Re: KCSAN: data-race in __alloc_file / __alloc_file

From: Alexei Starovoitov <alexei.starovoitov@gmail.com>
To: Eric Dumazet <edumazet@google.com>
Cc: Alan Stern <stern@rowland.harvard.edu>,
	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>
Subject: Re: KCSAN: data-race in __alloc_file / __alloc_file
Date: Tue, 12 Nov 2019 14:44:43 -0800	[thread overview]
Message-ID: <20191112224441.2kxmt727qy4l4ncb@ast-mbp.dhcp.thefacebook.com> (raw)
In-Reply-To: <CANn89iKjWH86kChzPiVtCgVpt3GookwGk2x1YCTMeBSPpKU+Ww@mail.gmail.com>

On Tue, Nov 12, 2019 at 02:07:03PM -0800, Eric Dumazet wrote:
> 
> 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.

Annotating every line where tsk->min_flt is used with a comment
or explicit macro seems like a lot of churn.
How about adding an attribute to a field ?
Or an attribute to a type?

clang attributes can be easily exteneded. We add bpf specific attributes
that are known to clang only when 'clang -target bpf' is used.
There could be x86 or generic attributes.
Then one can do:
typedef unsigned long __attribute__((ignore_data_race)) racy_u64;
struct task_struct { 
   racy_u64 min_flt;
};

Hopefully less churn and clear signal to clang.