[PATCH 1/2] btrfs: catch cow on deleting snapshots

[Josef Bacik <josef@toxicpanda.com>]

From: Josef Bacik <jbacik@fb.com>

When debugging some weird extent reference bug I suspected that we were
changing a snapshot while we were deleting it, which could explain my
bug.  This was indeed what was happening, and this patch helped me
verify my theory.  It is never correct to modify the snapshot once it's
being deleted, so mark the root when we are deleting it and make sure we
complain about it when it happens.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
---
 fs/btrfs/ctree.c       | 3 +++
 fs/btrfs/ctree.h       | 1 +
 fs/btrfs/extent-tree.c | 9 +++++++++
 3 files changed, 13 insertions(+)

diff --git a/fs/btrfs/ctree.c b/fs/btrfs/ctree.c
index 5912a97b07a6..5f82f86085e8 100644
--- a/fs/btrfs/ctree.c
+++ b/fs/btrfs/ctree.c
@@ -1440,6 +1440,9 @@ noinline int btrfs_cow_block(struct btrfs_trans_handle *trans,
 	u64 search_start;
 	int ret;
 
+	if (test_bit(BTRFS_ROOT_DELETING, &root->state))
+		WARN(1, KERN_CRIT "cow'ing blocks on a fs root thats being dropped\n");
+
 	if (trans->transaction != fs_info->running_transaction)
 		WARN(1, KERN_CRIT "trans %llu running %llu\n",
 		       trans->transid,
diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
index facde70c15ed..5a3a94ccb65c 100644
--- a/fs/btrfs/ctree.h
+++ b/fs/btrfs/ctree.h
@@ -1199,6 +1199,7 @@ enum {
 	BTRFS_ROOT_FORCE_COW,
 	BTRFS_ROOT_MULTI_LOG_TASKS,
 	BTRFS_ROOT_DIRTY,
+	BTRFS_ROOT_DELETING,
 };
 
 /*
diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
index 581c2a0b2945..dcb699dd57f3 100644
--- a/fs/btrfs/extent-tree.c
+++ b/fs/btrfs/extent-tree.c
@@ -9333,6 +9333,15 @@ int btrfs_drop_snapshot(struct btrfs_root *root,
 	if (block_rsv)
 		trans->block_rsv = block_rsv;
 
+	/*
+	 * This will help us catch people modifying the fs tree while we're
+	 * dropping it.  It is unsafe to mess with the fs tree while it's being
+	 * dropped as we unlock the root node and parent nodes as we walk down
+	 * the tree, assuming nothing will change.  If something does change
+	 * then we'll have stale information and drop references to blocks we've
+	 * already dropped.
+	 */
+	set_bit(BTRFS_ROOT_DELETING, &root->state);
 	if (btrfs_disk_key_objectid(&root_item->drop_progress) == 0) {
 		level = btrfs_header_level(root->node);
 		path->nodes[level] = btrfs_lock_root_node(root);
-- 
2.14.3