Btrfs aims for the mainline

I don't know that it'd cause things to grind to a halt on a NUMA under normal circumstances. The cost of migrating lines goes up, sure, but you only get extra traffic when the busywaiter arrives just ahead of a lock-release.

I'm sure it could cause some interesting thundering-herd behavior, though, if there are a lot of people waiting on the lock to release though. If a large number of CPUs are in the busywaiting loop when the lock releases, things get really fun I suppose.