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

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