linux-kernel - [PATCH tip/core/rcu 3/3] documentation: State that rcu

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Message-Id: <1431465787-32541-3-git-send-email-paulmck@linux.vnet.ibm.com>
Date:	Tue, 12 May 2015 14:23:07 -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, 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,
	bobby.prani@...il.com, Milos Vyletel <milos@...hat.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: [PATCH tip/core/rcu 3/3] documentation: State that rcu_dereference() reloads pointer

From: Milos Vyletel <milos@...hat.com>

Make a note stating that repeated calls of rcu_dereference() may not
return the same pointer if update happens while in critical section.

Reported-by: Jeff Haran <jeff.haran@...rix.com>
Signed-off-by: Milos Vyletel <milos@...hat.com>
Reviewed-by: Steven Rostedt <rostedt@...dmis.org>
Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
---
 Documentation/RCU/whatisRCU.txt | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/Documentation/RCU/whatisRCU.txt b/Documentation/RCU/whatisRCU.txt
index 88dfce182f66..16622c9e86b5 100644
--- a/Documentation/RCU/whatisRCU.txt
+++ b/Documentation/RCU/whatisRCU.txt
@@ -256,7 +256,9 @@ rcu_dereference()
 	If you are going to be fetching multiple fields from the
 	RCU-protected structure, using the local variable is of
 	course preferred.  Repeated rcu_dereference() calls look
-	ugly and incur unnecessary overhead on Alpha CPUs.
+	ugly, do not guarantee that the same pointer will be returned
+	if an update happened while in the critical section, and incur
+	unnecessary overhead on Alpha CPUs.
 
 	Note that the value returned by rcu_dereference() is valid
 	only within the enclosing RCU read-side critical section.
-- 
1.8.1.5

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