nstree: add unified namespace list

Allow to walk the unified namespace list completely locklessly.

Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Christian Brauner <[e-mail]>

include/linux/ns_common.h

···
 			u64 ns_id;
 			struct /* global namespace rbtree and list */ {
 				struct rb_node ns_unified_tree_node;
+				struct list_head ns_unified_list_node;
 			};
 			struct /* per type rbtree and list */ {
 				struct rb_node ns_tree_node;
···
 	.ns_list_node		= LIST_HEAD_INIT(nsname.ns.ns_list_node),		\
 	.ns_owner_entry		= LIST_HEAD_INIT(nsname.ns.ns_owner_entry),		\
 	.ns_owner		= LIST_HEAD_INIT(nsname.ns.ns_owner),			\
+	.ns_unified_list_node	= LIST_HEAD_INIT(nsname.ns.ns_unified_list_node),	\
 }
 
 #define ns_common_init(__ns)                     \

kernel/nscommon.c

···
 	RB_CLEAR_NODE(&ns->ns_unified_tree_node);
 	RB_CLEAR_NODE(&ns->ns_owner_tree_node);
 	INIT_LIST_HEAD(&ns->ns_list_node);
+	INIT_LIST_HEAD(&ns->ns_unified_list_node);
 	ns->ns_owner_tree = RB_ROOT;
 	INIT_LIST_HEAD(&ns->ns_owner);
 	INIT_LIST_HEAD(&ns->ns_owner_entry);

kernel/nstree.c

···
 
 static __cacheline_aligned_in_smp DEFINE_SEQLOCK(ns_tree_lock);
 static struct rb_root ns_unified_tree = RB_ROOT; /* protected by ns_tree_lock */
+static LIST_HEAD(ns_unified_list); /* protected by ns_tree_lock */
 
 /**
  * struct ns_tree - Namespace tree
···
 	else
 		list_add_rcu(&ns->ns_list_node, &node_to_ns(prev)->ns_list_node);
 
+	/* Add to unified tree and list */
 	rb_find_add_rcu(&ns->ns_unified_tree_node, &ns_unified_tree, ns_cmp_unified);
+	prev = rb_prev(&ns->ns_unified_tree_node);
+	if (!prev)
+		list_add_rcu(&ns->ns_unified_list_node, &ns_unified_list);
+	else
+		list_add_rcu(&ns->ns_unified_list_node, &node_to_ns_unified(prev)->ns_unified_list_node);
 
 	if (ops) {
 		struct user_namespace *user_ns;
···
 
 	write_seqlock(&ns_tree_lock);
 	rb_erase(&ns->ns_tree_node, &ns_tree->ns_tree);
-	rb_erase(&ns->ns_unified_tree_node, &ns_unified_tree);
 	RB_CLEAR_NODE(&ns->ns_tree_node);
 
 	list_bidir_del_rcu(&ns->ns_list_node);
+
+	rb_erase(&ns->ns_unified_tree_node, &ns_unified_tree);
+	RB_CLEAR_NODE(&ns->ns_unified_tree_node);
+
+	list_bidir_del_rcu(&ns->ns_unified_list_node);
 
 	/* Remove from owner's rbtree if this namespace has an owner */
 	if (ops) {