The SMP / multithread design has several levels in the ALSA project codebase. The kernel drivers are fully SMP / thread safe. The alsa-lib was designed to allow grouping of commands for returned handle to optimize pthread locking - so the serialization should be done inside an application. The snd_*_open() calls are thread safe. The use of returned handles must be serialized in the application using own locking scheme.
If you find a bug in kernel or alsa-lib which breaks these rules, please, let us know.