linux-kernel - [PATCH 3.12 61/82] rcu: Provide counterpart to rcu

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Date:	Mon, 24 Aug 2015 11:09:21 +0200
From:	Jiri Slaby <jslaby@...e.cz>
To:	stable@...r.kernel.org
Cc:	linux-kernel@...r.kernel.org,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Al Viro <viro@...iv.linux.org.uk>, Jiri Slaby <jslaby@...e.cz>
Subject: [PATCH 3.12 61/82] rcu: Provide counterpart to rcu_dereference() for non-RCU situations

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

3.12-stable review patch.  If anyone has any objections, please let me know.

===============

commit 54ef6df3f3f1353d99c80c437259d317b2cd1cbd upstream.

Although rcu_dereference() and friends can be used in situations where
object lifetimes are being managed by something other than RCU, the
resulting sparse and lockdep-RCU noise can be annoying.  This commit
therefore supplies a lockless_dereference(), which provides the
protection for dereferences without the RCU-related debugging noise.

Reported-by: Al Viro <viro@...IV.linux.org.uk>
Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
Signed-off-by: Al Viro <viro@...iv.linux.org.uk>
Signed-off-by: Jiri Slaby <jslaby@...e.cz>
---
 include/linux/rcupdate.h | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index f1f1bc39346b..4912c953cab7 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -554,6 +554,20 @@ static inline void rcu_preempt_sleep_check(void)
 		(p) = (typeof(*v) __force space *)(v); \
 	} while (0)
 
+/**
+ * lockless_dereference() - safely load a pointer for later dereference
+ * @p: The pointer to load
+ *
+ * Similar to rcu_dereference(), but for situations where the pointed-to
+ * object's lifetime is managed by something other than RCU.  That
+ * "something other" might be reference counting or simple immortality.
+ */
+#define lockless_dereference(p) \
+({ \
+	typeof(p) _________p1 = ACCESS_ONCE(p); \
+	smp_read_barrier_depends(); /* Dependency order vs. p above. */ \
+	(_________p1); \
+})
 
 /**
  * rcu_access_pointer() - fetch RCU pointer with no dereferencing
-- 
2.5.0

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