On Sat, Jan 27, 2024 at 05:31:39PM +0100, Oleg Nesterov wrote: > On 01/27, Tycho Andersen wrote: > > > > It seems like (and the current pidfd_test enforces for some cases) > > Which pidfd_test ? I was thinking of poll_pidfd() in pidfd_test.c, but, > > we > > want exactly one notification for a task dying. > > This can't be right. EVERY user of poll_wait() or wait_event/etc > must handle/tolerate the false wakeups. you're right, it doesn't enforce exactly once. > > I don't understand > > how we guarantee this now, with all of these calls. > > I don't understand why do we need or even want to guarantee this. > > The extra wakeup must be always fine correctness-wise. Sure, it > would be nice to avoid the unnecessary wakeups, and perhaps we > can change wake_up_all() to pass a key to, say, only wake_up the > PIDFD_THREAD waiters from exit_notify(). But certainly this is > outside the scope of PIDFD_THREAD change we discuss. > > The changes in do_notify_parent() (I have already sent the patch) and > in exit_notify() (proposed in my previous email) just ensure that, > with the minimal changes, we avoid 2 do_notify_pidfd's from the same > exit_notify() path. Sounds good. > > > exit_notify() is called after exit_files(). pidfd_getfd() returns > > > ESRCH if the exiting thread completes release_task(), otherwise it > > > returns EBADF because ->files == NULL. This too doesn't really > > > depend on PIDFD_THREAD. > > > > Yup, understood. It just seems like an inconsistency we might want to > > fix. > > Not sure this worth "fixing"... Yep, maybe not. Just wanted to point it out. Tycho
