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Date:   Wed, 11 May 2022 09:11:02 +0100
From:   Mark Rutland <mark.rutland@arm.com>
To:     Thomas Gleixner <tglx@linutronix.de>
Cc:     Lukas Wunner <lukas@wunner.de>, maz@kernel.org,
        Linus Walleij <linus.walleij@linaro.org>,
        Bartosz Golaszewski <brgl@bgdev.pl>,
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        Octavian Purdila <octavian.purdila@nxp.com>,
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        stefan.kristiansson@saunalahti.fi, tsbogend@alpha.franken.de,
        vgupta@kernel.org, vladimir.murzin@arm.com, will@kernel.org
Subject: Re: [PATCH v2 17/17] irq: remove handle_domain_{irq,nmi}()
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On Wed, May 11, 2022 at 02:11:52AM +0200, Thomas Gleixner wrote:
> On Tue, May 10 2022 at 15:15, Mark Rutland wrote:
> > On Tue, May 10, 2022 at 02:13:20PM +0200, Lukas Wunner wrote:
> >> Actually, since you're mentioning the in_nmi() check, I suspect
> >> there's another problem here:
> >> 
> >> generic_handle_domain_nmi() warns if !in_nmi(), then calls down 
> >> to handle_irq_desc() which warns if !in_hardirq().  Doesn't this
> >> cause a false-positive !in_hardirq() warning for a NMI on GIC/GICv3?
> >
> > I agree that doesn't look right.
> >
> >> The only driver calling request_nmi() or request_percpu_nmi() is
> >> drivers/perf/arm_pmu.c.  So that's the only one affected.
> >> You may want to test if that driver indeed exhibits such a
> >> false-positive warning since c16816acd086.
> >
> > In testing with v5.18-rc5, I can't see that going wrong.
> >
> > I also hacked the following in:
> >
> > -------->8--------
> > diff --git a/kernel/irq/irqdesc.c b/kernel/irq/irqdesc.c
> > index 939d21cd55c38..3c85608a8779f 100644
> > --- a/kernel/irq/irqdesc.c
> > +++ b/kernel/irq/irqdesc.c
> > @@ -718,6 +718,7 @@ EXPORT_SYMBOL_GPL(generic_handle_domain_irq);
> >  int generic_handle_domain_nmi(struct irq_domain *domain, unsigned int hwirq)
> >  {
> >         WARN_ON_ONCE(!in_nmi());
> > +       WARN_ON_ONCE(!in_hardirq());
> >         return handle_irq_desc(irq_resolve_mapping(domain, hwirq));
> 
> which is pointless because NMI entry code has to invoke [__]nmi_enter()
> before invoking this function. [__]nmi_enter() does:
> 
>     __preempt_count_add(NMI_OFFSET + HARDIRQ_OFFSET);
> 
> So it's more than bloody obvious why there is no warning triggered for a
> regular hardware induced NMI invocation.

Ugh, yes; clearly I need new eyes and/or more sleep. I entirely missed that we
treat an NMI as *also* being a hardirq rather than something completely
independent, and that means that this is *not* a problem for NMI.

Thanks for pointing that out!

> For a software invocation from the wrong context it does not matter how
> many redundant WARN_ONs you add. The existing ones are covering it
> nicely already.

Yup; as above I was clearly not thinknig straight here.

Mark.