linux-kernel - Re: KCSAN: data-race in __alloc_file / __alloc

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Message-ID: <CANn89i+RrngUr11_iOYDuqDvAZnPfG3ieJR025M78uhiwEPuvQ@mail.gmail.com>
Date:   Fri, 8 Nov 2019 09:55:50 -0800
From:   Eric Dumazet <edumazet@...gle.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Eric Dumazet <eric.dumazet@...il.com>,
        syzbot <syzbot+3ef049d50587836c0606@...kaller.appspotmail.com>,
        Marco Elver <elver@...gle.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
        Al Viro <viro@...iv.linux.org.uk>
Subject: Re: KCSAN: data-race in __alloc_file / __alloc_file

On Fri, Nov 8, 2019 at 9:53 AM Eric Dumazet <edumazet@...gle.com> wrote:
>
> On Fri, Nov 8, 2019 at 9:39 AM Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
>
> > I'd hope that there is some way to mark the cases we know about where
> > we just have a flag. I'm not sure what KCSAN uses right now - is it
> > just the "volatile" that makes KCSAN ignore it, or are there other
> > ways to do it?
>
> I dunno, Marco will comment on this.
>
> I personally like WRITE_ONCE() since it adds zero overhead on generated code,
> and is the facto accessor we used for many years (before KCSAN was conceived)
>

BTW, I would love an efficient ADD_ONCE(variable, value)

Using WRITE_ONCE(variable, variable + value) is not good, since it can
not use the
optimized instructions operating directly on memory.