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Date:   Fri, 6 Oct 2023 08:49:41 -0700
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     Andrea Parri <parri.andrea@gmail.com>
Cc:     linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org,
        linux-doc@vger.kernel.org, Alan Stern <stern@rowland.harvard.edu>,
        Will Deacon <will@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Boqun Feng <boqun.feng@gmail.com>,
        Nicholas Piggin <npiggin@gmail.com>,
        David Howells <dhowells@redhat.com>,
        Jade Alglave <j.alglave@ucl.ac.uk>,
        Luc Maranget <luc.maranget@inria.fr>,
        Akira Yokosawa <akiyks@gmail.com>,
        Daniel Lustig <dlustig@nvidia.com>,
        Joel Fernandes <joel@joelfernandes.org>,
        Jonathan Corbet <corbet@lwn.net>
Subject: Re: [PATCH memory-model] docs: memory-barriers: Add note on compiler
 transformation and address deps
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On Fri, Oct 06, 2023 at 04:24:38PM +0200, Andrea Parri wrote:
> On Thu, Oct 05, 2023 at 09:53:12AM -0700, Paul E. McKenney wrote:
> > The compiler has the ability to cause misordering by destroying
> > address-dependency barriers if comparison operations are used. Add a
> > note about this to memory-barriers.txt in the beginning of both the
> > historical address-dependency sections and point to rcu-dereference.rst
> > for more information.
> > 
> > Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
> > Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> 
> Reviewed-by: Andrea Parri <parri.andrea@gmail.com>

Thank you, and I will apply on my next rebase.

							Thanx, Paul

> Thanks,
>   Andrea
> 
> 
> > diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
> > index 06e14efd8662..d414e145f912 100644
> > --- a/Documentation/memory-barriers.txt
> > +++ b/Documentation/memory-barriers.txt
> > @@ -396,6 +396,10 @@ Memory barriers come in four basic varieties:
> >  
> >  
> >   (2) Address-dependency barriers (historical).
> > +     [!] This section is marked as HISTORICAL: For more up-to-date
> > +     information, including how compiler transformations related to pointer
> > +     comparisons can sometimes cause problems, see
> > +     Documentation/RCU/rcu_dereference.rst.
> >  
> >       An address-dependency barrier is a weaker form of read barrier.  In the
> >       case where two loads are performed such that the second depends on the
> > @@ -556,6 +560,9 @@ There are certain things that the Linux kernel memory barriers do not guarantee:
> >  
> >  ADDRESS-DEPENDENCY BARRIERS (HISTORICAL)
> >  ----------------------------------------
> > +[!] This section is marked as HISTORICAL: For more up-to-date information,
> > +including how compiler transformations related to pointer comparisons can
> > +sometimes cause problems, see Documentation/RCU/rcu_dereference.rst.
> >  
> >  As of v4.15 of the Linux kernel, an smp_mb() was added to READ_ONCE() for
> >  DEC Alpha, which means that about the only people who need to pay attention