Re: KCSAN: data-race in __alloc_file / __alloc_file

From: Eric Dumazet <edumazet@google.com>
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>
Subject: Re: KCSAN: data-race in __alloc_file / __alloc_file
Date: Fri, 8 Nov 2019 09:55:50 -0800	[thread overview]
Message-ID: <CANn89i+RrngUr11_iOYDuqDvAZnPfG3ieJR025M78uhiwEPuvQ@mail.gmail.com> (raw)
In-Reply-To: <CANn89iJJiB6avNtZ1qQNTeJwyjW32Pxk_2CwvEJxgQ==kgY0fA@mail.gmail.com>

On Fri, Nov 8, 2019 at 9:53 AM Eric Dumazet <edumazet@google.com> wrote:
>
> 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)
>

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.