Make klist_invalidate() more careful and general. Acquire knotes before

changing them, to synchronize with kqueue_register() and kqueue_scan().
Detach the knotes from the original knote list, change the filterops
to one that always indicates EOF condition, and activate in one-shot
mode.

The detaching allows the original knote list head to be deleted after
klist_invalidate() returns. The knotes are activated to make the EOF
condition visible to the event subscribers as soon as possible. As the
knotes are detached from the list, klist_invalidate() does not have to
wait for userspace to process them.

The use of the special filterops minimizes the need to handle klist
invalidation in actual implementations of filterops.

Tested by Greg Steuck
OK mpi@

1 files changed, 35 insertions, 4 deletions

diff --git a/sys/kern/kern_event.c b/sys/kern/kern_event.c
index 828b2ffa06c..b84a674e38b 100644
--- a/sys/kern/kern_event.c
+++ b/sys/kern/kern_event.c
@@ -1,4 +1,4 @@
-/*	$OpenBSD: kern_event.c,v 1.120 2020/01/13 13:57:19 visa Exp $	*/
+/*	$OpenBSD: kern_event.c,v 1.121 2020/01/18 08:59:48 visa Exp $	*/
 
 /*-
  * Copyright (c) 1999,2000,2001 Jonathan Lemon <jlemon@FreeBSD.org>
@@ -465,6 +465,27 @@ seltrue_kqfilter(dev_t dev, struct knote *kn)
 	return (0);
 }
 
+static int
+filt_dead(struct knote *kn, long hint)
+{
+	kn->kn_flags |= (EV_EOF | EV_ONESHOT);
+	kn->kn_data = 0;
+	return (1);
+}
+
+static void
+filt_deaddetach(struct knote *kn)
+{
+	/* Nothing to do */
+}
+
+static const struct filterops dead_filtops = {
+	.f_isfd		= 1,
+	.f_attach	= NULL,
+	.f_detach	= filt_deaddetach,
+	.f_event	= filt_dead,
+};
+
 int
 sys_kqueue(struct proc *p, void *v, register_t *retval)
 {
@@ -1315,8 +1336,18 @@ klist_invalidate(struct klist *list)
 {
 	struct knote *kn;
 
-	SLIST_FOREACH(kn, list, kn_selnext) {
-		kn->kn_status |= KN_DETACHED;
-		kn->kn_flags |= EV_EOF | EV_ONESHOT;
+	/*
+	 * NET_LOCK() must not be held because it can block another thread
+	 * in f_event with a knote acquired.
+	 */
+	NET_ASSERT_UNLOCKED();
+
+	while ((kn = SLIST_FIRST(list)) != NULL) {
+		if (knote_acquire(kn) == 0)
+			continue;
+		kn->kn_fop->f_detach(kn);
+		kn->kn_fop = &dead_filtops;
+		knote_activate(kn);
+		knote_release(kn);
 	}
 }