当子线程退出时，父线程是如何通知的?

Can someone help me understand what wakes up the main thread after the child thread has exited? My understanding is that join calls futex_wait, causing the main thread to wait until the child thread has finished. Presumably there is a call to futex_wake to allow the master thread to resume but after running strace on both threads, I can't see where the syscall to wake up the parent thread is. Can someone help me understand what wakes up th