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From:   Eric Dumazet <edumazet@google.com>
Date:   Mon, 11 Nov 2019 13:53:07 -0800
Message-ID: <CANn89iJsh97aaAHhPTtkPjz4QFJgi9WHs3bbSTnLGrdE8qrJaA@mail.gmail.com>
Subject: Re: KCSAN: data-race in __alloc_file / __alloc_file
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Alan Stern <stern@rowland.harvard.edu>,
        Marco Elver <elver@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>,
        Andrea Parri <parri.andrea@gmail.com>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        LKMM Maintainers -- Akira Yokosawa <akiyks@gmail.com>
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On Mon, Nov 11, 2019 at 12:47 PM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> On Mon, Nov 11, 2019 at 12:43 PM Linus Torvalds
> <torvalds@linux-foundation.org> wrote:
> >
> > Yeah, maybe we could have some model for marking "this is statistics,
> > doesn't need to be exact".
>
> Side note: that marking MUST NOT be "READ_ONCE + WRITE_ONCE", because
> that makes gcc create horrible code, and only makes the race worse.
>
> At least with a regular add, it might stay as a single r-m-w
> instruction on architectures that have that, and makes the quality of
> the statistics slightly better (no preemption etc).
>
> So that's an excellent example of where changing code to use
> WRITE_ONCE actually makes the code objectively worse in practice -
> even if it might be the same in theory.

Yes, I believe that was the rationale of the ADD_ONCE() thing I
mentioned earlier.

I do not believe we have a solution right now ?

We have similar non atomic increments in some virtual network drivers
doing "dev->stats.tx_errors++;"  in their error path.