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

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Message-ID: <CANn89iK9mTJ4BN-X3MeSx5LGXGYafXkhZyaUpdXDjVivTwA6Jg@mail.gmail.com>
Date:   Fri, 8 Nov 2019 09:22:41 -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 9:01 AM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> On Fri, Nov 8, 2019 at 5:28 AM Eric Dumazet <eric.dumazet@...il.com> wrote:
> >
> > Linus, what do you think of the following fix ?
>
> I think it's incredibly ugly.
>
> I realize that avoiding the cacheline dirtying might be worth it, but
> I'd like to see some indication that it actually matters and helps
> from a performance angle. We've already dirtied memory fairly close,
> even if it might not share a cacheline (that structure is randomized,
> we've touched - or will touch - 'cred->usage') too.
>
> Honestly, I don't think get_cred() is even in a hotpath. Most cred use
> just use the current cred that doesn't need the 'get'. So the
> optimization looks somewhat questionable - for all we know it just
> makes things worse.
>
> I also don't like using a "WRITE_ONCE()" without a reason for it. In
> this case, the only "reason" is that KCSAN special-cases that thing.
> I'd much rather have some other way to mark it.
>
> So it just looks hacky to me.
>
> I like that people are looking at KCSAN, but I get a very strong
> feeling that right now the workarounds for KCSAN false-positives are
> incredibly ugly, and not always appropriate.
>
> There is absolutely zero need for a WRITE_ONCE() in this case. The
> code would work fine if the compiler did the zero write fifty times,
> and re-ordered it wildly. We have a flag that starts out set, and we
> clear it.  There's really no "write-once" about it.
>

Ok, so what do you suggest next ?

Declare KCSAN useless because too many false positives ?