linux-kernel - Re: [PATCH] irqchip/mips: Fix RCU violation when using irqdomain lookup on interrupt entry

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Date:   Thu, 08 Jul 2021 17:39:28 +0100
From:   Marc Zyngier <maz@...nel.org>
To:     Serge Semin <fancer.lancer@...il.com>
Cc:     linux-mips@...r.kernel.org, linux-kernel@...r.kernel.org,
        Thomas Gleixner <tglx@...utronix.de>, kernel-team@...roid.com,
        Guenter Roeck <linux@...ck-us.net>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>
Subject: Re: [PATCH] irqchip/mips: Fix RCU violation when using irqdomain lookup on interrupt entry

Hi Sergey,

On Thu, 08 Jul 2021 10:46:08 +0100,
Serge Semin <fancer.lancer@...il.com> wrote:
> 
> Hi Marc,
> Thanks for the fix.
> 
> On Tue, Jul 06, 2021 at 12:06:47PM +0100, Marc Zyngier wrote:
> > Since d4a45c68dc81 ("irqdomain: Protect the linear revmap with RCU"),
> > any irqdomain lookup requires the RCU read lock to be held.
> > 
> > This assumes that the architecture code will be structured such as
> > irq_enter() will be called *before* the interrupt is looked up
> > in the irq domain. However, this isn't the case for MIPS, and a number
> > of drivers are structured to do it the other way around when handling
> > an interrupt in their root irqchip (secondary irqchips are OK by
> > construction).
> > 
> > This results in a RCU splat on a lockdep-enabled kernel when the kernel
> > takes an interrupt from idle, as reported by Guenter Roeck.
> 
> Alas I am still on 5.12-rc4, so can't test it out at the moment. Soon
> after getting further on the modern kernel version I'll give this
> patch a try on my hw and send a report.

It is likely that I'll send a pull request to Thomas with this
shortly, given that it affects existing systems and that this patch
does address the issue (see Guenter's report). We can always amend
things once you've had the time to upgrade your kernel to the latest.

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.