@haypo: It's a socketpair. It works fine when I set up a toy test case using set_wakeup_fd + select, and it works fine in my real code when I use CFFI cleverness to register a signal handler that manually writes a byte to my wakeup socket, but when I pass that exact same socket to set_wakeup_fd in my real code, it doesn't work. It's pretty mysterious, and I have no particular reason to think that the problem is in CPython as opposed to some stupid mistake I'm making.