linux-kernel - Re: [PATCH v2 1/8] kcsan: Add Kernel Concurrency Sanitizer infrastructure

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Date:   Thu, 24 Oct 2019 13:02:03 +0200
From:   Marco Elver <elver@...gle.com>
To:     Oleg Nesterov <oleg@...hat.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 <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>,
        Peter Zijlstra <peterz@...radead.org>,
        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 v2 1/8] kcsan: Add Kernel Concurrency Sanitizer infrastructure

On Wed, 23 Oct 2019 at 18:24, Oleg Nesterov <oleg@...hat.com> wrote:
>
> On 10/22, Marco Elver wrote:
> >
> > On Tue, 22 Oct 2019 at 17:49, Oleg Nesterov <oleg@...hat.com> wrote:
> > >
> > > Just for example. Suppose that task->state = TASK_UNINTERRUPTIBLE, this task
> > > does __set_current_state(TASK_RUNNING), another CPU does wake_up_process(task)
> > > which does the same UNINTERRUPTIBLE -> RUNNING transition.
> > >
> > > Looks like, this is the "data race" according to kcsan?
> >
> > Yes, they are "data races". They are probably not "race conditions" though.
> >
> > This is a fair distinction to make, and we never claimed to find "race
> > conditions" only
>
> I see, thanks, just wanted to be sure...
>
> > KCSAN's goal is to find *data races* according to the LKMM.  Some data
> > races are race conditions (usually the more interesting bugs) -- but
> > not *all* data races are race conditions. Those are what are usually
> > referred to as "benign", but they can still become bugs on the wrong
> > arch/compiler combination. Hence, the need to annotate these accesses
> > with READ_ONCE, WRITE_ONCE or use atomic_t:
>
> Well, if I see READ_ONCE() in the code I want to understand why it was
> used. Is it really needed for correctness or we want to shut up kcsan?
> Say, why should wait_event(wq, *ptr) use READ_ONCE()? Nevermind, please
> forget.
>
> Btw, why __kcsan_check_watchpoint() does user_access_save() before
> try_consume_watchpoint() ?

Instrumentation is added in UACCESS regions. Since we do not access
user-memory, we do user_access_save to ensure everything is safe
(otherwise objtool complains that we do calls to non-whitelisted
functions). I will try to optimize this a bit, but we can't avoid it.

> Oleg.
>