How does CRuby ensure thread safety of internal VM state under the Global VM Lock (GVL)?

GVL serializes access to the Ruby VM, ensuring only one thread mutates VM state at a time.

CRuby avoids fine-grained locking inside the VM by using a single Global VM Lock. This ensures that internal structures like object heap, method tables, and GC metadata are never concurrently mutated. Threads only execute Ruby bytecode when holding the GVL. During blocking I/O or sleep, the GVL is released so other threads can run. This design simplifies correctness but limits CPU parallelism.