[3.16.y-ckt stable] Patch "rcu: Provide counterpart to rcu_dereference() for non-RCU situations" has been added to staging queue

[Luis Henriques]

This is a note to let you know that I have just added a patch titled

    rcu: Provide counterpart to rcu_dereference() for non-RCU situations

to the linux-3.16.y-queue branch of the 3.16.y-ckt extended stable tree 
which can be found at:

    http://kernel.ubuntu.com/git/ubuntu/linux.git/log/?h=linux-3.16.y-queue

This patch is scheduled to be released in version 3.16.7-ckt17.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.16.y-ckt tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Luis

------

>From 4903dc9f830525b03f6c86ed6563489453feb00f Mon Sep 17 00:00:00 2001
From: "Paul E. McKenney" <paulmck at linux.vnet.ibm.com>
Date: Mon, 27 Oct 2014 21:11:27 -0700
Subject: rcu: Provide counterpart to rcu_dereference() for non-RCU situations

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 at ZenIV.linux.org.uk>
Signed-off-by: Paul E. McKenney <paulmck at linux.vnet.ibm.com>
Signed-off-by: Al Viro <viro at zeniv.linux.org.uk>
[ luis: 3.16 prereq for:
  37868fe113ff "x86/ldt: Make modify_ldt synchronous" ]
Signed-off-by: Luis Henriques <luis.henriques at 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)