linux-kernel - Re: [PATCH v2 03/23] kcsan: Avoid checking scoped accesses from nested contexts

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Date:   Mon, 29 Nov 2021 15:42:59 +0100
From:   Marco Elver <elver@...gle.com>
To:     Boqun Feng <boqun.feng@...il.com>
Cc:     "Paul E. McKenney" <paulmck@...nel.org>,
        Alexander Potapenko <glider@...gle.com>,
        Borislav Petkov <bp@...en8.de>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Ingo Molnar <mingo@...nel.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Mark Rutland <mark.rutland@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Waiman Long <longman@...hat.com>,
        Will Deacon <will@...nel.org>, kasan-dev@...glegroups.com,
        linux-arch@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-kbuild@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, x86@...nel.org
Subject: Re: [PATCH v2 03/23] kcsan: Avoid checking scoped accesses from
 nested contexts

On Mon, 29 Nov 2021 at 15:27, Boqun Feng <boqun.feng@...il.com> wrote:
[...]
> > This case is also possible:
> >
> >       static int v;
> >       static int x;
> >       int foo(..)
> >       {
> >               ASSERT_EXCLUSIVE_ACCESS_SCOPED(v);
> >               x++; // preempted during watchpoint for 'v' after checking x++
> >       }
> >
> > Here, all we need is for the scoped access to be checked after x++, end
> > up with a watchpoint for it, then enter scheduler code, which then
> > checked 'v', sees the conflicting watchpoint, and reports a nonsensical
> > race again.
> >
>
> Just to be clear, in both examples, the assumption is that 'v' is a
> variable that scheduler code doesn't access, right? Because if scheduler
> code does access 'v', then it's a problem that KCSAN should report. Yes,
> I don't know any variable that scheduler exports, just to make sure
> here.

Right. We might miss such cases where an ASSERT_EXCLUSIVE*_SCOPED()
could have pointed out a legitimate race with a nested context that
share ctx, like in scheduler, where the only time to detect it is if
some state change later in the scope makes a concurrent access
possible from that point in the scope. I'm willing to bet that there's
an extremely small chance we'll ever encounter such a case (famous
last words ;-)), i.e. the initial check_access() in
kcsan_begin_scoped_access() wouldn't detect it nor would the problem
manifest as a regular data race.

Thanks,
-- Marco