rcu: Upgrade rcu_swap_protected() to rcu_replace_pointer()

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_pointer() 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_pointer(),
rcu_swap_protected() will be removed.

Link: https://lore.kernel.org/lkml/CAHk-=wiAsJLw1egFEE=Z7-GGtM6wcvtyytXZA1+BHqta4gg6Hw@mail.gmail.com/
Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
[ paulmck: From rcu_replace() to rcu_replace_pointer() per Ingo Molnar. ]
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Bart Van Assche <bart.vanassche@wdc.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Hannes Reinecke <hare@suse.de>
Cc: Johannes Thumshirn <jthumshirn@suse.de>
Cc: Shane M Seymour <shane.seymour@hpe.com>
Cc: Martin K. Petersen <martin.petersen@oracle.com>

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 75a2ede..185dd97 100644 (file)
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -383,6 +383,24 @@ do {                                                                             \
 } while (0)
 
 /**
+ * rcu_replace_pointer() - 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_pointer(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