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

From: Joel Fernandes (Google)
Date: Wed Aug 02 2023 - 23:24:34 EST

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