fs: lockless mntns lookup for nsfs

We already made the rbtree lookup lockless for the simple lookup case.
However, walking the list of mount namespaces via nsfs still happens
with taking the read lock blocking concurrent additions of new mount
namespaces pointlessly. Plus, such additions are rare anyway so allow
lockless lookup of the previous and next mount namespace by keeping a
separate list. This also allows to make some things simpler in the code.

Link: https://lore.kernel.org/r/20241213-work-mount-rbtree-lockless-v3-5-6e3cdaf9b280@kernel.org
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Suggested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>

1 files changed, 4 insertions, 9 deletions

diff --git a/fs/mount.h b/fs/mount.h
index bd8d6c36b421..e9f48e563c0f 100644
--- a/fs/mount.h
+++ b/fs/mount.h
@@ -20,6 +20,7 @@ struct mnt_namespace {
 	unsigned int		nr_mounts; /* # of mounts in the namespace */
 	unsigned int		pending_mounts;
 	struct rb_node		mnt_ns_tree_node; /* node in the mnt_ns_tree */
+	struct list_head	mnt_ns_list; /* entry in the sequential list of mounts namespace */
 	refcount_t		passive; /* number references not pinning @mounts */
 } __randomize_layout;
 
@@ -160,15 +161,9 @@ static inline void move_from_ns(struct mount *mnt, struct list_head *dt_list)
 }
 
 bool has_locked_children(struct mount *mnt, struct dentry *dentry);
-struct mnt_namespace *__lookup_next_mnt_ns(struct mnt_namespace *mnt_ns, bool previous);
-static inline struct mnt_namespace *lookup_next_mnt_ns(struct mnt_namespace *mntns)
-{
-	return __lookup_next_mnt_ns(mntns, false);
-}
-static inline struct mnt_namespace *lookup_prev_mnt_ns(struct mnt_namespace *mntns)
-{
-	return __lookup_next_mnt_ns(mntns, true);
-}
+struct mnt_namespace *get_sequential_mnt_ns(struct mnt_namespace *mnt_ns,
+					    bool previous);
+
 static inline struct mnt_namespace *to_mnt_ns(struct ns_common *ns)
 {
 	return container_of(ns, struct mnt_namespace, ns);