linux-kernel - Re: [PATCH v3 26/39] arm64: mte: Add in-kernel tag fault handler

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Date:   Fri, 25 Sep 2020 13:35:04 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     Andrey Konovalov <andreyknvl@...gle.com>
Cc:     Dmitry Vyukov <dvyukov@...gle.com>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        kasan-dev <kasan-dev@...glegroups.com>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Alexander Potapenko <glider@...gle.com>,
        Marco Elver <elver@...gle.com>,
        Evgenii Stepanov <eugenis@...gle.com>,
        Elena Petrova <lenaptr@...gle.com>,
        Branislav Rankov <Branislav.Rankov@....com>,
        Kevin Brodsky <kevin.brodsky@....com>,
        Will Deacon <will.deacon@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Memory Management List <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 26/39] arm64: mte: Add in-kernel tag fault handler

On Fri, Sep 25, 2020 at 01:52:56PM +0200, Andrey Konovalov wrote:
> On Fri, Sep 25, 2020 at 1:47 PM Catalin Marinas <catalin.marinas@....com> wrote:
> > On Fri, Sep 25, 2020 at 01:26:02PM +0200, Andrey Konovalov wrote:
> > > On Fri, Sep 25, 2020 at 12:49 PM Catalin Marinas
> > > <catalin.marinas@....com> wrote:
> > > > > +
> > > > >  static void __do_kernel_fault(unsigned long addr, unsigned int esr,
> > > > >                             struct pt_regs *regs)
> > > > >  {
> > > > > @@ -641,10 +647,40 @@ static int do_sea(unsigned long addr, unsigned int esr, struct pt_regs *regs)
> > > > >       return 0;
> > > > >  }
> > > > >
> > > > > +static void do_tag_recovery(unsigned long addr, unsigned int esr,
> > > > > +                        struct pt_regs *regs)
> > > > > +{
> > > > > +     static bool reported = false;
> > > > > +
> > > > > +     if (!READ_ONCE(reported)) {
> > > > > +             report_tag_fault(addr, esr, regs);
> > > > > +             WRITE_ONCE(reported, true);
> > > > > +     }
> > > >
> > > > I don't mind the READ_ONCE/WRITE_ONCE here but not sure what they help
> > > > with.
> > >
> > > The fault can happen on multiple cores at the same time, right? In
> > > that case without READ/WRITE_ONCE() we'll have a data-race here.
> >
> > READ/WRITE_ONCE won't magically solve such races. If two CPUs enter
> > simultaneously in do_tag_recovery(), they'd both read 'reported' as
> > false and both print the fault info.
> 
> They won't solve the race condition, but they will solve the data
> race. I guess here we don't really care about the race condition, as
> printing a tag fault twice is OK. But having a data race here will
> lead to KCSAN reports, although won't probably break anything in
> practice.

I agree, in practice it should be fine. Anyway, it doesn't hurt leaving
them in place.

-- 
Catalin