I don't think you understood my point. So let me give you a more concrete example.

The Linux kernel uses RCU to manage data structures that are concurrently read and updated. When an old value (after an update) is no longer needed, a thread can either:

(a) block for a while to make sure no other thread is using it using synchronize_rcu() and then free the memory (see https://www.kernel.org/doc/html/latest/core-api/kernel-api.h...)

(b) if the thread cannot block, it will use call_rcu to register a callback to free the memory at a later time (https://www.kernel.org/doc/html/latest/core-api/kernel-api.h...). That callback generally runs in some other thread to do the cleanup.

Now, moving the concepts to user space, a typical user space implementation will just launch a dedicated thread to free all memory that is no longer needed by any RCU data structures.