[PATCH 4.16 30/52] btrfs: Take trans lock before access running trans in check_delayed_ref

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

4.16-stable review patch.  If anyone has any objections, please let me know.

------------------

From: ethanwu <ethanwu@synology.com>

commit 998ac6d21cfd6efd58f5edf420bae8839dda9f2a upstream.

In preivous patch:
Btrfs: kill trans in run_delalloc_nocow and btrfs_cross_ref_exist
We avoid starting btrfs transaction and get this information from
fs_info->running_transaction directly.

When accessing running_transaction in check_delayed_ref, there's a
chance that current transaction will be freed by commit transaction
after the NULL pointer check of running_transaction is passed.

After looking all the other places using fs_info->running_transaction,
they are either protected by trans_lock or holding the transactions.

Fix this by using trans_lock and increasing the use_count.

Fixes: e4c3b2dcd144 ("Btrfs: kill trans in run_delalloc_nocow and btrfs_cross_ref_exist")
CC: stable@vger.kernel.org # 4.14+
Signed-off-by: ethanwu <ethanwu@synology.com>
Signed-off-by: David Sterba <dsterba@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 fs/btrfs/extent-tree.c |    7 +++++++
 1 file changed, 7 insertions(+)

--- a/fs/btrfs/extent-tree.c
+++ b/fs/btrfs/extent-tree.c
@@ -3155,7 +3155,11 @@ static noinline int check_delayed_ref(st
 	struct rb_node *node;
 	int ret = 0;
 
+	spin_lock(&root->fs_info->trans_lock);
 	cur_trans = root->fs_info->running_transaction;
+	if (cur_trans)
+		refcount_inc(&cur_trans->use_count);
+	spin_unlock(&root->fs_info->trans_lock);
 	if (!cur_trans)
 		return 0;
 
@@ -3164,6 +3168,7 @@ static noinline int check_delayed_ref(st
 	head = btrfs_find_delayed_ref_head(delayed_refs, bytenr);
 	if (!head) {
 		spin_unlock(&delayed_refs->lock);
+		btrfs_put_transaction(cur_trans);
 		return 0;
 	}
 
@@ -3180,6 +3185,7 @@ static noinline int check_delayed_ref(st
 		mutex_lock(&head->mutex);
 		mutex_unlock(&head->mutex);
 		btrfs_put_delayed_ref_head(head);
+		btrfs_put_transaction(cur_trans);
 		return -EAGAIN;
 	}
 	spin_unlock(&delayed_refs->lock);
@@ -3212,6 +3218,7 @@ static noinline int check_delayed_ref(st
 	}
 	spin_unlock(&head->lock);
 	mutex_unlock(&head->mutex);
+	btrfs_put_transaction(cur_trans);
 	return ret;
 }