Running an rsync process writing to the btrfs filesystem deadlocked with a bunch of kworkers in leaf_space_used and one in btrfs_tree_read_lock(). The first stack trace in the attched file, hitting btrfs_lock_root_node(), was about 2 minutes before the deadlock when all activity on the filesystem stopped.
Created attachment 280299 [details]
dmesg stack traces just before deadlock
Thanks for the report. Fix https://patchwork.kernel.org/patch/10751909/ will go to 5.0-rc and then to stable 4.19.x.