devlink: expose get/put functions

Allow those who hold implicit reference on a devlink instance
to try to take a full ref on it. This will be used from netdev
code which has an implicit ref because of driver call ordering.

Note that after recent changes devlink_unregister() may happen
before netdev unregister, but devlink_free() should still happen
after, so we are safe to try, but we can't just refcount_inc()
and assume it's not zero.

Signed-off-by: Jakub Kicinski <[e-mail]>
Reviewed-by: Leon Romanovsky <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

include/net/devlink.h

···
 
 #if IS_ENABLED(CONFIG_NET_DEVLINK)
 
+struct devlink *__must_check devlink_try_get(struct devlink *devlink);
+void devlink_put(struct devlink *devlink);
+
 void devlink_compat_running_version(struct net_device *dev,
 				    char *buf, size_t len);
 int devlink_compat_flash_update(struct net_device *dev, const char *file_name);
···
 				 struct netdev_phys_item_id *ppid);
 
 #else
+
+static inline struct devlink *devlink_try_get(struct devlink *devlink)
+{
+	return NULL;
+}
+
+static inline void devlink_put(struct devlink *devlink)
+{
+}
 
 static inline void
 devlink_compat_running_version(struct net_device *dev, char *buf, size_t len)

net/core/devlink.c

···
 }
 EXPORT_SYMBOL_GPL(devlink_net);
 
-static void devlink_put(struct devlink *devlink)
+void devlink_put(struct devlink *devlink)
 {
 	if (refcount_dec_and_test(&devlink->refcount))
 		complete(&devlink->comp);
 }
 
-static bool __must_check devlink_try_get(struct devlink *devlink)
+struct devlink *__must_check devlink_try_get(struct devlink *devlink)
 {
-	return refcount_inc_not_zero(&devlink->refcount);
+	if (refcount_inc_not_zero(&devlink->refcount))
+		return devlink;
+	return NULL;
 }
 
 static struct devlink *devlink_get_from_attrs(struct net *net,