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

Catalin Marinas
2020-09-25T12:35:11Z

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@arm.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@arm.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