linux-kernel - Re: [PATCH 1/8] kcsan: Add Kernel Concurrency Sanitizer infrastructure

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Date:   Thu, 17 Oct 2019 09:47:30 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Marco Elver <elver@...gle.com>
Cc:     LKMM Maintainers -- Akira Yokosawa <akiyks@...il.com>,
        Alan Stern <stern@...land.harvard.edu>,
        Alexander Potapenko <glider@...gle.com>,
        Andrea Parri <parri.andrea@...il.com>,
        Andrey Konovalov <andreyknvl@...gle.com>,
        Andy Lutomirski <luto@...nel.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Arnd Bergmann <arnd@...db.de>,
        Boqun Feng <boqun.feng@...il.com>,
        Borislav Petkov <bp@...en8.de>, Daniel Axtens <dja@...ens.net>,
        Daniel Lustig <dlustig@...dia.com>,
        dave.hansen@...ux.intel.com, David Howells <dhowells@...hat.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        Jade Alglave <j.alglave@....ac.uk>,
        Joel Fernandes <joel@...lfernandes.org>,
        Jonathan Corbet <corbet@....net>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Luc Maranget <luc.maranget@...ia.fr>,
        Mark Rutland <mark.rutland@....com>,
        Nicholas Piggin <npiggin@...il.com>,
        "Paul E. McKenney" <paulmck@...ux.ibm.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Will Deacon <will@...nel.org>,
        kasan-dev <kasan-dev@...glegroups.com>,
        linux-arch <linux-arch@...r.kernel.org>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        linux-efi@...r.kernel.org,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux Memory Management List <linux-mm@...ck.org>,
        the arch/x86 maintainers <x86@...nel.org>
Subject: Re: [PATCH 1/8] kcsan: Add Kernel Concurrency Sanitizer
 infrastructure

On Wed, Oct 16, 2019 at 09:34:05PM +0200, Marco Elver wrote:
> On Wed, 16 Oct 2019 at 20:44, Peter Zijlstra <peterz@...radead.org> wrote:
> > > +     /*
> > > +      * Disable interrupts & preemptions, to ignore races due to accesses in
> > > +      * threads running on the same CPU.
> > > +      */
> > > +     local_irq_save(irq_flags);
> > > +     preempt_disable();
> >
> > Is there a point to that preempt_disable() here?
> 
> We want to avoid being preempted while the watchpoint is set up;
> otherwise, we would report data-races for CPU-local data, which is
> incorrect.

Disabling IRQs already very much disables preemption. There is
absolutely no point in doing preempt_disable() when the whole section
already runs with IRQs disabled.