rcu: Open-code the rcu_cblist_empty() function

Because the rcu_cblist_empty() just samples the ->head pointer, and
because the rcu_cblist structure is quite straightforward, it makes
sense to open-code rcu_cblist_empty(p) as !p->head, cutting out a
level of indirection.  This commit makes this change.

Reported-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>

kernel/rcu/rcu_segcblist.h

@@ -22,12 +22,6 @@
 
 #include <linux/rcu_segcblist.h>
 
-/* Is simple callback list empty? */
-static inline bool rcu_cblist_empty(struct rcu_cblist *rclp)
-{
-	return !rclp->head;
-}
-
 /* Return number of callbacks in simple callback list. */
 static inline long rcu_cblist_n_cbs(struct rcu_cblist *rclp)
 {
@@ -66,7 +60,7 @@ static inline struct rcu_head *rcu_cblist_head(struct rcu_cblist *rclp)
  */
 static inline struct rcu_head **rcu_cblist_tail(struct rcu_cblist *rclp)
 {
-	WARN_ON_ONCE(rcu_cblist_empty(rclp));
+	WARN_ON_ONCE(!rclp->head);
 	return rclp->tail;
 }
 

kernel/rcu/tree.c

@@ -2647,9 +2647,9 @@ static void rcu_adopt_orphan_cbs(struct rcu_state *rsp, unsigned long flags)
 
 	/* First adopt the ready-to-invoke callbacks, then the done ones. */
 	rcu_segcblist_insert_done_cbs(&rdp->cblist, &rsp->orphan_done);
-	WARN_ON_ONCE(!rcu_cblist_empty(&rsp->orphan_done));
+	WARN_ON_ONCE(rsp->orphan_done.head);
 	rcu_segcblist_insert_pend_cbs(&rdp->cblist, &rsp->orphan_pend);
-	WARN_ON_ONCE(!rcu_cblist_empty(&rsp->orphan_pend));
+	WARN_ON_ONCE(rsp->orphan_pend.head);
 	WARN_ON_ONCE(rcu_segcblist_empty(&rdp->cblist) !=
 		     !rcu_segcblist_n_cbs(&rdp->cblist));
 }
@@ -2800,9 +2800,8 @@ static void rcu_do_batch(struct rcu_state *rsp, struct rcu_data *rdp)
 
 	local_irq_save(flags);
 	count = -rcu_cblist_n_cbs(&rcl);
-	trace_rcu_batch_end(rsp->name, count, !rcu_cblist_empty(&rcl),
-			    need_resched(), is_idle_task(current),
-			    rcu_is_callbacks_kthread());
+	trace_rcu_batch_end(rsp->name, count, !!rcl.head, need_resched(),
+			    is_idle_task(current), rcu_is_callbacks_kthread());
 
 	/* Update counts and requeue any remaining callbacks. */
 	rcu_segcblist_insert_done_cbs(&rdp->cblist, &rcl);

kernel/rcu/tree_plugin.h

@@ -1942,12 +1942,12 @@ static bool __maybe_unused rcu_nocb_adopt_orphan_cbs(struct rcu_state *rsp,
 		return false;
 
 	/* First, enqueue the donelist, if any.  This preserves CB ordering. */
-	if (!rcu_cblist_empty(&rsp->orphan_done)) {
+	if (rsp->orphan_done.head) {
 		__call_rcu_nocb_enqueue(rdp, rcu_cblist_head(&rsp->orphan_done),
 					rcu_cblist_tail(&rsp->orphan_done),
 					ql, qll, flags);
 	}
-	if (!rcu_cblist_empty(&rsp->orphan_pend)) {
+	if (rsp->orphan_pend.head) {
 		__call_rcu_nocb_enqueue(rdp, rcu_cblist_head(&rsp->orphan_pend),
 					rcu_cblist_tail(&rsp->orphan_pend),
 					ql, qll, flags);