rails: Deadlock between ParamsWrapper#model and Loadable#require_or_load

Steps to reproduce

1. Create a Rails 5.1.2 app with Postgres DB (pg 0.20), puma server (puma 3.9.1, 5 threads).
2. Add some models and RESTful controllers. Keep default wrap_parameters format: [:json].
3. Make a large number (e.g., 5) of simultaneous GET requests to the controllers with Accept: application/json and Content-Type: application/json; charset=utf-8 headers.

Expected behavior

Requests succeed.

Actual behavior

Requests cause a hangup almost consistently. Further investigation with ActionDispatch::DebugLocks shows threads end up deadlocked.

System configuration

Rails version: 5.1.2
Ruby version: 2.4.1