linux-kernel - [PATCH locking/Documentation 2/2] No speculated stores

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Message-Id: <20160929155506.GA5255@linux.vnet.ibm.com>
Date:   Thu, 29 Sep 2016 08:55:06 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     linux-kernel@...r.kernel.org
Cc:     mingo@...nel.org, dhowells@...hat.com, will.deacon@....com,
        peterz@...radead.org
Subject: [PATCH locking/Documentation 2/2] No speculated stores

This commit reworks an erroneous example that claims that dependency
barriers are needed to prevent speculation of dependent stores.

Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
---
 Documentation/memory-barriers.txt | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
index a57679ec9441..b6307139b81a 100644
--- a/Documentation/memory-barriers.txt
+++ b/Documentation/memory-barriers.txt
@@ -598,7 +598,9 @@ between the address load and the data load:
 This enforces the occurrence of one of the two implications, and prevents the
 third possibility from arising.
 
-A data-dependency barrier must also order against dependent writes:
+However, writes are never speculated, so it is not necessary (but is
+good documentation practice) to use data-dependency barrier to order
+against dependent writes:
 
 	CPU 1		      CPU 2
 	===============	      ===============
@@ -607,11 +609,11 @@ A data-dependency barrier must also order against dependent writes:
 	<write barrier>
 	WRITE_ONCE(P, &B);
 			      Q = READ_ONCE(P);
-			      <data dependency barrier>
 			      *Q = 5;
 
-The data-dependency barrier must order the read into Q with the store
-into *Q.  This prohibits this outcome:
+The prohibition against speculating writes means that even without a
+data-dependency barrier, the system must order the read into Q with the
+store into *Q.  This prohibits this outcome:
 
 	(Q == &B) && (B == 4)
 
-- 
2.5.2