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[PATCH 3.16.y-ckt 097/130] rcu: Provide counterpart to rcu_dereference() for non-RCU situations

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  [PATCH 3.16.y-ckt 097/130] rcu: Provide counterpart to rcu_dereference() for non-RCU situations Luis Henriques <luis.henriques@canonical.com> - 2015-09-04 15:30 +0200

#1218959 — [PATCH 3.16.y-ckt 097/130] rcu: Provide counterpart to rcu_dereference() for non-RCU situations

3.16.7-ckt17 -stable review patch.  If anyone has any objections, please let me know.

------------------

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

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@ZenIV.linux.org.uk>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
[ luis: 3.16 prereq for:
  37868fe113ff "x86/ldt: Make modify_ldt synchronous" ]
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>
---
 include/linux/rcupdate.h | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 6a94cc8b1ca0..53ea8bed34bc 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -593,6 +593,21 @@ static inline void rcu_preempt_sleep_check(void)
 #define RCU_INITIALIZER(v) (typeof(*(v)) __force __rcu *)(v)
 
 /**
+ * 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_assign_pointer() - assign to RCU-protected pointer
  * @p: pointer to assign to
  * @v: value to assign (publish)
--
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