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Date:      Tue, 20 Nov 2018 01:45:00 -0500
From:      Mark Johnston <markj@freebsd.org>
To:        Sylvain GALLIANO <sg@efficientip.com>
Cc:        freebsd-current@freebsd.org
Subject:   Re: Panic on kern_event.c
Message-ID:  <20181120064500.GA2371@raichu>
In-Reply-To: <CAHdyrksNptg1cMorN9Y5xMdQb2UoZPBLOQRZ7huwMS=Afjt5%2BQ@mail.gmail.com>
References:  <CAHdyrkvqGp8PGFaCSGgeDFC7wBhjnHK4eL99WM5fMO_yZ_u5KA@mail.gmail.com> <20181107043503.GB30861@raichu> <CAHdyrkt42cn8%2BKqhp-jQ9iZNnreypMT1qybNTcFtx8JivKggZA@mail.gmail.com> <20181115221019.GA2514@raichu> <CAHdyrksHLvzXDkjoy2PpiTgb%2BmEKHJ979rwcW3RJx32qdAyJzg@mail.gmail.com> <20181116154210.GB17379@raichu> <CAHdyrku4OPRr1Ku0WF3XT3vK_dqvNzWN%2BMYz7pXTkiNJakfJGQ@mail.gmail.com> <20181118003554.GC2799@raichu> <CAHdyrksNptg1cMorN9Y5xMdQb2UoZPBLOQRZ7huwMS=Afjt5%2BQ@mail.gmail.com>
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On Mon, Nov 19, 2018 at 10:26:51AM +0100, Sylvain GALLIANO wrote:
> With this latest patch, after stressing syslog-ng few minutes, it do not
> log anymore and a simple kill do not work (I have to do kill -9)

Thanks for your patience.  I finally managed to reproduce the problem
and can see the bug now.  Please try this patch instead.

diff --git a/sys/kern/kern_event.c b/sys/kern/kern_event.c
index d9c670e29d60..0be765a040ed 100644
--- a/sys/kern/kern_event.c
+++ b/sys/kern/kern_event.c
@@ -1538,6 +1538,10 @@ kqueue_register(struct kqueue *kq, struct kevent *kev, struct thread *td, int wa
 			kn_enter_flux(kn);
 
 			error = knote_attach(kn, kq);
+			if ((kev->flags & EV_ENABLE) != 0)
+				kn->kn_status &= ~KN_DISABLED;
+			else if ((kev->flags & EV_DISABLE) != 0)
+				kn->kn_status |= KN_DISABLED;
 			KQ_UNLOCK(kq);
 			if (error != 0) {
 				tkn = kn;
@@ -1570,6 +1574,11 @@ kqueue_register(struct kqueue *kq, struct kevent *kev, struct thread *td, int wa
 		KNOTE_ACTIVATE(kn, 1);
 	}
 
+	if ((kev->flags & EV_ENABLE) != 0)
+		kn->kn_status &= ~KN_DISABLED;
+	else if ((kev->flags & EV_DISABLE) != 0)
+		kn->kn_status |= KN_DISABLED;
+
 	/*
 	 * The user may change some filter values after the initial EV_ADD,
 	 * but doing so will not reset any filter which has already been
@@ -1595,11 +1604,9 @@ kqueue_register(struct kqueue *kq, struct kevent *kev, struct thread *td, int wa
 	 * kn_knlist.
 	 */
 done_ev_add:
-	if ((kev->flags & EV_ENABLE) != 0)
-		kn->kn_status &= ~KN_DISABLED;
-	else if ((kev->flags & EV_DISABLE) != 0)
-		kn->kn_status |= KN_DISABLED;
-
+	/*
+	 * KN_DISABLED will be stable while the knote is in flux.
+	 */
 	if ((kn->kn_status & KN_DISABLED) == 0)
 		event = kn->kn_fop->f_event(kn, 0);
 	else
@@ -1861,6 +1868,8 @@ kqueue_scan(struct kqueue *kq, int maxevents, struct kevent_copyops *k_ops,
 		}
 
 		TAILQ_REMOVE(&kq->kq_head, kn, kn_tqe);
+		KASSERT(kn == marker || (kn->kn_status & KN_QUEUED) != 0,
+		    ("knote %p not queued", kn));
 		if ((kn->kn_status & KN_DISABLED) == KN_DISABLED) {
 			kn->kn_status &= ~KN_QUEUED;
 			kq->kq_count--;

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