kernfs: Use a per-fs rwsem to protect per-fs list of kernfs_super_info.

Right now per-fs kernfs_rwsem protects list of kernfs_super_info instances
for a kernfs_root. Since kernfs_rwsem is used to synchronize several other
operations across kernfs and since most of these operations don't impact
kernfs_super_info, we can use a separate per-fs rwsem to synchronize access
to list of kernfs_super_info.
This helps in reducing contention around kernfs_rwsem and also allows
operations that change/access list of kernfs_super_info to proceed without
contending for kernfs_rwsem.
Signed-off-by: Imran Khan <imran.f.khan@oracle.com>
Link: https://lore.kernel.org/r/20230309110932.2889010-3-imran.f.khan@oracle.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

fs/kernfs/dir.c

@@ -944,6 +944,7 @@ struct kernfs_root *kernfs_create_root(struct kernfs_syscall_ops *scops,
 	idr_init(&root->ino_idr);
 	init_rwsem(&root->kernfs_rwsem);
 	init_rwsem(&root->kernfs_iattr_rwsem);
+	init_rwsem(&root->kernfs_supers_rwsem);
 	INIT_LIST_HEAD(&root->supers);
 
 	/*

fs/kernfs/file.c

@@ -922,8 +922,8 @@ static void kernfs_notify_workfn(struct work_struct *work)
 
 	root = kernfs_root(kn);
 	/* kick fsnotify */
-	down_write(&root->kernfs_rwsem);
 
+	down_read(&root->kernfs_supers_rwsem);
 	list_for_each_entry(info, &kernfs_root(kn)->supers, node) {
 		struct kernfs_node *parent;
 		struct inode *p_inode = NULL;
@@ -960,7 +960,7 @@ static void kernfs_notify_workfn(struct work_struct *work)
 		iput(inode);
 	}
 
-	up_write(&root->kernfs_rwsem);
+	up_read(&root->kernfs_supers_rwsem);
 	kernfs_put(kn);
 	goto repeat;
 }

fs/kernfs/kernfs-internal.h

@@ -48,6 +48,7 @@ struct kernfs_root {
 	wait_queue_head_t	deactivate_waitq;
 	struct rw_semaphore	kernfs_rwsem;
 	struct rw_semaphore	kernfs_iattr_rwsem;
+	struct rw_semaphore	kernfs_supers_rwsem;
 };
 
 /* +1 to avoid triggering overflow warning when negating it */

fs/kernfs/mount.c

@@ -351,9 +351,9 @@ int kernfs_get_tree(struct fs_context *fc)
 		}
 		sb->s_flags |= SB_ACTIVE;
 
-		down_write(&root->kernfs_rwsem);
+		down_write(&root->kernfs_supers_rwsem);
 		list_add(&info->node, &info->root->supers);
-		up_write(&root->kernfs_rwsem);
+		up_write(&root->kernfs_supers_rwsem);
 	}
 
 	fc->root = dget(sb->s_root);
@@ -380,9 +380,9 @@ void kernfs_kill_sb(struct super_block *sb)
 	struct kernfs_super_info *info = kernfs_info(sb);
 	struct kernfs_root *root = info->root;
 
-	down_write(&root->kernfs_rwsem);
+	down_write(&root->kernfs_supers_rwsem);
 	list_del(&info->node);
-	up_write(&root->kernfs_rwsem);
+	up_write(&root->kernfs_supers_rwsem);
 
 	/*
 	 * Remove the superblock from fs_supers/s_instances