linux-kernel - Re: seqcount usage in xt_replace

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Date:   Wed, 9 Jan 2019 12:55:27 +0100
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Andrea Parri <andrea.parri@...rulasolutions.com>
Cc:     Anatol Pomozov <anatol.pomozov@...il.com>,
        Florian Westphal <fw@...len.de>,
        Paul McKenney <paulmck@...ux.ibm.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Andrey Konovalov <andreyknvl@...gle.com>
Subject: Re: seqcount usage in xt_replace_table()

On Wed, Jan 9, 2019 at 12:24 PM Andrea Parri
<andrea.parri@...rulasolutions.com> wrote:
>
> On Tue, Jan 08, 2019 at 04:36:46PM -0800, Anatol Pomozov wrote:
> > Hello
> >
> > On Tue, Jan 8, 2019 at 4:02 PM Andrea Parri
> > <andrea.parri@...rulasolutions.com> wrote:
> > >
> > > Hi Anatol,
> > >
> > > On Tue, Jan 08, 2019 at 11:33:39AM -0800, Anatol Pomozov wrote:
> > > > Hello folks,
> > > >
> > > > A bit of context what I am doing. I am trying to port KTSAN (Kernel
> > > > Thread Sanitizer) tool to v4.20. That tool tracks shared data usage
> > > > and makes sure it is accessed in a thread-safe manner.
> > >
> > > Interesting!  FYI, some LKMM's maintainers (Paul included) had and
> > > continued to have some "fun" discussing topics related to "thread-
> > > safe memory accesses": I'm sure that they'll be very interested in
> > > such work of yours and eager to discuss your results.
> >
> > Thread Sanitizer is a great tool to find thread-safety issues with
> > user-space code. The tool been developed by a team of smart people
> > from Google [1].
> >
> > KTSAN is an attempt to bring the same ideas to Linux kernel [2]. A
> > bunch of work been done there but the project is still at
> > proof-of-concept point.
>
> Yes, I have been aware of these tools since at least  ;-)
>
>   https://groups.google.com/forum/#!msg/ktsan/bVZ1c6H2NE0/Dxrw55bfBAAJ
>
>
> >
> > I am not a part of Google's dynamic tools team. But I've decided to
> > pick something to do during the New Year holidays so started porting
> > KTSAN from v4.2 to v4.20. The work is "almost completed" but I need to
> > fix a few crashes [3].
>
> I guess my first reaction would remain
>
>   "it's kind of hard (to use an euphemism) to review 7,582 additions
>   or so for a data race detector without a clear/an accepted (by the
>   community) notion of data race..."

Tsan's notion of a data race is basically the C/C++'s notion:
concurrent/unsynchronized non-atomic access in different threads at
least one of which is a write.
Tremendous (for such a project) benefits of automatic data race
detection is a good motivation to finally agree on and accept a
practically useful notion of a data race.