kqueue: add some kn_knlist assertions around knlist_(add|remove)

We currently assert that kn_status is accurate, but there's more room
for error.  Neither of these are very likely, but currently we'd blow up
in SLIST*() macros instead of providing more obvious diagnostics.  It's
perhaps only worth testing these because knlist_remove() requires
getting logic across both f_attach() and f_detach() correct.

Reviewed by:	kib, markj
Differential Revision:	https://reviews.freebsd.org/D56211

1 files changed, 4 insertions, 0 deletions

diff --git a/sys/kern/kern_event.c b/sys/kern/kern_event.c
index 485123989319..deb23ae96f9a 100644
--- a/sys/kern/kern_event.c
+++ b/sys/kern/kern_event.c
@@ -2617,6 +2617,8 @@ knlist_add(struct knlist *knl, struct knote *kn, int islocked)
 	KASSERT(kn_in_flux(kn), ("knote %p not in flux", kn));
 	KASSERT((kn->kn_status & KN_DETACHED) != 0,
 	    ("knote %p was not detached", kn));
+	KASSERT(kn->kn_knlist == NULL,
+	    ("knote %p was already on knlist %p", kn, kn->kn_knlist));
 	if (!islocked)
 		knl->kl_lock(knl->kl_lockarg);
 	SLIST_INSERT_HEAD(&knl->kl_list, kn, kn_selnext);
@@ -2639,6 +2641,8 @@ knlist_remove_kq(struct knlist *knl, struct knote *kn, int knlislocked,
 	KASSERT(kqislocked || kn_in_flux(kn), ("knote %p not in flux", kn));
 	KASSERT((kn->kn_status & KN_DETACHED) == 0,
 	    ("knote %p was already detached", kn));
+	KASSERT(kn->kn_knlist == knl,
+	    ("knote %p was not on knlist %p", kn, knl));
 	if (!knlislocked)
 		knl->kl_lock(knl->kl_lockarg);
 	SLIST_REMOVE(&knl->kl_list, kn, knote, kn_selnext);