[PATCH 7/8] super: switch list manipulation to _rcu primitives

[Christian Brauner <brauner@kernel.org>]

Swap the list/hlist write-side operations on @super_blocks and
@fs_type->fs_supers over to their _rcu variants. All three call sites
still hold sb_lock; this is a purely mechanical change that
establishes the writer-side memory ordering lockless RCU readers can
rely on in the next patch.

The affected sites are sget_fc() (list_add_tail() and
hlist_add_head() at the publish step), __put_super()
(list_del_init() -> list_bidir_del_rcu() of s_list when the last
temporary reference is dropped) and kill_super_notify()
(hlist_del_init() -> hlist_del_rcu() of s_instances).

@super_blocks gets list_bidir_del_rcu() rather than list_del_rcu()
because the next patch walks the list backward for
filesystems_freeze() and do_emergency_remount(). list_del_rcu()
preserves the unlinked entry's ->next pointer but poisons ->prev with
LIST_POISON2, which would crash any concurrent reverse traversal that
landed on the just-unlinked entry between the SB_DYING check and the
cursor advance. list_bidir_del_rcu() preserves both ->next and
->prev so reverse traversal stays safe. See kernel/nstree.c for the
canonical bidirectional-RCU list pattern.

The "_init" half of the deletions is not used elsewhere on these list
nodes after removal so dropping it is fine. The entry is about to be
freed via call_rcu(destroy_super_rcu) (for s_list) or to disappear
with the superblock (for s_instances, once the list has done its job
notifying SB_DEAD waiters).

Iterators keep using plain list_for_each_entry() and
hlist_for_each_entry() under sb_lock. Their conversion to lockless
RCU traversal with refcount_inc_not_zero() is the next patch.

Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
---
 fs/super.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/fs/super.c b/fs/super.c
index 2fa7023010ec..8c01b95be717 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -407,7 +407,7 @@ static struct super_block *alloc_super(struct file_system_type *type, int flags,
 static void __put_super(struct super_block *s)
 {
 	if (refcount_dec_and_test(&s->s_count)) {
-		list_del_init(&s->s_list);
+		list_bidir_del_rcu(&s->s_list);
 		WARN_ON(s->s_dentry_lru.node);
 		WARN_ON(s->s_inode_lru.node);
 		WARN_ON(s->s_mounts);
@@ -445,7 +445,7 @@ static void kill_super_notify(struct super_block *sb)
 	 * SB_DEAD.
 	 */
 	spin_lock(&sb_lock);
-	hlist_del_init(&sb->s_instances);
+	hlist_del_rcu(&sb->s_instances);
 	spin_unlock(&sb_lock);
 
 	/*
@@ -784,8 +784,8 @@ struct super_block *sget_fc(struct fs_context *fc,
 	 * It's in a nascent state and users should wait on SB_BORN or
 	 * SB_DYING to be set.
 	 */
-	list_add_tail(&s->s_list, &super_blocks);
-	hlist_add_head(&s->s_instances, &s->s_type->fs_supers);
+	list_add_tail_rcu(&s->s_list, &super_blocks);
+	hlist_add_head_rcu(&s->s_instances, &s->s_type->fs_supers);
 	spin_unlock(&sb_lock);
 	get_filesystem(s->s_type);
 	shrinker_register(s->s_shrink);

-- 
2.47.3