Re: locking down kqueue (was some other completely unrelated topic)

From: Brian F. Feldman (green_at_freebsd.org)
Date: 04/16/04

  • Next message: Bruce Evans: "Re: bin/41071: make NO to NO_ transition patch" 
    To: Robert Watson <rwatson@freebsd.org>, Seigo Tanimura <tanimura@tanimura.dyndns.org>, Pawel Jakub Dawidek <pjd@freebsd.org>, freebsd-arch@freebsd.org
Date: Thu, 15 Apr 2004 20:38:58 -0400

    John-Mark Gurney <gurney_j@efn.org> wrote:
    > Brian F. Feldman wrote this message on Thu, Apr 15, 2004 at 16:36 -0400:
    > > Nothing is currently designed to work with anything even remotely not
    > > looking like spl(), so we have to either flatten it out (using workqueues)
    > > or change semantics so that when KNOTE() is called it acts like the closure
    > > that we pretend it is. Of course, the easy way to do this is with a worker
    > > queue/condvar/mutex/thread. What other ways do we have available to turn
    > > KNOTE() into a closure, bearing in mind that the entire point of the
    > > mechanism is that there is no memory allocation at the time of event
    > > generation -- only when events are defined (by the user or recursively by
    > > other events).
    >
    > The proc case should be treated special as it is an [ab]use of the kevent
    > system with following children... I'm looking at it more..

    It's stupid, hopeless, and an abomination. I'm removing it. The only way
    it can possibly work to report failure to the "parent" knote at the same
    time is if KNOTE() and kqueue_register() have the exact same locks -- not
    gonna happen if there are ANY non-global kqueue locks at all! I'm going to
    replace it with what it SHOULD have been: implemented, at the end of
    kevent() after kqueue_scan() as a shortcut that doesn't leave the kernel but
    does not do any of the horrible, evil, SINGLE special case that makes me
    have to try to do this to accomodate it:

                    if (kn->kn_fop->f_event(kn, hint) ||
                        kn->kn_kq->kq_state & KQ_UPCALL) {
                            int enqueued = 0, upcall;

                            upcall = kn->kn_kq->kq_state & KQ_UPCALL;
                            KNOTE_ACTIVATE(kn, enqueued);
                            if (upcall || enqueued) {
                                    struct kevent upkev;
                                    long uperrnote;

                                    if (upcall) {
                                            upkev = kn->kn_kq->kq_upcall;
                                            uperrnote = kn->kn_kq->kq_uperror;
                                            kn->kn_kq->kq_state &= ~KQ_UPCALL;
                                    }
                                    mtx_unlock(&knote_mtx);
                                    mtx_unlock(&kn->kn_kq->kq_mtx);
                                    mtx_unlock(&klist_mtx);
                                    if (upcall) {
                                            if (kqueue_register(kn->kn_kq,
                                                &upkev, curthread) != 0) {
                                            }
                                    } 

    -- 
Brian Fundakowski Feldman                           \'[ FreeBSD ]''''''''''\
  <> green@FreeBSD.org                               \  The Power to Serve! \
 Opinions expressed are my own.                       \,,,,,,,,,,,,,,,,,,,,,,\
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