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

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Message-ID: <20230803032408.2514989-2-joel@joelfernandes.org>
Date:   Thu,  3 Aug 2023 03:24:07 +0000
From:   "Joel Fernandes (Google)" <joel@...lfernandes.org>
To:     linux-kernel@...r.kernel.org, rcu@...r.kernel.org
Cc:     "Joel Fernandes (Google)" <joel@...lfernandes.org>,
        Alan Stern <stern@...land.harvard.edu>,
        Andrea Parri <parri.andrea@...il.com>,
        Will Deacon <will@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Nicholas Piggin <npiggin@...il.com>,
        David Howells <dhowells@...hat.com>,
        Jade Alglave <j.alglave@....ac.uk>,
        Luc Maranget <luc.maranget@...ia.fr>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Akira Yokosawa <akiyks@...il.com>,
        Daniel Lustig <dlustig@...dia.com>,
        Jonathan Corbet <corbet@....net>
Subject: [PATCH 2/2] docs: memory-barriers: Add note on plain-accesses to address-dependency barriers

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@...lfernandes.org>
---
 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