linux-kernel - Re: [PATCH 1/2] docs: rcu: Add cautionary note on plain-accesses to requirements

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Date:   Fri, 4 Aug 2023 12:17:58 -0400
From:   Joel Fernandes <joel@...lfernandes.org>
To:     paulmck@...nel.org
Cc:     Alan Huang <mmpgouride@...il.com>, linux-kernel@...r.kernel.org,
        rcu@...r.kernel.org, Will Deacon <will@...nel.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        Neeraj Upadhyay <quic_neeraju@...cinc.com>,
        Josh Triplett <josh@...htriplett.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Zqiang <qiang.zhang1211@...il.com>,
        Jonathan Corbet <corbet@....net>
Subject: Re: [PATCH 1/2] docs: rcu: Add cautionary note on plain-accesses to requirements

[...]
> > >> However, the kernel consider the volatile access to be atomic, right?
> > >
> > > The compiler must therefore act as if a volatile access to an aligned
> > > machine-word size location is atomic.  To see this, consider accesses
> > > to memory that is shared by a device driver and that device's firmware,
> > > both of which are written in either C or C++.
> >
> > Btw it appears TSAN complaints bitterly on even volatile 4 byte data races.
> > Hence we have to explicitly use atomic API for data race accesses in Chrome.
>
> That might have been a conscious and deliberate choice on the part of
> the TSAN guys.  Volatile does not imply any ordering in the standard
> (other than the compiler avoiding emitting volatile operations out of
> order), but some compilers did emit memory-barrier instructions for
> volatile accesses.  Which resulted in a lot of problems when such code
> found compilers that did not cause the CPU to order volatile operations.
>
> So a lot of people decided to thrown the volatile baby out with the
> unordered bathwather.  ;-)

Thanks for the input, I think TSAN was indeed worried about
memory-ordering even if relaxed ordering was intended. I think there
is a way to tell TSAN to shut-up in such situations but in my last
Chrome sprint, I just used the atomic API with relaxed ordering and
called it a day. :-)

 - Joel