[PATCH] nilfs2: fix checkpoint root lifetime on sysfs errors

[Aldo Ariel Panzardo]

nilfs_find_or_create_root() publishes a new root in the checkpoint
tree before creating its sysfs object.  If sysfs registration fails,
the root is freed while it is still reachable from the tree.  Merely
erasing it in the error path is insufficient because a concurrent
lookup may already hold a reference.

Serialize root creation and removal, finish sysfs registration before
publishing the root, and wait for the embedded kobject release before
freeing its container.  The wait also makes normal root removal safe
when kobject release is delayed.

Fixes: dd70edbde262 ("nilfs2: integrate sysfs support into driver")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Aldo Ariel Panzardo <qwe.aldo@xxxxxxxxx>
---
 fs/nilfs2/the_nilfs.c | 38 +++++++++++++++++++++++++-------------
 fs/nilfs2/the_nilfs.h |  3 +++
 2 files changed, 28 insertions(+), 13 deletions(-)

diff --git a/fs/nilfs2/the_nilfs.c b/fs/nilfs2/the_nilfs.c
index 7b23e373a..01389dba2 100644
--- a/fs/nilfs2/the_nilfs.c
+++ b/fs/nilfs2/the_nilfs.c
@@ -71,6 +71,7 @@ struct the_nilfs *alloc_nilfs(struct super_block *sb)
 	spin_lock_init(&nilfs->ns_last_segment_lock);
 	nilfs->ns_cptree = RB_ROOT;
 	spin_lock_init(&nilfs->ns_cptree_lock);
+	mutex_init(&nilfs->ns_cptree_mutex);
 	init_rwsem(&nilfs->ns_segctor_sem);
 	nilfs->ns_sb_update_freq = NILFS_SB_FREQ;
 
@@ -881,8 +882,15 @@ nilfs_find_or_create_root(struct the_nilfs *nilfs, __u64 cno)
 	if (!new)
 		return NULL;
 
-	spin_lock(&nilfs->ns_cptree_lock);
+	new->cno = cno;
+	new->ifile = NULL;
+	new->nilfs = nilfs;
+	refcount_set(&new->count, 1);
+	atomic64_set(&new->inodes_count, 0);
+	atomic64_set(&new->blocks_count, 0);
 
+	mutex_lock(&nilfs->ns_cptree_mutex);
+	spin_lock(&nilfs->ns_cptree_lock);
 	p = &nilfs->ns_cptree.rb_node;
 	parent = NULL;
 
@@ -897,29 +905,28 @@ nilfs_find_or_create_root(struct the_nilfs *nilfs, __u64 cno)
 		} else {
 			refcount_inc(&root->count);
 			spin_unlock(&nilfs->ns_cptree_lock);
+			mutex_unlock(&nilfs->ns_cptree_mutex);
 			kfree(new);
 			return root;
 		}
 	}
-
-	new->cno = cno;
-	new->ifile = NULL;
-	new->nilfs = nilfs;
-	refcount_set(&new->count, 1);
-	atomic64_set(&new->inodes_count, 0);
-	atomic64_set(&new->blocks_count, 0);
-
-	rb_link_node(&new->rb_node, parent, p);
-	rb_insert_color(&new->rb_node, &nilfs->ns_cptree);
-
 	spin_unlock(&nilfs->ns_cptree_lock);
 
 	err = nilfs_sysfs_create_snapshot_group(new);
 	if (err) {
+		mutex_unlock(&nilfs->ns_cptree_mutex);
+		wait_for_completion(&new->snapshot_kobj_unregister);
 		kfree(new);
-		new = NULL;
+		return NULL;
 	}
 
+	spin_lock(&nilfs->ns_cptree_lock);
+	rb_link_node(&new->rb_node, parent, p);
+	rb_insert_color(&new->rb_node, &nilfs->ns_cptree);
+
+	spin_unlock(&nilfs->ns_cptree_lock);
+	mutex_unlock(&nilfs->ns_cptree_mutex);
+
 	return new;
 }
 
@@ -927,13 +934,18 @@ void nilfs_put_root(struct nilfs_root *root)
 {
 	struct the_nilfs *nilfs = root->nilfs;
 
+	mutex_lock(&nilfs->ns_cptree_mutex);
 	if (refcount_dec_and_lock(&root->count, &nilfs->ns_cptree_lock)) {
 		rb_erase(&root->rb_node, &nilfs->ns_cptree);
 		spin_unlock(&nilfs->ns_cptree_lock);
 
 		nilfs_sysfs_delete_snapshot_group(root);
+		mutex_unlock(&nilfs->ns_cptree_mutex);
+		wait_for_completion(&root->snapshot_kobj_unregister);
 		iput(root->ifile);
 
 		kfree(root);
+	} else {
+		mutex_unlock(&nilfs->ns_cptree_mutex);
 	}
 }
diff --git a/fs/nilfs2/the_nilfs.h b/fs/nilfs2/the_nilfs.h
index 4776a70f0..72affad9e 100644
--- a/fs/nilfs2/the_nilfs.h
+++ b/fs/nilfs2/the_nilfs.h
@@ -68,6 +68,7 @@ enum {
  * @ns_sufile: segusage file inode
  * @ns_cptree: rb-tree of all mounted checkpoints (nilfs_root)
  * @ns_cptree_lock: lock protecting @ns_cptree
+ * @ns_cptree_mutex: mutex serializing checkpoint root creation and removal
  * @ns_dirty_files: list of dirty files
  * @ns_inode_lock: lock protecting @ns_dirty_files
  * @ns_gc_inodes: dummy inodes to keep live blocks
@@ -151,6 +152,8 @@ struct the_nilfs {
 	/* Checkpoint tree */
 	struct rb_root		ns_cptree;
 	spinlock_t		ns_cptree_lock;
+	/* Serialize root creation and removal. */
+	struct mutex		ns_cptree_mutex;
 
 	/* Dirty inode list */
 	struct list_head	ns_dirty_files;
-- 
2.43.0