Btrfs: fix file loss on log replay after renaming a file and fsync

We have two cases where we end up deleting a file at log replay time
when we should not. For this to happen the file must have been renamed
and a directory inode must have been fsynced/logged.

Two examples that exercise these two cases are listed below.

  Case 1)

  $ mkfs.btrfs -f /dev/sdb
  $ mount /dev/sdb /mnt
  $ mkdir -p /mnt/a/b
  $ mkdir /mnt/c
  $ touch /mnt/a/b/foo
  $ sync
  $ mv /mnt/a/b/foo /mnt/c/
  # Create file bar just to make sure the fsync on directory a/ does
  # something and it's not a no-op.
  $ touch /mnt/a/bar
  $ xfs_io -c "fsync" /mnt/a
  < power fail / crash >

  The next time the filesystem is mounted, the log replay procedure
  deletes file foo.

  Case 2)

  $ mkfs.btrfs -f /dev/sdb
  $ mount /dev/sdb /mnt
  $ mkdir /mnt/a
  $ mkdir /mnt/b
  $ mkdir /mnt/c
  $ touch /mnt/a/foo
  $ ln /mnt/a/foo /mnt/b/foo_link
  $ touch /mnt/b/bar
  $ sync
  $ unlink /mnt/b/foo_link
  $ mv /mnt/b/bar /mnt/c/
  $ xfs_io -c "fsync" /mnt/a/foo
  < power fail / crash >

  The next time the filesystem is mounted, the log replay procedure
  deletes file bar.

The reason why the files are deleted is because when we log inodes
other then the fsync target inode, we ignore their last_unlink_trans
value and leave the log without enough information to later replay the
rename operations. So we need to look at the last_unlink_trans values
and fallback to a transaction commit if they are greater than the
id of the last committed transaction.

So fix this by looking at the last_unlink_trans values and fallback to
transaction commits when needed. Also, when logging other inodes (for
case 1 we logged descendants of the fsync target inode while for case 2
we logged ascendants) we need to care about concurrent tasks updating
the last_unlink_trans of inodes we are logging (which was already an
existing problem in check_parent_dirs_for_sync()). Since we can not
acquire their inode mutex (vfs' struct inode ->i_mutex), as that causes
deadlocks with other concurrent operations that acquire the i_mutex of
2 inodes (other fsyncs or renames for example), we need to serialize on
the log_mutex of the inode we are logging. A task setting a new value for
an inode's last_unlink_trans must acquire the inode's log_mutex and it
must do this update before doing the actual unlink operation (which is
already the case except when deleting a snapshot). Conversely the task
logging the inode must first log the inode and then check the inode's
last_unlink_trans value while holding its log_mutex, as if its value is
not greater then the id of the last committed transaction it means it
logged a safe state of the inode's items, while if its value is not
smaller then the id of the last committed transaction it means the inode
state it has logged might not be safe (the concurrent task might have
just updated last_unlink_trans but hasn't done yet the unlink operation)
and therefore a transaction commit must be done.

Test cases for xfstests follow in separate patches.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Chris Mason <clm@fb.com>

fs/btrfs/ioctl.c

@@ -2471,6 +2471,8 @@ static noinline int btrfs_ioctl_snap_destroy(struct file *file,
 	trans->block_rsv = &block_rsv;
 	trans->bytes_reserved = block_rsv.size;
 
+	btrfs_record_snapshot_destroy(trans, dir);
+
 	ret = btrfs_unlink_subvol(trans, root, dir,
 				dest->root_key.objectid,
 				dentry->d_name.name,
@@ -2522,8 +2524,6 @@ static noinline int btrfs_ioctl_snap_destroy(struct file *file,
 out_end_trans:
 	trans->block_rsv = NULL;
 	trans->bytes_reserved = 0;
-	if (!err)
-		btrfs_record_snapshot_destroy(trans, dir);
 	ret = btrfs_end_transaction(trans, root);
 	if (ret && !err)
 		err = ret;

