Is it OK to use multi-thread in Postgresql extension?

I'd like to improve performance of a Postgresql extension and certainly multi-thread is considered. While roughly, I know so far Postgresql is multi-process based, and in the extension, Postgresql things like SRF_FIRSTCALL_INIT(), SRF_PERCALL_SETUP, MemoryContextSwitchTo(), CreateTemplateTupleDesc(), palloc(), Int64GetDatum and so on are used, so my questions: - Are above APIs thread-safe? - Is it OK to use multi-thread in Postgresql extension? If so, would you provide any example, or provide any suggestion? - Is there any multiple process framework is actually preferable for extension? Like LaunchParallelWorker() and RegisterBackgroundWorker(), are they applicable for extension? - Is there any method to perform something in parallel in extension?