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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Fri, 8 Nov 2019 12:26:18 -0800
Message-ID: <CAHk-=wiGfnHopopFDhcGp1=wg7XY8iGm7tDjgf_zfZZy5tdRjA@mail.gmail.com>
Subject: Re: KCSAN: data-race in __alloc_file / __alloc_file
To:     Marco Elver <elver@google.com>
Cc:     Eric Dumazet <edumazet@google.com>,
        Eric Dumazet <eric.dumazet@gmail.com>,
        syzbot <syzbot+3ef049d50587836c0606@syzkaller.appspotmail.com>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        syzkaller-bugs <syzkaller-bugs@googlegroups.com>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Alan Stern <stern@rowland.harvard.edu>,
        Andrea Parri <parri.andrea@gmail.com>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        LKMM Maintainers -- Akira Yokosawa <akiyks@gmail.com>
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On Fri, Nov 8, 2019 at 11:48 AM Marco Elver <elver@google.com> wrote:
>
> It's not explicitly aware of initialization or release. We rely on
> compiler instrumentation for all memory accesses; KCSAN then sets up
> "watchpoints" for sampled memory accesses, delaying execution, and
> checking if a concurrent access is observed.

Ok.

> This same approach could be used to ignore "idempotent writes" where
> we would otherwise report a data race; i.e. if there was a concurrent
> write, but the data value did not change, do not report the race. I'm
> happy to add this feature if this should always be ignored.

Hmm. I don't think it's valid in general, but it might be useful
enough in practice, at least as an option to lower the false
positives.

               Linus