On Sat, Jan 27, 2024 at 11:54:32AM +0100, Oleg Nesterov wrote: > Hi Tycho, > > On 01/26, Tycho Andersen wrote: > > > > On Thu, Jan 25, 2024 at 03:08:31PM +0100, Oleg Nesterov wrote: > > > What do you think? > > > > Thank you, it passes all my tests. > > Great, thanks! > > OK, I'll make v2 on top of the recent > "pidfd: cleanup the usage of __pidfd_prepare's flags" > > but we need to finish our discussion with Christian about the > usage of O_EXCL. Just write the patch exactly like you want. If it's as a little a detail as the uapi flag value we can just always change that when applying. There's no reason to introduce artificial delays because of my preference here.. > > As for clone(CLONE_PIDFD | CLONE_THREAD), this is trivial but > I think this needs another discussion too, lets do this later. > > > > + /* unnecessary if do_notify_parent() was already called, > > > + we can do better */ > > > + do_notify_pidfd(tsk); > > > > "do better" here could be something like, > > > > [...snip...] > > No, no, please see below. > > For the moment, please forget about PIDFD_THREAD, lets discuss > the current behaviour. > > > but even with that, there's other calls in the tree to > > do_notify_parent() that might double notify. > > Yes, and we can't avoid this. Well, perhaps do_notify_parent() > can do something like > > if (ptrace_reparented()) > do_notify_pidfd(); > > so that only the "final" do_notify_parent() does do_notify_pidfd() > but this needs another discussion and in fact I don't think this > would be right or make much sense. Lets forget this for now. > > Now. Even without PIDFD_THREAD, I think it makes sense to change > do_notify_parent() to do > > if (thread_group_empty(tsk)) > do_notify_pidfd(tsk); > > thread_group_empty(tsk) can only be true if tsk is a group leader > and it is the last thread. And this is exactly what pidfd_poll() > currently needs. > > In fact I'd even prefer to do this in a separate patch for the > documentation purposes. > > Now, PIDFD_THREAD can just add > > if (!thread_group_empty(tsk)) > do_notify_pidfd(tsk); > > right after "tsk->exit_state = EXIT_ZOMBIE", that is all. Sounds good. > > This also preserves the do_notify_pidfd/__wake_up_parent ordering. > Not that I think this is important, just for consistency. > > > This brings up another interesting behavior that I noticed while > > testing this, if you do a poll() on pidfd, followed quickly by a > > pidfd_getfd() on the same thread you just got an event on, you can > > sometimes get an EBADF from __pidfd_fget() instead of the more > > expected ESRCH higher up the stack. > > 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. > > > I wonder if it makes sense to abuse ->f_flags to add a PIDFD_NOTIFIED? > > Then we can refuse further pidfd syscall operations in a sane way, and > > But how? We only have "struct pid *", how can we find all files > "attached" to this pid? We can't. There's some use-cases that would make this desirable but that would mean we would need another data structure to track this. I once toyed with a patch for this but never got so excited about it to actually finish it.
