[PATCH tip/core/rcu 1/9] rcu: Upgrade rcu_swap_protected() to rcu_replace()

[paulmck]

From: "Paul E. McKenney" <paulmck@xxxxxxxxxx>

Although the rcu_swap_protected() macro follows the example of swap(),
the interactions with RCU make its update of its argument somewhat
counter-intuitive.  This commit therefore introduces an rcu_replace()
that returns the old value of the RCU pointer instead of doing the
argument update.  Once all the uses of rcu_swap_protected() are updated
to instead use rcu_replace(), rcu_swap_protected() will be removed.

Link: https://lore.kernel.org/lkml/CAHk-=wiAsJLw1egFEE=Z7-GGtM6wcvtyytXZA1+BHqta4gg6Hw@xxxxxxxxxxxxxx/
Reported-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
Cc: Bart Van Assche <bart.vanassche@xxxxxxx>
Cc: Christoph Hellwig <hch@xxxxxx>
Cc: Hannes Reinecke <hare@xxxxxxx>
Cc: Johannes Thumshirn <jthumshirn@xxxxxxx>
Cc: Shane M Seymour <shane.seymour@xxxxxxx>
Cc: Martin K. Petersen <martin.petersen@xxxxxxxxxx>
---
 include/linux/rcupdate.h | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 75a2ede..3b73287 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -383,6 +383,24 @@ do {									      \
 } while (0)
 
 /**
+ * rcu_replace() - replace an RCU pointer, returning its old value
+ * @rcu_ptr: RCU pointer, whose old value is returned
+ * @ptr: regular pointer
+ * @c: the lockdep conditions under which the dereference will take place
+ *
+ * Perform a replacement, where @rcu_ptr is an RCU-annotated
+ * pointer and @c is the lockdep argument that is passed to the
+ * rcu_dereference_protected() call used to read that pointer.  The old
+ * value of @rcu_ptr is returned, and @rcu_ptr is set to @ptr.
+ */
+#define rcu_replace(rcu_ptr, ptr, c)					\
+({									\
+	typeof(ptr) __tmp = rcu_dereference_protected((rcu_ptr), (c));	\
+	rcu_assign_pointer((rcu_ptr), (ptr));				\
+	__tmp;								\
+})
+
+/**
  * rcu_swap_protected() - swap an RCU and a regular pointer
  * @rcu_ptr: RCU pointer
  * @ptr: regular pointer
-- 
2.9.5