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>

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 61a9716..cac24f5 100644 (file)
--- a/kernel/rcu/tree.c
+++ b/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;