Add kqueue_scan_state struct

The struct keeps track of the end point of an event queue scan by
persisting the end marker. This will be needed when kqueue_scan() is
called repeatedly to complete a scan in a piecewise fashion. The end
marker has to be preserved between calls because otherwise the scan
might collect an event more than once. If a collected event gets
reactivated during scanning, it will be added at the tail of the queue,
out of reach because of the end marker.

OK mpi@

2 files changed, 84 insertions, 27 deletions

diff --git a/sys/kern/kern_event.c b/sys/kern/kern_event.c
index 3613f30fad0..0bed8e5f671 100644
--- a/sys/kern/kern_event.c
+++ b/sys/kern/kern_event.c
@@ -1,4 +1,4 @@
-/*	$OpenBSD: kern_event.c,v 1.131 2020/04/07 13:27:51 visa Exp $	*/
+/*	$OpenBSD: kern_event.c,v 1.132 2020/05/17 10:53:14 visa Exp $	*/
 
 /*-
  * Copyright (c) 1999,2000,2001 Jonathan Lemon <jlemon@FreeBSD.org>
@@ -62,7 +62,7 @@ void	KQREF(struct kqueue *);
 void	KQRELE(struct kqueue *);
 
 int	kqueue_sleep(struct kqueue *, struct timespec *);
-int	kqueue_scan(struct kqueue *kq, int maxevents,
+int	kqueue_scan(struct kqueue_scan_state *scan, int maxevents,
 		    struct kevent *ulistp, struct timespec *timeout,
 		    struct proc *p, int *retval);
 
@@ -529,6 +529,7 @@ out:
 int
 sys_kevent(struct proc *p, void *v, register_t *retval)
 {
+	struct kqueue_scan_state scan;
 	struct filedesc* fdp = p->p_fd;
 	struct sys_kevent_args /* {
 		syscallarg(int)	fd;
@@ -612,8 +613,10 @@ sys_kevent(struct proc *p, void *v, register_t *retval)
 
 	KQREF(kq);
 	FRELE(fp, p);
-	error = kqueue_scan(kq, SCARG(uap, nevents), SCARG(uap, eventlist),
+	kqueue_scan_setup(&scan, kq);
+	error = kqueue_scan(&scan, SCARG(uap, nevents), SCARG(uap, eventlist),
 	    tsp, p, &n);
+	kqueue_scan_finish(&scan);
 	KQRELE(kq);
 	*retval = n;
 	return (error);
@@ -870,11 +873,12 @@ kqueue_sleep(struct kqueue *kq, struct timespec *tsp)
 }
 
 int
-kqueue_scan(struct kqueue *kq, int maxevents, struct kevent *ulistp,
-    struct timespec *tsp, struct proc *p, int *retval)
+kqueue_scan(struct kqueue_scan_state *scan, int maxevents,
+    struct kevent *ulistp, struct timespec *tsp, struct proc *p, int *retval)
 {
 	struct kevent *kevp;
-	struct knote mend, mstart, *kn;
+	struct knote *kn;
+	struct kqueue *kq = scan->kqs_kq;
 	int s, count, nkev = 0, error = 0;
 	struct kevent kev[KQ_NEVENTS];
 
@@ -882,9 +886,6 @@ kqueue_scan(struct kqueue *kq, int maxevents, struct kevent *ulistp,
 	if (count == 0)
 		goto done;
 
-	memset(&mstart, 0, sizeof(mstart));
-	memset(&mend, 0, sizeof(mend));
-
 retry:
 	if (kq->kq_state & KQ_DYING) {
 		error = EBADF;
@@ -894,7 +895,8 @@ retry:
 	kevp = &kev[0];
 	s = splhigh();
 	if (kq->kq_count == 0) {
-		if (tsp != NULL && !timespecisset(tsp)) {
+		if ((tsp != NULL && !timespecisset(tsp)) ||
+		    scan->kqs_nevent != 0) {
 			splx(s);
 			error = 0;
 			goto done;
@@ -910,27 +912,40 @@ retry:
 		goto done;
 	}
 
