rcu: Improve __call_rcu() debug-objects error message

The "__call_rcu(): Leaked duplicate callback" error message from
__call_rcu() has proven to be unhelpful.  This commit therefore changes
it to "__call_rcu(): Double-freed CB" and adds the value of the pointer
passed in.  The value of the pointer improves debuggability by allowing
correlation with tracing output, for example, the rcu:rcu_callback trace
event.

Reported-by: Vegard Nossum <vegard.nossum@oracle.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

kernel/rcu/tree.c

@@ -3161,9 +3161,14 @@ __call_rcu(struct rcu_head *head, rcu_callback_t func,
 	WARN_ON_ONCE((unsigned long)head & (sizeof(void *) - 1));
 
 	if (debug_rcu_head_queue(head)) {
-		/* Probable double call_rcu(), so leak the callback. */
+		/*
+		 * Probable double call_rcu(), so leak the callback.
+		 * Use rcu:rcu_callback trace event to find the previous
+		 * time callback was passed to __call_rcu().
+		 */
+		WARN_ONCE(1, "__call_rcu(): Double-freed CB %p->%pF()!!!\n",
+			  head, head->func);
 		WRITE_ONCE(head->func, rcu_leak_callback);
-		WARN_ONCE(1, "__call_rcu(): Leaked duplicate callback\n");
 		return;
 	}
 	head->func = func;