security,rcu: convert call_rcu(user_update_rcu_disposal) to kfree_rcu()

index 02807fb..71e538a 100644
@@ -69,18 +69,6 @@ error:
 EXPORT_SYMBOL_GPL(user_instantiate);
 
 /*
- * dispose of the old data from an updated user defined key
- */
-static void user_update_rcu_disposal(struct rcu_head *rcu)
-{
-	struct user_key_payload *upayload;
-
-	upayload = container_of(rcu, struct user_key_payload, rcu);
-
-	kfree(upayload);
-}
-
-/*
  * update a user defined key
  * - the key's semaphore is write-locked
  */

@@ -114,7 +102,7 @@ int user_update(struct key *key, const void *data, size_t datalen)
 		key->expiry = 0;
 	}
 
-	call_rcu(&zap->rcu, user_update_rcu_disposal);
+	kfree_rcu(zap, rcu);
 
 error:
 	return ret;

@@ -145,7 +133,7 @@ void user_revoke(struct key *key)
 
 	if (upayload) {
 		rcu_assign_pointer(key->payload.data, NULL);
-		call_rcu(&upayload->rcu, user_update_rcu_disposal);
+		kfree_rcu(upayload, rcu);
 	}
 }
 

index af56148..70c932f 100644
@@ -777,4 +777,60 @@ static inline void debug_rcu_head_unqueue(struct rcu_head *head)
 }
 #endif	/* #else !CONFIG_DEBUG_OBJECTS_RCU_HEAD */
 
+static __always_inline bool __is_kfree_rcu_offset(unsigned long offset)
+{
+	return offset < 4096;
+}
+
+static __always_inline
+void __kfree_rcu(struct rcu_head *head, unsigned long offset)
+{
+	typedef void (*rcu_callback)(struct rcu_head *);
+
+	BUILD_BUG_ON(!__builtin_constant_p(offset));
+
+	/* See the kfree_rcu() header comment. */
+	BUILD_BUG_ON(!__is_kfree_rcu_offset(offset));
+
+	call_rcu(head, (rcu_callback)offset);
+}
+
+extern void kfree(const void *);
+
+static inline void __rcu_reclaim(struct rcu_head *head)
+{
+	unsigned long offset = (unsigned long)head->func;
+
+	if (__is_kfree_rcu_offset(offset))
+		kfree((void *)head - offset);
+	else
+		head->func(head);
+}
+
+/**
+ * kfree_rcu() - kfree an object after a grace period.
+ * @ptr:	pointer to kfree
+ * @rcu_head:	the name of the struct rcu_head within the type of @ptr.
+ *
+ * Many rcu callbacks functions just call kfree() on the base structure.
+ * These functions are trivial, but their size adds up, and furthermore
+ * when they are used in a kernel module, that module must invoke the
+ * high-latency rcu_barrier() function at module-unload time.
+ *
+ * The kfree_rcu() function handles this issue.  Rather than encoding a
+ * function address in the embedded rcu_head structure, kfree_rcu() instead
+ * encodes the offset of the rcu_head structure within the base structure.
+ * Because the functions are not allowed in the low-order 4096 bytes of
+ * kernel virtual memory, offsets up to 4095 bytes can be accommodated.
+ * If the offset is larger than 4095 bytes, a compile-time error will
+ * be generated in __kfree_rcu().  If this error is triggered, you can
+ * either fall back to use of call_rcu() or rearrange the structure to
+ * position the rcu_head structure into the first 4096 bytes.
+ *
+ * Note that the allowable offset might decrease in the future, for example,
+ * to allow something like kmem_cache_free_rcu().
+ */
+#define kfree_rcu(ptr, rcu_head)					\
+	__kfree_rcu(&((ptr)->rcu_head), offsetof(typeof(*(ptr)), rcu_head))
+
 #endif /* __LINUX_RCUPDATE_H */

index 0c343b9..4d60fbc 100644
@@ -167,7 +167,7 @@ static void rcu_process_callbacks(struct rcu_ctrlblk *rcp)
 		prefetch(next);
 		debug_rcu_head_unqueue(list);
 		local_bh_disable();
-		list->func(list);
+		__rcu_reclaim(list);
 		local_bh_enable();
 		list = next;
 		RCU_TRACE(cb_count++);

index 89e7dbb..4e1d925 100644
@@ -1161,7 +1161,7 @@ static void rcu_do_batch(struct rcu_state *rsp, struct rcu_data *rdp)
 		next = list->next;
 		prefetch(next);
 		debug_rcu_head_unqueue(list);
-		list->func(list);
+		__rcu_reclaim(list);
 		list = next;
 		if (++count >= rdp->blimit)
 			break;