPLE(), I *think* we
would be fine with just:

rcu_irq_enter();
__irq_enter_raw();

generic_handle_irq_desc()

__irq_exit_raw();
rcu_irq_exit();

I tested that and it didn't explode (though I haven't managed to make
CONFIG_RCU_EQS_DEBUG squeal). Also please note RCU isn't my forte, so the
above may contain traces of bullcrap.

> } else {
> - generic_handle_irq(irq);
> + irq_enter();
> + generic_handle_irq_desc(desc);
> + irq_exit();
> }
>
> - irq_exit();
> +out:
> set_irq_regs(old_regs);
> return ret;
> }
[...]
> @@ -180,3 +182,16 @@ static inline bool irq_settings_is_hidden(struct irq_desc *desc)
> {
> return desc->status_use_accessors & _IRQ_HIDDEN;
> }
> +
> +static inline bool irq_settings_is_raw(struct irq_desc *desc)
> +{
> + if (IS_ENABLED(CONFIG_ARCH_WANTS_IRQ_RAW))
> + return desc->status_use_accessors & _IRQ_RAW;
> +
> + /*
> + * Using IRQ_RAW on architectures that don't expect it is
> + * likely to be wrong.
> + */
> + WARN_ON_ONCE(1);

Per __handle_domain_irq()'s short-circuit evaluation, this is only entered
when the above config is enabled. Perhaps a better place to check for this
would be in __irq_settings_clr_and_set().

> + return false;
> +}