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From:   Thomas Gleixner <tglx@linutronix.de>
To:     Mark Rutland <mark.rutland@arm.com>, Lukas Wunner <lukas@wunner.de>
Cc:     maz@kernel.org, Linus Walleij <linus.walleij@linaro.org>,
        Bartosz Golaszewski <brgl@bgdev.pl>,
        linux-gpio@vger.kernel.org,
        Octavian Purdila <octavian.purdila@nxp.com>,
        linux-kernel@vger.kernel.org, aou@eecs.berkeley.edu,
        catalin.marinas@arm.com, deanbo422@gmail.com, green.hu@gmail.com,
        guoren@kernel.org, jonas@southpole.se, kernelfans@gmail.com,
        linux-arm-kernel@lists.infradead.org, linux@armlinux.org.uk,
        palmer@dabbelt.com, paul.walmsley@sifive.com, shorne@gmail.com,
        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}()
In-Reply-To: <Ynpzb5L53iGex14D@FVFF77S0Q05N>
References: <20211026092504.27071-1-mark.rutland@arm.com>
 <20211026092504.27071-18-mark.rutland@arm.com>
 <20220506203242.GA1855@wunner.de> <YnjWvbzn8ox+f2Y2@FVFF77S0Q05N>
 <20220510121320.GA3020@wunner.de> <Ynpzb5L53iGex14D@FVFF77S0Q05N>
Date:   Wed, 11 May 2022 02:11:52 +0200
Message-ID: <874k1xorlj.ffs@tglx>
MIME-Version: 1.0
Content-Type: text/plain
Precedence: bulk

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.

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.

Thanks,

        tglx