linux-kernel - Re: [PATCH v2 05/12] x86: rework arch_local_irq

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Date:   Wed, 9 Dec 2020 14:02:21 +0000
From:   Mark Rutland <mark.rutland@....com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Jürgen Groß <jgross@...e.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Stefano Stabellini <sstabellini@...nel.org>,
        "VMware, Inc." <pv-drivers@...are.com>, X86 ML <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux Virtualization <virtualization@...ts.linux-foundation.org>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        xen-devel <xen-devel@...ts.xenproject.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>
Subject: Re: [PATCH v2 05/12] x86: rework arch_local_irq_restore() to not use
 popf

On Wed, Dec 09, 2020 at 01:27:10PM +0000, Mark Rutland wrote:
> On Sun, Nov 22, 2020 at 01:44:53PM -0800, Andy Lutomirski wrote:
> > On Sat, Nov 21, 2020 at 10:55 PM Jürgen Groß <jgross@...e.com> wrote:
> > > On 20.11.20 12:59, Peter Zijlstra wrote:
> > > > If someone were to write horrible code like:
> > > >
> > > >       local_irq_disable();
> > > >       local_irq_save(flags);
> > > >       local_irq_enable();
> > > >       local_irq_restore(flags);
> > > >
> > > > we'd be up some creek without a paddle... now I don't _think_ we have
> > > > genius code like that, but I'd feel saver if we can haz an assertion in
> > > > there somewhere...

> I was just talking to Peter on IRC about implementing the same thing for
> arm64, so could we put this in the generic irqflags code? IIUC we can
> use raw_irqs_disabled() to do the check.
> 
> As this isn't really entry specific (and IIUC the cases this should
> catch would break lockdep today), maybe we should add a new
> DEBUG_IRQFLAGS for this, that DEBUG_LOCKDEP can also select?
> 
> Something like:
> 
> #define local_irq_restore(flags)                               \
>        do {                                                    \
>                if (!raw_irqs_disabled_flags(flags)) {          \
>                        trace_hardirqs_on();                    \
>                } else if (IS_ENABLED(CONFIG_DEBUG_IRQFLAGS) {  \
>                        if (unlikely(raw_irqs_disabled())       \

Whoops; that should be !raw_irqs_disabled().

>                                warn_bogus_irqrestore();        \
>                }                                               \
>                raw_local_irq_restore(flags);                   \
>         } while (0)
> 
> ... perhaps? (ignoring however we deal with once-ness).

If no-one shouts in the next day or two I'll spin this as its own patch.

Mark.