[jbn]

or a port of the lock-free hash map from https://preshing.com/20130605/the-worlds-simplest-lock-free-...

But really, the premise of using a shared map with concurrent readers and writers seems like a good generator of hard-to-reproduce bugs. IMHO shared-nothing (when feasible) is much easier to reason about, and possibly do periodic merging of thread-local updates, but I would avoid concurrent updates entirely (in particular if 2 threads race to update the same key... that goes to deeper design issues in the application).