[PATCH 3/5] rcu/tree: Add support for debug_objects debugging for kfree_rcu()

["Joel Fernandes (Google)" <joel@joelfernandes.org>]

Make use of RCU's debug_objects debugging support
(CONFIG_DEBUG_OBJECTS_RCU_HEAD) similar to call_rcu() and other flavors.
We queue the object during the kfree_rcu() call and dequeue it 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>
---
 kernel/rcu/tree.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 9b9ae4db1c2d..64568f12641d 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2757,6 +2757,7 @@ static void kfree_rcu_work(struct work_struct *work)
 	for (; head; head = next) {
 		next = head->next;
 		/* Could be possible to optimize with kfree_bulk in future */
+		debug_rcu_head_unqueue(head);
 		__rcu_reclaim(rcu_state.name, head);
 		cond_resched_tasks_rcu_qs();
 	}
@@ -2868,6 +2869,13 @@ void kfree_call_rcu(struct rcu_head *head, rcu_callback_t func)
 	if (rcu_scheduler_active != RCU_SCHEDULER_RUNNING)
 		return kfree_call_rcu_nobatch(head, func);
 
+	if (debug_rcu_head_queue(head)) {
+		/* Probable double kfree_rcu() */
+		WARN_ONCE(1, "kfree_call_rcu(): Double-freed call. rcu_head %p\n",
+			  head);
+		return;
+	}
+
 	head->func = func;
 
 	local_irq_save(flags);	/* For safely calling this_cpu_ptr(). */
-- 
2.23.0.187.g17f5b7556c-goog