15.4.1.1. Rules for Running Guarded Methods

ooRexx ensures that guarded methods of the same scope (defined for the same class) cannot execute concurrently to protect access to its attribute pool (object variable pool). To do so, a counter-based guard lock is maintained for each scope, which gets increased by one if a guarded method gets invoked and decreased by one upon return. If a guarded method for the same scope gets invoked from another thread and the scope's guard lock counter is not zero, it gets blocked because another guarded method holding the guard lock is currently running in the same scope. Once the scope's guard lock counter drops to 0 (no other guarded method runs currently), the blocked guarded method can acquire the guard lock, thereby increasing the guard lock counter to one and starting to run.

Invoking a guarded method from the same thread will increase the guard lock counter by one and, upon return, will decrease it by one. Invoking an unguarded method from the same thread will not change the guard lock counter.
Invoking a guarded method from another thread for a scope in which a guarded method is currently running will block the invocation until the scope's guard lock counter drops to 0 (no other guarded method for the same scope is running anymore) and the guard lock becomes free. In this situation, the blocked guarded method will succeed in acquiring the guard lock, increasing the guard lock counter to one, and starting running.
If a REPLY keyword statement is processed in a currently guarded method, the remaining instructions of the guarded method will remain guarded.
A GUARD ON keyword statement has no effect.
A GUARD OFF keyword statement changes the guarded method to an unguarded method from that point on, in the process reducing the guard lock counter by one and removing the guard lock.