In the Linux kernel 5.0.21 mounting a crafted btrfs filesystem image performing some operations and then making a syncfs system call can lead to a use-after-free in __mutex_lock in kernel/locking/mutex.c. This is related to mutex_can_spin_on_owner in kernel/locking/mutex.c __btrfs_qgroup_free_meta in fs/btrfs/qgroup.c and btrfs_insert_delayed_items in fs/btrfs/delayed-inode.c.