all SA updates in a object set are serialized with a global mutex. given SA is used to store regular attributes and EAs, it's very expensive to update any of them, especially in a multi-thread and/or multi-core environment.
the mutex (sa_lock) is used to protect a structure listing all known SA configurations.
it's possible to cache references to few frequently-used configurations on per-cpu basis and skip the mutex in the majorify of cases - this is proved by a prototype patch. the patch still needs some polishing and inspections on ZoL