fs: Avoid deadlock when renaming

fs/refs.c

@@ -131,8 +131,11 @@ void vfs_entry_replace_children(struct vfs_entry* dst, struct vfs_entry* src) {
     tailq_head(struct vfs_entry) stale;
     tailq_init(&stale);
 
-    mutex_lock(&src->lock);
+    // Avoid a potential deadlock when this function is called with (a, b) and (b, a) at the same time.
-    mutex_lock(&dst->lock);
+    struct vfs_entry* first = src < dst ? src : dst;
+    struct vfs_entry* second = src < dst ? dst : src;
+    mutex_lock(&first->lock);
+    mutex_lock(&second->lock);
     struct vfs_entry* child;
     while ((child = tailq_first(&dst->children))) {
         tailq_remove(&dst->children, child, child_link);
@@ -145,8 +148,8 @@ void vfs_entry_replace_children(struct vfs_entry* dst, struct vfs_entry* src) {
         vfs_entry_unref(src);
         tailq_insert_tail(&dst->children, child, child_link);
     }
-    mutex_unlock(&dst->lock);
+    mutex_unlock(&second->lock);
-    mutex_unlock(&src->lock);
+    mutex_unlock(&first->lock);
 
     while ((child = tailq_first(&stale))) {
         tailq_remove(&stale, child, child_link);