Market data service freezes

A Java market-data fan-out service freezes completely about once a day, it stops publishing updates, accepts no new work, and only a restart recovers it. There's no crash and no OOM. When it freezes, CPU goes to ~0% and stays there (not pegged). A thread dump taken during the freeze shows two threads each holding one lock and waiting on the other ('Found one Java-level deadlock' is printed by the JVM), involving a subscription-registry lock and a publisher lock. The deadlock appeared after a refactor that lets a publish callback re-enter and modify the subscription registry. Triage and design the fix.