Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
MIME-Version: 1.0
References: <000000000000c422a80596d595ee@google.com> <6bddae34-93df-6820-0390-ac18dcbf0927@gmail.com>
 <CAHk-=whh5bcxCecEL5Fy4XvQjgBTJ9uqvyp7dW=CLU6VNxS9iA@mail.gmail.com>
 <CANn89iK9mTJ4BN-X3MeSx5LGXGYafXkhZyaUpdXDjVivTwA6Jg@mail.gmail.com> <CAHk-=whNBL63qmO176qOQpkY16xvomog5ocvM=9K55hUgAgOPA@mail.gmail.com>
In-Reply-To: <CAHk-=whNBL63qmO176qOQpkY16xvomog5ocvM=9K55hUgAgOPA@mail.gmail.com>
From:   Eric Dumazet <edumazet@google.com>
Date:   Fri, 8 Nov 2019 09:53:01 -0800
Message-ID: <CANn89iJJiB6avNtZ1qQNTeJwyjW32Pxk_2CwvEJxgQ==kgY0fA@mail.gmail.com>
Subject: Re: KCSAN: data-race in __alloc_file / __alloc_file
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Eric Dumazet <eric.dumazet@gmail.com>,
        syzbot <syzbot+3ef049d50587836c0606@syzkaller.appspotmail.com>,
        Marco Elver <elver@google.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>
Content-Type: text/plain; charset="UTF-8"
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Fri, Nov 8, 2019 at 9:39 AM Linus Torvalds
<torvalds@linux-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)

>
> "volatile" has huge problems with code generation for gcc. It would
> probably be fine for "not_rcu" in this case, but I'd like to avoid it
> in general otherwise, which is why I wonder if there are other
> options.
>
> But worst comes to worst, I'd be ok with a WRITE_ONCE() and a comment
> about why (and the reason being KCSAN, not the questionable
> optimization).

Ok for a single WRITE_ONCE() with a comment.

Hmm, which questionable optimization are you referring to?