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 20:48:29 +0100 [thread overview]
Message-ID: <CANpmjNO6UgNS9h5ZwSV2c+uKz04ch96d+f0-jquDj_ekOjr5bQ@mail.gmail.com> (raw)
In-Reply-To: <CAHk-=wjodfXqd9=iW=ziFrfY7xqopgO3Ko_HrAUp-kUQHHyyqg@mail.gmail.com>
On Fri, 8 Nov 2019 at 19:40, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> On Fri, Nov 8, 2019 at 10:16 AM Marco Elver <elver@google.com> wrote:
> >
> > 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.
>
> Ok, so we'd have to do this in terms of ATOMIC_WRITE().
>
> One alternative might be KCSAN enhancement, where you notice the
> following pattern:
>
> - a field is initialized before the data structure gets exposed (I
> presume KCSAN already must understand about this issue -
> initializations are different and not atomic)
>
> - while the field is live, there are operations that write the same
> (let's call it "idempotent") value to the field under certain
> circumstances
>
> - at release time, after all the reference counts are gone, the field
> is read for whether that situation happened. I'm assuming KCSAN
> already understands about this case too?
It's not explicitly aware of initialization or release. We rely on
compiler instrumentation for all memory accesses; KCSAN then sets up
"watchpoints" for sampled memory accesses, delaying execution, and
checking if a concurrent access is observed.
We already have an option (currently disabled on syzbot) where KCSAN
infers a data race not because another instrumented accesses happened
concurrently, but because the data value changed during a watchpoint's
lifetime (e.g. due to uninstrumented write, device write etc.).
This same approach could be used to ignore "idempotent writes" where
we would otherwise report a data race; i.e. if there was a concurrent
write, but the data value did not change, do not report the race. I'm
happy to add this feature if this should always be ignored.
> So it would only be the "idempotent writes" thing that would be
> something KCSAN would have to realize do not involve a race - because
> it simply doesn't matter if two writes of the same value race against
> each other.
>
> But I guess we could also just do
>
> #define WRITE_IDEMPOTENT(x,y) WRITE_ONCE(x,y)
>
> and use that in the kernel to annotate these things. And if we have
> that kind of annotation, we could then possibly change it to
>
> #define WRITE_IDEMPOTENT(x,y) \
> if READ_ONCE(x)!=y WRITE_ONCE(x,y)
>
> if we have numbers that that actually helps (that macro written to be
> intentionally invalid C - it obviously needs statement protection and
> protection against evaluating the arguments multiple times etc).
>
> Linus
Thanks,
-- Marco
next prev parent reply other threads:[~2019-11-08 19:48 UTC|newest]
Thread overview: 67+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-11-08 13:16 KCSAN: data-race in __alloc_file / __alloc_file 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
2019-11-08 18:40 ` Linus Torvalds
2019-11-08 19:48 ` Marco Elver [this message]
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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