A race condition was discovered in get_old_root in fs/btrfs/ctree.c in the Linux kernel through 5.11.8. It allows attackers to cause a denial of service (BUG) because of a lack of locking on an extent buffer before a cloning operation, aka CID-dbcc7d57bffc.
https://security.netapp.com/advisory/ntap-20210430-0003/
https://lists.debian.org/debian-lts-announce/2021/06/msg00020.html
https://lists.debian.org/debian-lts-announce/2021/06/msg00019.html