rcu: Make call_rcu() print mem_dump_obj() info for double-freed callback

The debug-object double-free checks in __call_rcu() print out the
RCU callback function, which is usually sufficient to track down the
double free.  However, all uses of things like queue_rcu_work() will
have the same RCU callback function (rcu_work_rcufn() in this case),
so a diagnostic message for a double queue_rcu_work() needs more than
just the callback function.

This commit therefore calls mem_dump_obj() to dump out any additional
available information on the double-freed callback.

Cc: Christoph Lameter <cl@linux.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: David Rientjes <rientjes@google.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: <linux-mm@kvack.org>
Reported-by: Andrii Nakryiko <andrii@kernel.org>
Tested-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

kernel/rcu/tree.c

@@ -2941,6 +2941,7 @@ static void check_cb_ovld(struct rcu_data *rdp)
 static void
 __call_rcu(struct rcu_head *head, rcu_callback_t func)
 {
+	static atomic_t doublefrees;
 	unsigned long flags;
 	struct rcu_data *rdp;
 	bool was_alldone;
@@ -2954,8 +2955,10 @@ __call_rcu(struct rcu_head *head, rcu_callback_t func)
 		 * 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->%pS()!!!\n",
-			  head, head->func);
+		if (atomic_inc_return(&doublefrees) < 4) {
+			pr_err("%s(): Double-freed CB %p->%pS()!!!  ", __func__, head, head->func);
+			mem_dump_obj(head);
+		}
 		WRITE_ONCE(head->func, rcu_leak_callback);
 		return;
 	}