linux-kernel - RE: [RFC] IRQ handlers run with some high-priority interrupts(not NMI) enabled on some platform

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Date:   Thu, 18 Feb 2021 16:30:20 +1100 (AEDT)
From:   Finn Thain <fthain@...egraphics.com.au>
To:     "Song Bao Hua (Barry Song)" <song.bao.hua@...ilicon.com>
cc:     Arnd Bergmann <arnd@...nel.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        "arnd@...db.de" <arnd@...db.de>,
        "geert@...ux-m68k.org" <geert@...ux-m68k.org>,
        "funaho@...ai.org" <funaho@...ai.org>,
        "philb@....org" <philb@....org>, "corbet@....net" <corbet@....net>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "linux-m68k@...ts.linux-m68k.org" <linux-m68k@...ts.linux-m68k.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [RFC] IRQ handlers run with some high-priority interrupts(not
 NMI) enabled on some platform

On Wed, 17 Feb 2021, Song Bao Hua (Barry Song) wrote:

> > On Sat, 13 Feb 2021, Song Bao Hua (Barry Song) wrote:
> > 
> > >
> > > So what is really confusing and a pain to me is that: For years 
> > > people like me have been writing device drivers with the idea that 
> > > irq handlers run with interrupts disabled after those commits in 
> > > genirq. So I don't need to care about if some other IRQs on the same 
> > > cpu will jump out to access the data the current IRQ handler is 
> > > accessing.
> > >
> > > but it turns out the assumption is not true on some platform. So 
> > > should I start to program devices driver with the new idea 
> > > interrupts can actually come while irqhandler is running?
> > >
> > > That's the question which really bothers me.
> > >
> > 
> > That scenario seems a little contrived to me (drivers for two or more 
> > devices sharing state through their interrupt handlers). Is it real? I 
> > suppose every platform has its quirks. The irq lock in 
> > sonic_interrupt() is only there because of a platform quirk (the same 
> > device can trigger either of two IRQs). Anyway, no-one expects all 
> > drivers to work on all platforms; I don't know why it bothers you so 
> > much when platforms differ.
> 
> Basically, we wrote drivers with the assumption that this driver will be 
> cross-platform. (Of course there are some drivers which can only work on 
> one platform, for example, if the IP of the device is only used in one 
> platform as an internal component of a specific SoC.)
> 
> So once a device has two or more interrupts, we need to consider one 
> interrupt might preempt another one on m68k on the same cpu if we also 
> want to support this driver on m68k. this usually doesn't matter on 
> other platforms.
> 

When users show up who desire to run your drivers on their platform, you 
can expect them to bring patches and a MAINTAINERS file entry. AFAIK, 
Linux development has always worked that way.

Besides, not all m68k platforms implement priority masking. So there's no 
problem with portability to m68k per se.

> on the other hand, there are more than 400 irqs_disabled() in kernel, I 
> am really not sure if they are running with the knowledge that the true 
> irqs_disabled() actually means some interrupts are off and some others 
> are still open on m68k.

Firstly, use of irqs_disabled() is considered an antipattern by some 
developers. Please see, 
https://lore.kernel.org/linux-scsi/X8pfD5XtLoOygdez@lx-t490/
and
commit e6b6be53ec91 ("Merge branch 'net-in_interrupt-cleanup-and-fixes'")

This means that the differences in semantics between the irqs_disabled() 
implementations on various architectures are moot.

Secondly, the existence of irqs_disabled() call sites does not imply a 
flaw in your drivers nor in the m68k interrupt scheme. The actual semantic 
differences are immaterial at many (all?) of these call sites.

> Or they are running with the assumption that the true irqs_disabled() 
> means IRQ is totally quiet? If the latter is true, those drivers might 
> fail to work on m68k as well.
> 

Yes it's possible, and that was my fear too back in 2017 when I raised the 
same question with the m68k maintainer. But I never found any code making 
that assumption. If you know of such a bug, do tell. So far, your fears 
remain unfounded.

> Thanks
> Barry
>