rcu: Move lockless_dereference() out of rcupdate.h

commit 0a04b0166929405cd833c1cc40f99e862b965ddc upstream.

I want to use lockless_dereference() from seqlock.h, which would mean
including rcupdate.h from it, however rcupdate.h already includes
seqlock.h.

Avoid this by moving lockless_dereference() into compiler.h. This is
somewhat tricky since it uses smp_read_barrier_depends() which isn't
available there, but its a CPP macro so we can get away with it.

The alternative would be moving it into asm/barrier.h, but that would
be updating each arch (I can do if people feel that is more
appropriate).

Cc: Paul McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
[ luis: 3.16 prereq for:
  37868fe113ff "x86/ldt: Make modify_ldt synchronous" ]
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>

2 files changed, 15 insertions, 15 deletions

diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index d5ad7b1118fc..da293bf86575 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -380,6 +380,21 @@ void ftrace_likely_update(struct ftrace_branch_data *f, int val, int expect);
  */
 #define ACCESS_ONCE(x) (*(volatile typeof(x) *)&(x))
 
+/**
+ * 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); \
+})
+
 /* Ignore/forbid kprobes attach on very low level functions marked by this attribute: */
 #ifdef CONFIG_KPROBES
 # define __kprobes	__attribute__((__section__(".kprobes.text")))
diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 53ea8bed34bc..6a94cc8b1ca0 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -593,21 +593,6 @@ 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)