4 files changed, 26 insertions, 3 deletions

diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
index a84e59b..91a8ca7 100644
--- a/fs/btrfs/ctree.h
+++ b/fs/btrfs/ctree.h
@@ -1746,6 +1746,7 @@ struct btrfs_root {
 	int force_cow;
 
 	spinlock_t root_item_lock;
+	atomic_t refs;
 };
 
 struct btrfs_ioctl_defrag_range_args {
diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index c65a5aa..90b643e 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -1216,6 +1216,7 @@ static void __setup_root(u32 nodesize, u32 leafsize, u32 sectorsize,
 	atomic_set(&root->log_writers, 0);
 	atomic_set(&root->log_batch, 0);
 	atomic_set(&root->orphan_inodes, 0);
+	atomic_set(&root->refs, 1);
 	root->log_transid = 0;
 	root->last_log_commit = 0;
 	extent_io_tree_init(&root->dirty_log_pages,
@@ -2052,7 +2053,7 @@ static void del_fs_roots(struct btrfs_fs_info *fs_info)
 		} else {
 			free_extent_buffer(gang[0]->node);
 			free_extent_buffer(gang[0]->commit_root);
-			kfree(gang[0]);
+			btrfs_put_fs_root(gang[0]);
 		}
 	}
 
@@ -3417,7 +3418,7 @@ static void free_fs_root(struct btrfs_root *root)
 	kfree(root->free_ino_ctl);
 	kfree(root->free_ino_pinned);
 	kfree(root->name);
-	kfree(root);
+	btrfs_put_fs_root(root);
 }
 
 void btrfs_free_fs_root(struct btrfs_root *root)
diff --git a/fs/btrfs/disk-io.h b/fs/btrfs/disk-io.h
index 534d583..b71acd6e 100644
--- a/fs/btrfs/disk-io.h
+++ b/fs/btrfs/disk-io.h
@@ -76,6 +76,27 @@ void btrfs_btree_balance_dirty_nodelay(struct btrfs_root *root);
 void btrfs_drop_and_free_fs_root(struct btrfs_fs_info *fs_info,
 				 struct btrfs_root *root);
 void btrfs_free_fs_root(struct btrfs_root *root);
+
+/*
+ * This function is used to grab the root, and avoid it is freed when we
+ * access it. But it doesn't ensure that the tree is not dropped.
+ *
+ * If you want to ensure the whole tree is safe, you should use
+ * 	fs_info->subvol_srcu
+ */
+static inline struct btrfs_root *btrfs_grab_fs_root(struct btrfs_root *root)
+{
+	if (atomic_inc_not_zero(&root->refs))
+		return root;
+	return NULL;
+}
+
+static inline void btrfs_put_fs_root(struct btrfs_root *root)
+{
+	if (atomic_dec_and_test(&root->refs))
+		kfree(root);
+}
+
 void btrfs_mark_buffer_dirty(struct extent_buffer *buf);
 int btrfs_buffer_uptodate(struct extent_buffer *buf, u64 parent_transid,
 			  int atomic);
diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
index c989fe6..04066c2 100644
--- a/fs/btrfs/extent-tree.c
+++ b/fs/btrfs/extent-tree.c
@@ -7469,7 +7469,7 @@ int btrfs_drop_snapshot(struct btrfs_root *root,
 	} else {
 		free_extent_buffer(root->node);
 		free_extent_buffer(root->commit_root);
-		kfree(root);
+		btrfs_put_fs_root(root);
 	}
 out_end_trans:
 	btrfs_end_transaction_throttle(trans, tree_root);