In ChibiOS/RT threads are divided in two classes:

• Static threads. The memory used for static threads is allocated at compile time so static threads are always there, there is no management to be done.
• Dynamic threads. Dynamic threads are allocated at runtime from one of the available allocators.

Dynamic threads create the problem of who is responsible of releasing their memory because a thread cannot dispose its own memory, this is handled in ChibiOS/RT through the mechanism of “thread references”. When the CH_USE_DYNAMIC option is enabled the threads become objects with a reference counter. The memory of a dynamic thread is freed when the last reference to the thread is released while the thread is in its THD_STATE_FINAL state. The following diagram explains the mechanism: As you can see the easiest way to ensure that the memory is released is to make another thread perform a chThdWait() on the dynamic thread.
If all the references to the threads are released while the thread is still alive then the thread goes in a “detached” state and its memory cannot be recovered unless there is a dedicated task in the system that scans the threads through the registry subsystem, scanning the registry has the side effect to release the zombies (detached and then terminated threads).