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From:   Marco Elver <elver@google.com>
Date:   Thu, 24 Oct 2019 13:02:03 +0200
Message-ID: <CANpmjNOOT+KR7m8KpETk1czyJLr3TeHsvvejwyuY3JXKr=eajg@mail.gmail.com>
Subject: Re: [PATCH v2 1/8] kcsan: Add Kernel Concurrency Sanitizer infrastructure
To:     Oleg Nesterov <oleg@redhat.com>
Cc:     LKMM Maintainers -- Akira Yokosawa <akiyks@gmail.com>,
        Alan Stern <stern@rowland.harvard.edu>,
        Alexander Potapenko <glider@google.com>,
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On Wed, 23 Oct 2019 at 18:24, Oleg Nesterov <oleg@redhat.com> wrote:
>
> 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 = 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.
>