linux-kernel - [PATCH RFC tip/core/rcu 01/15] doc: READ_ONCE() now implies smp_barrier

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Date:   Mon,  9 Oct 2017 17:22:35 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     linux-kernel@...r.kernel.org
Cc:     mingo@...nel.org, torvalds@...ux-foundation.org,
        mark.rutland@....com, dhowells@...hat.com,
        linux-arch@...r.kernel.org, peterz@...radead.org,
        will.deacon@....com,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: [PATCH RFC tip/core/rcu 01/15] doc: READ_ONCE() now implies smp_barrier_depends()

This commit updates an example in memory-barriers.txt to account for
the fact that READ_ONCE() now implies smp_barrier_depends().

Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
---
 Documentation/memory-barriers.txt | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
index b759a60624fd..b592bb265e5c 100644
--- a/Documentation/memory-barriers.txt
+++ b/Documentation/memory-barriers.txt
@@ -227,17 +227,18 @@ There are some minimal guarantees that may be expected of a CPU:
  (*) On any given CPU, dependent memory accesses will be issued in order, with
      respect to itself.  This means that for:
 
-	Q = READ_ONCE(P); smp_read_barrier_depends(); D = READ_ONCE(*Q);
+	Q = READ_ONCE(P); D = READ_ONCE(*Q);
 
      the CPU will issue the following memory operations:
 
 	Q = LOAD P, D = LOAD *Q
 
      and always in that order.  On most systems, smp_read_barrier_depends()
-     does nothing, but it is required for DEC Alpha.  The READ_ONCE()
-     is required to prevent compiler mischief.  Please note that you
-     should normally use something like rcu_dereference() instead of
-     open-coding smp_read_barrier_depends().
+     does nothing, but it is required for DEC Alpha, and is supplied by
+     READ_ONCE().  The READ_ONCE() is also required to prevent compiler
+     mischief.  Please note that you should normally use something
+     like READ_ONCE() or rcu_dereference() instead of open-coding
+     smp_read_barrier_depends().
 
  (*) Overlapping loads and stores within a particular CPU will appear to be
      ordered within that CPU.  This means that for:
-- 
2.5.2