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From: Marco Elver <elver@google.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Eric Dumazet <edumazet@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>,
	Alan Stern <stern@rowland.harvard.edu>,
	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: Fri, 8 Nov 2019 19:15:58 +0100
Message-ID: <CANpmjNOuRp0gdekQeodXm8O_yiXm7mA8WZsXZNmFfJYMs93x8w@mail.gmail.com> (raw)
In-Reply-To: <CAHk-=wiZdSoweA-W_8iwLy6KLsd-DaZM0gN9_+f-aT4KL64U0g@mail.gmail.com>

On Fri, 8 Nov 2019 at 19:05, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> On Fri, Nov 8, 2019 at 9:53 AM Eric Dumazet <edumazet@google.com> wrote:
> >
> > 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)
>
> So I generally prefer WRITE_ONCE() over adding "volatile" to random
> data structure members.
>
> Because volatile *does* have potentially absolutely horrendous
> overhead on generated code. It just happens to be ok for the simple
> case of writing once to a variable.
>
> In fact, you bring that up yourself in your next email when you ask
> for "ADD_ONCE()". Exactly because gcc generates absolutely horrendous
> garbage for volatiles, for no actual good reason. Gcc *could* generate
> a single add-to-memory instruction. But no, that's not at all what gcc
> does.
>
> So for the kernel, we've generally had the rule to avoid 'volatile'
> data structures as much as humanly possible, because it actually does
> something much worse than it could do, and the source code _looks_
> simple when the volatile is hidden in the data structures.
>
> Which is why we have READ_ONCE/WRITE_ONCE - it puts the volatile in
> the code, and makes it clear not only what is going on, but also the
> impact it has on code generation.
>
> But at the same time, I don't love WRITE_ONCE() when it's not actually
> about writing once. It might be better to have another way to show
> "this variable is a flag that we set to a single value". Even if maybe
> the implementation is then the same (ie we use a 'volatile' assignment
> to make KCSAN happy).

(+some LKMM folks, in case I missed something on what the LKMM defines
as data race.)

KCSAN does not use volatile to distinguish accesses. Right now
READ_ONCE, WRITE_ONCE, atomic bitops, atomic_t (+ some arch specific
primitives) are treated as marked atomic operations.

The goal is to cover all primitives that the LKMM declares as
marked/atomic. A data race is then detected for concurrent conflicting
accesses where at least one is plain unmarked. In the end the LKMM
should decide what KCSAN determines as a data race. As far as I can
tell, none of the reported data races so far are false positives in
that sense.

Many thanks,
-- Marco

  reply index

Thread overview: 67+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-11-08 13:16 syzbot
2019-11-08 13:28 ` Eric Dumazet
2019-11-08 17:01   ` Linus Torvalds
2019-11-08 17:22     ` Eric Dumazet
2019-11-08 17:38       ` Linus Torvalds
2019-11-08 17:53         ` Eric Dumazet
2019-11-08 17:55           ` Eric Dumazet
2019-11-08 18:02             ` Eric Dumazet
2019-11-08 18:12               ` Linus Torvalds
2019-11-08 20:30             ` Linus Torvalds
2019-11-08 20:53               ` Eric Dumazet
2019-11-08 21:36                 ` Linus Torvalds
2019-11-08 18:05           ` Linus Torvalds
2019-11-08 18:15             ` Marco Elver [this message]
2019-11-08 18:40               ` Linus Torvalds
2019-11-08 19:48                 ` Marco Elver
2019-11-08 20:26                   ` Linus Torvalds
2019-11-08 21:57                     ` Alan Stern
2019-11-08 22:06                       ` Linus Torvalds
2019-11-09 23:08                         ` Alan Stern
     [not found] <CAHk-=wjB61GNmqpX0BLA5tpL4tsjWV7akaTc2Roth7uGgax+mw@mail.gmail.com>
2019-11-10 16:09 ` Alan Stern
2019-11-10 19:10   ` Marco Elver
2019-11-11 15:51     ` Alan Stern
2019-11-11 16:51       ` Linus Torvalds
2019-11-11 17:52         ` Eric Dumazet
2019-11-11 18:04           ` Linus Torvalds
2019-11-11 18:31             ` Eric Dumazet
2019-11-11 18:44               ` Eric Dumazet
2019-11-11 19:00                 ` Linus Torvalds
2019-11-11 19:13                   ` Eric Dumazet
2019-11-11 20:43                     ` Linus Torvalds
2019-11-11 20:46                       ` Linus Torvalds
2019-11-11 21:53                         ` Eric Dumazet
2019-11-11 23:51                   ` Linus Torvalds
2019-11-12 16:50                     ` Kirill Smelkov
2019-11-12 17:23                       ` Linus Torvalds
2019-11-12 17:36                         ` Linus Torvalds
2019-11-17 18:56                           ` Kirill Smelkov
2019-11-17 19:20                             ` Linus Torvalds
2019-11-11 18:50               ` Linus Torvalds
2019-11-11 18:59                 ` Marco Elver
2019-11-11 18:59                 ` Eric Dumazet
2019-11-10 19:12   ` Linus Torvalds
2019-11-10 19:20     ` Linus Torvalds
2019-11-10 20:44       ` Paul E. McKenney
2019-11-10 21:10         ` Linus Torvalds
2019-11-10 21:31           ` Paul E. McKenney
2019-11-11 14:17         ` Marco Elver
2019-11-11 14:31           ` Paul E. McKenney
2019-11-11 15:10             ` Marco Elver
2019-11-13  0:25               ` Paul E. McKenney
2019-11-12 19:14     ` Alan Stern
2019-11-12 19:47       ` Linus Torvalds
2019-11-12 20:29         ` Alan Stern
2019-11-12 20:58           ` Linus Torvalds
2019-11-12 21:13             ` Linus Torvalds
2019-11-12 22:05               ` Marco Elver
2019-11-12 21:48             ` Alan Stern
2019-11-12 22:07               ` Eric Dumazet
2019-11-12 22:44                 ` Alexei Starovoitov
2019-11-12 23:17                   ` Eric Dumazet
2019-11-12 23:40                     ` Linus Torvalds
2019-11-13 15:00                       ` Marco Elver
2019-11-13 16:57                         ` Linus Torvalds
2019-11-13 21:33                           ` Marco Elver
2019-11-13 21:50                             ` Alan Stern
2019-11-13 22:48                               ` Marco Elver

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