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Message-ID: <ceaeba0a-fc30-4635-802a-668c859a58b2@paulmck-laptop>
Date: Thu, 5 Oct 2023 09:53:12 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
linux-doc@...r.kernel.org
Cc: 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>,
Akira Yokosawa <akiyks@...il.com>,
Daniel Lustig <dlustig@...dia.com>,
Joel Fernandes <joel@...lfernandes.org>,
Jonathan Corbet <corbet@....net>
Subject: [PATCH memory-model] 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@...lfernandes.org>
Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
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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