rcu: Add support for debug_objects debugging for kfree_rcu()

This commit applies RCU's debug_objects debugging to the new batched
kfree_rcu() implementations.  The object is queued at the kfree_rcu()
call and dequeued during reclaim.

Tested that enabling CONFIG_DEBUG_OBJECTS_RCU_HEAD successfully detects
double kfree_rcu() calls.

Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
[ paulmck: Fix IRQ per kbuild test robot <lkp@intel.com> feedback. ]
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

kernel/rcu/tree.c

@@ -2749,6 +2749,7 @@ static void kfree_rcu_work(struct work_struct *work)
 	for (; head; head = next) {
 		next = head->next;
 		// Potentially optimize with kfree_bulk in future.
+		debug_rcu_head_unqueue(head);
 		__rcu_reclaim(rcu_state.name, head);
 		cond_resched_tasks_rcu_qs();
 	}
@@ -2855,6 +2856,12 @@ void kfree_call_rcu(struct rcu_head *head, rcu_callback_t func)
 		spin_lock(&krcp->lock);
 
 	// Queue the object but don't yet schedule the batch.
+	if (debug_rcu_head_queue(head)) {
+		// Probable double kfree_rcu(), just leak.
+		WARN_ONCE(1, "%s(): Double-freed call. rcu_head %p\n",
+			  __func__, head);
+		goto unlock_return;
+	}
 	head->func = func;
 	head->next = krcp->head;
 	krcp->head = head;
@@ -2866,6 +2873,7 @@ void kfree_call_rcu(struct rcu_head *head, rcu_callback_t func)
 		schedule_delayed_work(&krcp->monitor_work, KFREE_DRAIN_JIFFIES);
 	}
 
+unlock_return:
 	if (krcp->initialized)
 		spin_unlock(&krcp->lock);
 	local_irq_restore(flags);