Re: [PATCH memory-model] docs: memory-barriers: Add note on compiler transformation and address deps

[Paul E. McKenney]

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@xxxxxxxxxxxxxxxxx>
> > Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
> 
> Reviewed-by: Andrea Parri <parri.andrea@xxxxxxxxx>

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