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From: Thomas Gleixner <tglx@linutronix.de>
To: Leonardo Bras <leobras@redhat.com>, Greg Kroah-Hartman
 <gregkh@linuxfoundation.org>, Jiri Slaby <jirislaby@kernel.org>, Ilpo
 =?utf-8?Q?J=C3=A4rvinen?= <ilpo.jarvinen@linux.intel.com>, Andy Shevchenko
 <andriy.shevchenko@linux.intel.com>, Florian Fainelli
 <f.fainelli@gmail.com>, John Ogness <john.ogness@linutronix.de>, Tony
 Lindgren <tony@atomide.com>, Marcelo Tosatti <mtosatti@redhat.com>
Cc: linux-kernel@vger.kernel.org, linux-serial@vger.kernel.org, Sebastian
 Andrzej Siewior <bigeasy@linutronix.de>
Subject: Re: [RESEND RFC PATCH v1 1/2] irq/spurious: Reset irqs_unhandled if
 an irq_thread handles one IRQ request
In-Reply-To: <20240116073701.2356171-2-leobras@redhat.com>
References: <20240116073701.2356171-1-leobras@redhat.com>
 <20240116073701.2356171-2-leobras@redhat.com>
Date: Wed, 17 Jan 2024 23:08:44 +0100
Message-ID: <87ttnbfw9f.ffs@tglx>
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On Tue, Jan 16 2024 at 04:36, Leonardo Bras wrote:
> This IRQ line disable bug can be easily reproduced with a serial8250
> console on a PREEMPT_RT kernel: it only takes the user to print a lot
> of text to the console (or to ttyS0): around 300k chars should be
> enough.

That has nothing to do with RT, it's a problem of force threaded
interrupts in combination with an edge type interrupt line and a
hardware which keeps firing interrupts forever.

> To fix this bug, reset irqs_unhandled whenever irq_thread handles at least
> one IRQ request.

This papers over the symptom and makes runaway detection way weaker for
all interrupts or breaks it completely.

The problem with edge type interrupts is that we cannot mask them like
we do with level type interrupts in the hard interrupt handler and
unmask them once the threaded handler finishes.

So yes, we need special rules here when:

   1) The interrupt handler is force threaded

   2) The interrupt line is edge type

   3) The accumulated unhandled interrupts are within a sane margin

Thanks,

        tglx