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>

2 files changed, 29 insertions, 0 deletions

diff --git a/fs/btrfs/send.c b/fs/btrfs/send.c
index 5b6978516461..4e2461b857f3 100644
--- a/fs/btrfs/send.c
+++ b/fs/btrfs/send.c
@@ -4753,6 +4753,7 @@ long btrfs_ioctl_send(struct file *mnt_file, void __user *arg_)
         u64 *clone_sources_tmp = NULL;
         int clone_sources_to_rollback = 0;
         int sort_clone_roots = 0;
+        int index;
 
         if (!capable(CAP_SYS_ADMIN))
                 return -EPERM;
@@ -4893,8 +4894,12 @@ long btrfs_ioctl_send(struct file *mnt_file, void __user *arg_)
                         key.objectid = clone_sources_tmp[i];
                         key.type = BTRFS_ROOT_ITEM_KEY;
                         key.offset = (u64)-1;
+
+                        index = srcu_read_lock(&fs_info->subvol_srcu);
+
                         clone_root = btrfs_read_fs_root_no_name(fs_info, &key);
                         if (IS_ERR(clone_root)) {
+                                srcu_read_unlock(&fs_info->subvol_srcu, index);
                                 ret = PTR_ERR(clone_root);
                                 goto out;
                         }
@@ -4903,10 +4908,13 @@ long btrfs_ioctl_send(struct file *mnt_file, void __user *arg_)
                         clone_root->send_in_progress++;
                         if (!btrfs_root_readonly(clone_root)) {
                                 spin_unlock(&clone_root->root_item_lock);
+                                srcu_read_unlock(&fs_info->subvol_srcu, index);
                                 ret = -EPERM;
                                 goto out;
                         }
                         spin_unlock(&clone_root->root_item_lock);
+                        srcu_read_unlock(&fs_info->subvol_srcu, index);
+
                         sctx->clone_roots[i].root = clone_root;
                 }
                 vfree(clone_sources_tmp);
@@ -4917,19 +4925,27 @@ long btrfs_ioctl_send(struct file *mnt_file, void __user *arg_)
                 key.objectid = arg->parent_root;
                 key.type = BTRFS_ROOT_ITEM_KEY;
                 key.offset = (u64)-1;
+
+                index = srcu_read_lock(&fs_info->subvol_srcu);
+
                 sctx->parent_root = btrfs_read_fs_root_no_name(fs_info, &key);
                 if (IS_ERR(sctx->parent_root)) {
+                        srcu_read_unlock(&fs_info->subvol_srcu, index);
                         ret = PTR_ERR(sctx->parent_root);
                         goto out;
                 }
+
                 spin_lock(&sctx->parent_root->root_item_lock);
                 sctx->parent_root->send_in_progress++;
                 if (!btrfs_root_readonly(sctx->parent_root)) {
                         spin_unlock(&sctx->parent_root->root_item_lock);
+                        srcu_read_unlock(&fs_info->subvol_srcu, index);
                         ret = -EPERM;
                         goto out;
                 }
                 spin_unlock(&sctx->parent_root->root_item_lock);
+
+                srcu_read_unlock(&fs_info->subvol_srcu, index);
         }
 
         /*
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);