rcu: Add data structures for synchronize_rcu()

The synchronize_rcu() call is going to be reworked, thus
this patch adds dedicated fields into the rcu_state structure.
Reviewed-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>

kernel/rcu/tree.h

@@ -315,6 +315,13 @@ do {									\
 	__set_current_state(TASK_RUNNING);				\
 } while (0)
 
+#define SR_NORMAL_GP_WAIT_HEAD_MAX 5
+
+struct sr_wait_node {
+	atomic_t inuse;
+	struct llist_node node;
+};
+
 /*
  * RCU global state, including node hierarchy.  This hierarchy is
  * represented in "heap" form in a dense array.  The root (first level)
@@ -400,6 +407,13 @@ struct rcu_state {
 						/* Synchronize offline with */
 						/*  GP pre-initialization. */
 	int nocb_is_setup;			/* nocb is setup from boot */
+
+	/* synchronize_rcu() part. */
+	struct llist_head srs_next;	/* request a GP users. */
+	struct llist_node *srs_wait_tail; /* wait for GP users. */
+	struct llist_node *srs_done_tail; /* ready for GP users. */
+	struct sr_wait_node srs_wait_nodes[SR_NORMAL_GP_WAIT_HEAD_MAX];
+	struct work_struct srs_cleanup_work;
 };
 
 /* Values for rcu_state structure's gp_flags field. */