Btrfs: tree mod log sanity checks in join_transaction

When a fresh transaction begins, the tree mod log must be clean. Users of
the tree modification log must ensure they never span across transaction
boundaries.

We reset the sequence to 0 in this safe situation to make absolutely sure
overflow can't happen.
Signed-off-by: Jan Schmidt <list.btrfs@jan-o-sch.net>

fs/btrfs/transaction.c

@@ -122,6 +122,24 @@ static noinline int join_transaction(struct btrfs_root *root, int nofail)
 	cur_trans->delayed_refs.flushing = 0;
 	cur_trans->delayed_refs.run_delayed_start = 0;
 	cur_trans->delayed_refs.seq = 1;
+
+	/*
+	 * although the tree mod log is per file system and not per transaction,
+	 * the log must never go across transaction boundaries.
+	 */
+	smp_mb();
+	if (!list_empty(&fs_info->tree_mod_seq_list)) {
+		printk(KERN_ERR "btrfs: tree_mod_seq_list not empty when "
+			"creating a fresh transaction\n");
+		WARN_ON(1);
+	}
+	if (!RB_EMPTY_ROOT(&fs_info->tree_mod_log)) {
+		printk(KERN_ERR "btrfs: tree_mod_log rb tree not empty when "
+			"creating a fresh transaction\n");
+		WARN_ON(1);
+	}
+	atomic_set(&fs_info->tree_mod_seq, 0);
+
 	init_waitqueue_head(&cur_trans->delayed_refs.seq_wait);
 	spin_lock_init(&cur_trans->commit_lock);
 	spin_lock_init(&cur_trans->delayed_refs.lock);