Put back-to-back invalidated locks back on list

A lock that is undergoing invalidation is put on a list of locks in the
super block.  Invalidation requests put locks on the list.  While locks
are invalidated they're temporarily put on a private list.

To support a request arriving while the lock is being processed we
carefully manage the invalidation fields in the lock between the
invalidation worker and the incoming request.  The worker correctly
noticed that a new invalidation request had arrived but it left the lock
on its private list instead of putting it back on the invalidation list
for further processing.  The lock was unreachable, wouldn't get
invalidated, and caused everyone trying to use the lock to block
indefinitely.

When the worker sees another request arrive for an invalidating lock it
needs to move the lock from the private list back to the invalidation
list.

Signed-off-by: Zach Brown <zab@versity.com>

kmod/src/lock.c

@@ -894,6 +894,9 @@ static void lock_invalidate_worker(struct work_struct *work)
 			list_del_init(&lock->inv_head);
 			lock->invalidate_pending = 0;
 			wake_up(&lock->waitq);
+		} else {
+			/* another request filled nl/net_id, put it back on the list */
+			list_move_tail(&lock->inv_head, &linfo->inv_list);
 		}
 		put_lock(linfo, lock);
 	}