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Date:   Wed, 23 Oct 2019 18:24:32 +0200
From:   Oleg Nesterov <oleg@redhat.com>
To:     Marco Elver <elver@google.com>
Cc:     LKMM Maintainers -- Akira Yokosawa <akiyks@gmail.com>,
        Alan Stern <stern@rowland.harvard.edu>,
        Alexander Potapenko <glider@google.com>,
        Andrea Parri <parri.andrea@gmail.com>,
        Andrey Konovalov <andreyknvl@google.com>,
        Andy Lutomirski <luto@kernel.org>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Arnd Bergmann <arnd@arndb.de>,
        Boqun Feng <boqun.feng@gmail.com>,
        Borislav Petkov <bp@alien8.de>, Daniel Axtens <dja@axtens.net>,
        Daniel Lustig <dlustig@nvidia.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        David Howells <dhowells@redhat.com>,
        Dmitry Vyukov <dvyukov@google.com>,
        "H. Peter Anvin" <hpa@zytor.com>, Ingo Molnar <mingo@redhat.com>,
        Jade Alglave <j.alglave@ucl.ac.uk>,
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        Peter Zijlstra <peterz@infradead.org>,
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Subject: Re: [PATCH v2 1/8] kcsan: Add Kernel Concurrency Sanitizer
 infrastructure
Message-ID: <20191023162432.GC14327@redhat.com>
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On 10/22, Marco Elver wrote:
>
> On Tue, 22 Oct 2019 at 17:49, Oleg Nesterov <oleg@redhat.com> wrote:
> >
> > Just for example. Suppose that task->state =3D TASK_UNINTERRUPTIBLE, th=
is task
> > does __set_current_state(TASK_RUNNING), another CPU does wake_up_proces=
s(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" thoug=
h.
>
> 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() ?

Oleg.