objc4

Hmm, if I'm understanding right, rather than passively checking whether any of the other threads are in the critical section, it's actually forcing all the threads to undergo a context switch so that the PC (instruction pointer) can be read. So long as the context switch is prevented until the thread is out of the critical section, the actual value of the PC is irrelevant so long as the call succeeds: #define PC_SENTINEL 1 unsigned int count = x86_THREAD_STATE64_COUNT; kern_return_t okay = thread_get_state (thread, x86_THREAD_STATE64, (thread_state_t)&state, &count); return (okay == KERN_SUCCESS) ? state.__rip : PC_SENTINEL; https://github.com/opensource-apple/objc4/blob/master/runtim... If you have a lot of threads running, this could get really expensive. I presume it works, but this doesn't seem like an efficient approach

You can do a variant of this in userspace as well: https://github.com/opensource-apple/objc4/blob/master/runtim... Basically, issue a syscall that acquires the PC of all threads. When none of the PCs are in the critical section, then you can garbage collect old objects.