fs/btrfs/tree-log.c

@@ -4771,6 +4771,42 @@ out_unlock:
 	return err;
 }
 
+/*
+ * Check if we must fallback to a transaction commit when logging an inode.
+ * This must be called after logging the inode and is used only in the context
+ * when fsyncing an inode requires the need to log some other inode - in which
+ * case we can't lock the i_mutex of each other inode we need to log as that
+ * can lead to deadlocks with concurrent fsync against other inodes (as we can
+ * log inodes up or down in the hierarchy) or rename operations for example. So
+ * we take the log_mutex of the inode after we have logged it and then check for
+ * its last_unlink_trans value - this is safe because any task setting
+ * last_unlink_trans must take the log_mutex and it must do this before it does
+ * the actual unlink operation, so if we do this check before a concurrent task
+ * sets last_unlink_trans it means we've logged a consistent version/state of
+ * all the inode items, otherwise we are not sure and must do a transaction
+ * commit (the concurrent task migth have only updated last_unlink_trans before
+ * we logged the inode or it might have also done the unlink).
+ */
+static bool btrfs_must_commit_transaction(struct btrfs_trans_handle *trans,
+					  struct inode *inode)
+{
+	struct btrfs_fs_info *fs_info = BTRFS_I(inode)->root->fs_info;
+	bool ret = false;
+
+	mutex_lock(&BTRFS_I(inode)->log_mutex);
+	if (BTRFS_I(inode)->last_unlink_trans > fs_info->last_trans_committed) {
+		/*
+		 * Make sure any commits to the log are forced to be full
+		 * commits.
+		 */
+		btrfs_set_log_full_commit(fs_info, trans);
+		ret = true;
+	}
+	mutex_unlock(&BTRFS_I(inode)->log_mutex);
+
+	return ret;
+}
+
 /*
  * follow the dentry parent pointers up the chain and see if any
  * of the directories in it require a full commit before they can
@@ -4784,7 +4820,6 @@ static noinline int check_parent_dirs_for_sync(struct btrfs_trans_handle *trans,
 					       u64 last_committed)
 {
 	int ret = 0;
-	struct btrfs_root *root;
 	struct dentry *old_parent = NULL;
 	struct inode *orig_inode = inode;
 
@@ -4816,14 +4851,7 @@ static noinline int check_parent_dirs_for_sync(struct btrfs_trans_handle *trans,
 			BTRFS_I(inode)->logged_trans = trans->transid;
 		smp_mb();
 
-		if (BTRFS_I(inode)->last_unlink_trans > last_committed) {
-			root = BTRFS_I(inode)->root;
-
-			/*
-			 * make sure any commits to the log are forced
-			 * to be full commits
-			 */
-			btrfs_set_log_full_commit(root->fs_info, trans);
+		if (btrfs_must_commit_transaction(trans, inode)) {
 			ret = 1;
 			break;
 		}
@@ -4982,6 +5010,9 @@ process_leaf:
 			btrfs_release_path(path);
 			ret = btrfs_log_inode(trans, root, di_inode,
 					      log_mode, 0, LLONG_MAX, ctx);
+			if (!ret &&
+			    btrfs_must_commit_transaction(trans, di_inode))
+				ret = 1;
 			iput(di_inode);
 			if (ret)
 				goto next_dir_inode;
@@ -5096,6 +5127,9 @@ static int btrfs_log_all_parents(struct btrfs_trans_handle *trans,
 
 			ret = btrfs_log_inode(trans, root, dir_inode,
 					      LOG_INODE_ALL, 0, LLONG_MAX, ctx);
+			if (!ret &&
+			    btrfs_must_commit_transaction(trans, dir_inode))
+				ret = 1;
 			iput(dir_inode);
 			if (ret)
 				goto out;
@@ -5447,6 +5481,9 @@ error:
  * They revolve around files there were unlinked from the directory, and
  * this function updates the parent directory so that a full commit is
  * properly done if it is fsync'd later after the unlinks are done.
+ *
+ * Must be called before the unlink operations (updates to the subvolume tree,
+ * inodes, etc) are done.
  */
 void btrfs_record_unlink_dir(struct btrfs_trans_handle *trans,
 			     struct inode *dir, struct inode *inode,
@@ -5462,8 +5499,11 @@ void btrfs_record_unlink_dir(struct btrfs_trans_handle *trans,
 	 * into the file.  When the file is logged we check it and
 	 * don't log the parents if the file is fully on disk.
 	 */
-	if (S_ISREG(inode->i_mode))
+	if (S_ISREG(inode->i_mode)) {
+		mutex_lock(&BTRFS_I(inode)->log_mutex);
 		BTRFS_I(inode)->last_unlink_trans = trans->transid;
+		mutex_unlock(&BTRFS_I(inode)->log_mutex);
+	}
 
 	/*
 	 * if this directory was already logged any new
@@ -5494,7 +5534,9 @@ void btrfs_record_unlink_dir(struct btrfs_trans_handle *trans,
 	return;
 
 record:
+	mutex_lock(&BTRFS_I(dir)->log_mutex);
 	BTRFS_I(dir)->last_unlink_trans = trans->transid;
+	mutex_unlock(&BTRFS_I(dir)->log_mutex);
 }
 
 /*
@@ -5505,11 +5547,16 @@ record:
  * corresponding to the deleted snapshot's root, which could lead to replaying
  * it after replaying the log tree of the parent directory (which would replay
  * the snapshot delete operation).
+ *
+ * Must be called before the actual snapshot destroy operation (updates to the
+ * parent root and tree of tree roots trees, etc) are done.
  */
 void btrfs_record_snapshot_destroy(struct btrfs_trans_handle *trans,
 				   struct inode *dir)
 {
+	mutex_lock(&BTRFS_I(dir)->log_mutex);
 	BTRFS_I(dir)->last_unlink_trans = trans->transid;
+	mutex_unlock(&BTRFS_I(dir)->log_mutex);
 }
 
 /*