Do locks on readonly secondary databases propagate to the Read/Write Database
AlwaysOn secondaries map all reads to versioned reads, which are (almost) lock-free
Even with versioned reads there are some (few) locks in place, eg. schema-stability locks on obejcts. There is never any sort of propagation of locks from secondaries to primaries. Furthermore, there cannot be any propagation of locks from secondaries to primaries simply because the secondary is a back-in-time copy of the primary. There is no point in propagating a lock on object 'foo' from secondary to primary because the primary may had dropped 'foo' 2 hours ago and the secondary did not catch up yet. The propagation of locks downstream from primary to secondary goes through the log records and as such the locks are re-acquired on the secondaries when the log is redone, therefore the lock is always applied to the appropriate structure (if 'foo' existed on the primary when the lock was acquired, then by definition exists on the secondary at the moment the lock is re-played).
Secondary reads may block redo (eg. by holding on a schema-stability lock when the redo is trying to acquire a schema-modification). The redo resumes when the secondary reads releases the lock. And 'redo' in this context means 'applying log from the primary on the secondary', ie. catching up.