[PATCH 2/2] docs: memory-barriers: Add note on plain-accesses to address-dependency barriers

[Joel Fernandes (Google)]

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 and point to rcu-dereference.rst
for more information.

Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
---
 Documentation/memory-barriers.txt | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
index 06e14efd8662..acc8ec5ce563 100644
--- a/Documentation/memory-barriers.txt
+++ b/Documentation/memory-barriers.txt
@@ -435,6 +435,11 @@ Memory barriers come in four basic varieties:
      variables such as READ_ONCE() and rcu_dereference() provide implicit
      address-dependency barriers.
 
+     [!] Note that address dependency barriers can be destroyed by comparison
+     of a pointer obtained by a marked accessor such as READ_ONCE() or
+     rcu_dereference() with some value.  For an example of this, see
+     rcu_dereference.rst (part where the comparison of pointers is discussed).
+
  (3) Read (or load) memory barriers.
 
      A read barrier is an address-dependency barrier plus a guarantee that all
-- 
2.41.0.585.gd2178a4bd4-goog