docs: memory-barriers: Add note on compiler transformation and address deps

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>
Cc: Alan Stern <stern@rowland.harvard.edu>
Cc: Will Deacon <will@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: David Howells <dhowells@redhat.com>
Cc: Jade Alglave <j.alglave@ucl.ac.uk>
Cc: Luc Maranget <luc.maranget@inria.fr>
Cc: Akira Yokosawa <akiyks@gmail.com>
Cc: Daniel Lustig <dlustig@nvidia.com>
Cc: Joel Fernandes <joel@joelfernandes.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Reviewed-by: Andrea Parri <parri.andrea@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

Author: joelagnel

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