linux-kernel - Re: [PATCH] irqchip/gic-v3-its: Don't try to move a disabled irq

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Message-ID: <87y2p5fatl.fsf@nanos.tec.linutronix.de>
Date:   Tue, 02 Jun 2020 22:54:14 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     "Herrenschmidt\, Benjamin" <benh@...zon.com>,
        "maz\@kernel.org" <maz@...nel.org>,
        "Saidi\, Ali" <alisaidi@...zon.com>
Cc:     "jason\@lakedaemon.net" <jason@...edaemon.net>,
        "linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-arm-kernel\@lists.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "Woodhouse\, David" <dwmw@...zon.co.uk>,
        "Zilberman\, Zeev" <zeev@...zon.com>,
        "Machulsky\, Zorik" <zorik@...zon.com>
Subject: Re: [PATCH] irqchip/gic-v3-its: Don't try to move a disabled irq

"Herrenschmidt, Benjamin" <benh@...zon.com> writes:
> On Sun, 2020-05-31 at 12:09 +0100, Marc Zyngier wrote:
>> > The semantic of activate/deactivate (which maps to started/shutdown
>> > in the IRQ code) is that the HW resources for a given interrupt are
>> > only committed when the interrupt is activated. Trying to perform
>> > actions involving the HW on an interrupt that isn't active cannot be
>> > guaranteed to take effect.
>> > 
>> > I'd rather address it in the core code, by preventing set_affinity (and
>> > potentially others) to take place when the interrupt is not in the
>> > STARTED state. Userspace would get an error, which is perfectly
>> > legitimate, and which it already has to deal with it for plenty of
>> > other
>> > reasons.
>
> So I finally found time to dig a bit in there :) Code has changed a bit
> since last I looked. But I have memories of the startup code messing
> around with the affinity, and here it is. In irq_startup() :
>
>
> 		switch (__irq_startup_managed(desc, aff, force)) {
> 		case IRQ_STARTUP_NORMAL:
> 			ret = __irq_startup(desc);
> 			irq_setup_affinity(desc);
> 			break;
> 		case IRQ_STARTUP_MANAGED:
> 			irq_do_set_affinity(d, aff, false);
> 			ret = __irq_startup(desc);
> 			break;
> 		case IRQ_STARTUP_ABORT:
> 			irqd_set_managed_shutdown(d);
> 			return 0;
>
> So we have two cases here. Normal and managed.
>
> In the managed case, we set the affinity before startup. I feel like your
> patch might break that or am I missing something ?

It will break stuff because the affinity is not stored in case that the
interrupt is not started.

I think we can fix this in the core code but that needs more thought.
__irq_can_set_affinity() is definitely the wrong place.

Thanks,

        tglx