drbd: Balance RCU calls in drbd_adm_dump_devices()

Make drbd_adm_dump_devices() call rcu_read_lock() before
rcu_read_unlock() is called. This has been detected by the Clang
thread-safety analyzer.

Tested-by: Christoph Böhmwalder <[email protected]>
Reviewed-by: Christoph Hellwig <[email protected]>
Cc: Andreas Gruenbacher <[email protected]>
Fixes: a55bbd3 ("drbd: Backport the "status" command")
Signed-off-by: Bart Van Assche <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Jens Axboe <[email protected]>

Author: bvanassche

drivers/block/drbd/drbd_nl.c

@@ -3443,8 +3443,10 @@ int drbd_adm_dump_devices(struct sk_buff *skb, struct netlink_callback *cb)
 		if (resource_filter) {
 			retcode = ERR_RES_NOT_KNOWN;
 			resource = drbd_find_resource(nla_data(resource_filter));
-			if (!resource)
+			if (!resource) {
+				rcu_read_lock();
 				goto put_result;
+			}
 			cb->args[0] = (long)resource;
 		}
 	}
@@ -3693,8 +3695,10 @@ int drbd_adm_dump_peer_devices(struct sk_buff *skb, struct netlink_callback *cb)
 		if (resource_filter) {
 			retcode = ERR_RES_NOT_KNOWN;
 			resource = drbd_find_resource(nla_data(resource_filter));
-			if (!resource)
+			if (!resource) {
+				rcu_read_lock();
 				goto put_result;
+			}
 		}
 		cb->args[0] = (long)resource;
 	}