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From:   Eric Dumazet <edumazet@google.com>
Date:   Tue, 12 Nov 2019 14:07:03 -0800
Message-ID: <CANn89iKjWH86kChzPiVtCgVpt3GookwGk2x1YCTMeBSPpKU+Ww@mail.gmail.com>
Subject: Re: KCSAN: data-race in __alloc_file / __alloc_file
To:     Alan Stern <stern@rowland.harvard.edu>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        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 Tue, Nov 12, 2019 at 1:48 PM Alan Stern <stern@rowland.harvard.edu> wrote:
>
> On Tue, 12 Nov 2019, Linus Torvalds wrote:
>
> > Honestly, my preferred model would have been to just add a comment,
> > and have the reporting tool know to then just ignore it. So something
> > like
> >
> > +               // Benign data-race on min_flt
> >                 tsk->min_flt++;
> >                 perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS_MIN, 1, regs, address);
> >
> > for the case that Eric mentioned - the tool would trigger on
> > "data-race", and the rest of the comment could/should be for humans.
> > Without making the code uglier, but giving the potential for a nice
> > leghibl.e explanation instead of a completely illegible "let's
> > randomly use WRITE_ONCE() here" or something like that.
>
> Just to be perfectly clear, then:
>
> Your feeling is that we don't need to tell the compiler anything at all
> about these races, because if a compiler generates code that is
> non-robust against such things then you don't want to use it for the
> kernel.
>

I would prefer some kind of explicit marking, instead of a comment.

Even if we prefer having a sane compiler, having these clearly
annotated can help
code readability quite a lot.

/*
 * To use when we are ok with minor races... bla bla bla
 */
static void inline add_relaxed(int *p, int x)
{
    x += __atomic_load_n(p, __ATOMIC_RELAXED);
    __atomic_store_n(p, x, __ATOMIC_RELAXED);
}

The actual implementation might depend on the compiler, and revert to something
without any constraint for old compilers  : *p += x;