From: Josef Bacik <jbacik@fb.com>
Date: Fri, 30 Nov 2018 11:52:13 -0500
Subject: btrfs: catch cow on deleting snapshots
Git-commit: 83354f0772cd07828b0fcdf1ef2872fd2032acb8
Patch-mainline: v5.0-rc1
References: bsc#1165949

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.

Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: David Sterba <dsterba@suse.com>
Acked-by: Nikolay Borisov <nborisov@suse.com>
---
 fs/btrfs/ctree.c       |    4 ++++
 fs/btrfs/ctree.h       |    1 +
 fs/btrfs/extent-tree.c |    9 +++++++++
 3 files changed, 14 insertions(+)

--- a/fs/btrfs/ctree.c
+++ b/fs/btrfs/ctree.c
@@ -1522,6 +1522,10 @@ noinline int btrfs_cow_block(struct btrf
 	u64 search_start;
 	int ret;
 
+	if (test_bit(BTRFS_ROOT_DELETING, &root->state))
+		btrfs_err(fs_info,
+			"COW'ing blocks on a fs root that's being dropped");
+
 	if (trans->transaction != fs_info->running_transaction)
 		WARN(1, KERN_CRIT "trans %llu running %llu\n",
 		       trans->transid,
--- a/fs/btrfs/ctree.h
+++ b/fs/btrfs/ctree.h
@@ -1179,6 +1179,7 @@ struct btrfs_subvolume_writers {
 #define BTRFS_ROOT_FORCE_COW		6
 #define BTRFS_ROOT_MULTI_LOG_TASKS	7
 #define BTRFS_ROOT_DIRTY		8
+#define BTRFS_ROOT_DELETING		9
 
 /*
  * in ram representation of the tree.  extent_root is used for all allocations
--- a/fs/btrfs/extent-tree.c
+++ b/fs/btrfs/extent-tree.c
@@ -9275,6 +9275,15 @@ int btrfs_drop_snapshot(struct btrfs_roo
 	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);