Thread bug 3 identified#

Mon Jul 19 01:39:59 2004

Thread bug 3 identified. Background: gc_stop_the_world is called from within a WITHOUT-INTERRUPTS form. WITHOUT-INTERRUPTS doesn't actually mask signals at the kernel level, but does set a flag that the C-level signal handlers check when deciding whether to defer a signal. SIG_THREAD_EXIT (the RT signal that the parent thread is sent when a child thread dies, a la SIGCHLD for traditional processes) is on the deferrable list (because it calls Lisp code), so if a thread dies between the start of WITHOUT-INTERRUPTS and the world getting stopped, there's nothing available to wait() for it. Thus it becomes a zombie and will never react to requests to suspend itself for GC.

Alternate strategy: make thread_exit_handler just wait() and then set thread->state=STATE_DEAD, so the GC can see the thread's no more, then do the Lisp-level stuff and destroy_thread at some later time. Now we can remove SIG_THREAD_EXIT from the deferrable list. Pretty arbitrarily, we've decided that at "after a GC" would be a reasonable later time; so now if you disable GC you have also disabled dead thread collection.