Btrfs: fix protection between send and root deletion

We should gurantee that parent and clone roots can not be destroyed
during send, for this we have two ideas.

1.by holding @subvol_sem, this might be a nightmare, because it will
block all subvolumes deletion for a long time.

2.Miao pointed out we can reuse @send_in_progress, that mean we will
skip snapshot deletion if root sending is in progress.

Here we adopt the second approach since it won't block other subvolumes
deletion for a long time.

Besides in btrfs_clean_one_deleted_snapshot(), we only check first root
, if this root is involved in send, we return directly rather than
continue to check.There are several reasons about it:

1.this case happen seldomly.
2.after sending,cleaner thread can continue to drop that root.
3.make code simple

Cc: David Sterba <dsterba@suse.cz>
Signed-off-by: Wang Shilong <wangsl.fnst@cn.fujitsu.com>
Reviewed-by: Miao Xie <miaox@cn.fujitsu.com>
Signed-off-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: Chris Mason <clm@fb.com>

1 files changed, 13 insertions, 0 deletions

diff --git a/fs/btrfs/transaction.c b/fs/btrfs/transaction.c
index e5fe801659ba..da2ac4c6d78b 100644
--- a/fs/btrfs/transaction.c
+++ b/fs/btrfs/transaction.c
@@ -1972,6 +1972,19 @@ int btrfs_clean_one_deleted_snapshot(struct btrfs_root *root)
         }
         root = list_first_entry(&fs_info->dead_roots,
                         struct btrfs_root, root_list);
+        /*
+         * Make sure root is not involved in send,
+         * if we fail with first root, we return
+         * directly rather than continue.
+         */
+        spin_lock(&root->root_item_lock);
+        if (root->send_in_progress) {
+                spin_unlock(&fs_info->trans_lock);
+                spin_unlock(&root->root_item_lock);
+                return 0;
+        }
+        spin_unlock(&root->root_item_lock);
+
         list_del_init(&root->root_list);
         spin_unlock(&fs_info->trans_lock);