linux-kernel - [PATCH tip/core/rcu 1/4] documentation: Clarify wake-up/memory-barrier relationship

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Date:	Mon,  7 Jul 2014 15:24:19 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	linux-kernel@...r.kernel.org
Cc:	mingo@...nel.org, laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	josh@...htriplett.org, niv@...ibm.com, tglx@...utronix.de,
	peterz@...radead.org, rostedt@...dmis.org, dhowells@...hat.com,
	edumazet@...gle.com, dvhart@...ux.intel.com, fweisbec@...il.com,
	oleg@...hat.com, sbw@....edu,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: [PATCH tip/core/rcu 1/4] documentation: Clarify wake-up/memory-barrier relationship

From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>

This commit adds an example demonstrating that if a wake_up() doesn't
actually wake something up, no memory ordering is provided.

Reported-by: Peter Zijlstra <peterz@...radead.org>
Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
---
 Documentation/memory-barriers.txt | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
index f1dc4a215593..a6ca533a73fc 100644
--- a/Documentation/memory-barriers.txt
+++ b/Documentation/memory-barriers.txt
@@ -1893,6 +1893,21 @@ between the STORE to indicate the event and the STORE to set TASK_RUNNING:
 	    <general barrier>		  STORE current->state
 	LOAD event_indicated
 
+To repeat, this write memory barrier is present if and only if something
+is actually awakened.  To see this, consider the following sequence of
+events, where X and Y are both initially zero:
+
+	CPU 1				CPU 2
+	===============================	===============================
+	X = 1;				STORE event_indicated
+	smp_mb();			wake_up();
+	Y = 1;				wait_event(wq, Y == 1);
+	wake_up();			  load from Y sees 1, no memory barrier
+					load from X might see 0
+
+In contrast, if a wakeup does occur, CPU 2's load from X would be guaranteed
+to see 1.
+
 The available waker functions include:
 
 	complete();
-- 
1.8.1.5

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