-	mstart.kn_filter = EVFILT_MARKER;
-	mstart.kn_status = KN_PROCESSING;
-	TAILQ_INSERT_HEAD(&kq->kq_head, &mstart, kn_tqe);
-	mend.kn_filter = EVFILT_MARKER;
-	mend.kn_status = KN_PROCESSING;
-	TAILQ_INSERT_TAIL(&kq->kq_head, &mend, kn_tqe);
+	/*
+	 * Put the end marker in the queue to limit the scan to the events
+	 * that are currently active.  This prevents events from being
+	 * recollected if they reactivate during scan.
+	 *
+	 * If a partial scan has been performed already but no events have
+	 * been collected, reposition the end marker to make any new events
+	 * reachable.
+	 */
+	if (!scan->kqs_queued) {
+		TAILQ_INSERT_TAIL(&kq->kq_head, &scan->kqs_end, kn_tqe);
+		scan->kqs_queued = 1;
+	} else if (scan->kqs_nevent == 0) {
+		TAILQ_REMOVE(&kq->kq_head, &scan->kqs_end, kn_tqe);
+		TAILQ_INSERT_TAIL(&kq->kq_head, &scan->kqs_end, kn_tqe);
+	}
+
+	TAILQ_INSERT_HEAD(&kq->kq_head, &scan->kqs_start, kn_tqe);
 	while (count) {
-		kn = TAILQ_NEXT(&mstart, kn_tqe);
+		kn = TAILQ_NEXT(&scan->kqs_start, kn_tqe);
 		if (kn->kn_filter == EVFILT_MARKER) {
-			if (kn == &mend) {
-				TAILQ_REMOVE(&kq->kq_head, &mend, kn_tqe);
-				TAILQ_REMOVE(&kq->kq_head, &mstart, kn_tqe);
+			if (kn == &scan->kqs_end) {
+				TAILQ_REMOVE(&kq->kq_head, &scan->kqs_start,
+				    kn_tqe);
 				splx(s);
-				if (count == maxevents)
+				if (scan->kqs_nevent == 0)
 					goto retry;
 				goto done;
 			}
 
 			/* Move start marker past another thread's marker. */
-			TAILQ_REMOVE(&kq->kq_head, &mstart, kn_tqe);
-			TAILQ_INSERT_AFTER(&kq->kq_head, kn, &mstart, kn_tqe);
+			TAILQ_REMOVE(&kq->kq_head, &scan->kqs_start, kn_tqe);
+			TAILQ_INSERT_AFTER(&kq->kq_head, kn, &scan->kqs_start,
+			    kn_tqe);
 			continue;
 		}
 
@@ -958,6 +973,9 @@ retry:
 		*kevp = kn->kn_kevent;
 		kevp++;
 		nkev++;
+		count--;
+		scan->kqs_nevent++;
+
 		if (kn->kn_flags & EV_ONESHOT) {
 			splx(s);
 			kn->kn_fop->f_detach(kn);
@@ -983,7 +1001,6 @@ retry:
 			knote_release(kn);
 		}
 		kqueue_check(kq);
-		count--;
 		if (nkev == KQ_NEVENTS) {
 			splx(s);
 #ifdef KTRACE
@@ -1000,8 +1017,7 @@ retry:
 				break;
 		}
 	}
-	TAILQ_REMOVE(&kq->kq_head, &mend, kn_tqe);
-	TAILQ_REMOVE(&kq->kq_head, &mstart, kn_tqe);
+	TAILQ_REMOVE(&kq->kq_head, &scan->kqs_start, kn_tqe);
 	splx(s);
 done:
 	if (nkev != 0) {
@@ -1016,6 +1032,36 @@ done:
 	return (error);
 }
 
+void
+kqueue_scan_setup(struct kqueue_scan_state *scan, struct kqueue *kq)
+{
+	memset(scan, 0, sizeof(*scan));
+	scan->kqs_kq = kq;
+	scan->kqs_start.kn_filter = EVFILT_MARKER;
+	scan->kqs_start.kn_status = KN_PROCESSING;
+	scan->kqs_end.kn_filter = EVFILT_MARKER;
+	scan->kqs_end.kn_status = KN_PROCESSING;
+}
+
+void
+kqueue_scan_finish(struct kqueue_scan_state *scan)
+{
+	struct kqueue *kq = scan->kqs_kq;
+	int s;
+
+	KASSERT(scan->kqs_start.kn_filter == EVFILT_MARKER);
+	KASSERT(scan->kqs_start.kn_status == KN_PROCESSING);
+	KASSERT(scan->kqs_end.kn_filter == EVFILT_MARKER);
+	KASSERT(scan->kqs_end.kn_status == KN_PROCESSING);
+
+	if (scan->kqs_queued) {
+		scan->kqs_queued = 0;
+		s = splhigh();
+		TAILQ_REMOVE(&kq->kq_head, &scan->kqs_end, kn_tqe);
+		splx(s);
+	}
+}
+
 /*
  * XXX
  * This could be expanded to call kqueue_scan, if desired.
diff --git a/sys/sys/event.h b/sys/sys/event.h
index 3f42b89aaa7..51487835ced 100644
--- a/sys/sys/event.h
+++ b/sys/sys/event.h
@@ -1,4 +1,4 @@
-/*	$OpenBSD: event.h,v 1.36 2020/05/10 00:56:06 guenther Exp $	*/
+/*	$OpenBSD: event.h,v 1.37 2020/05/17 10:53:14 visa Exp $	*/
 
 /*-
  * Copyright (c) 1999,2000,2001 Jonathan Lemon <jlemon@FreeBSD.org>
@@ -189,6 +189,15 @@ struct knote {
 #define kn_fp		kn_ptr.p_fp
 };
 
+struct kqueue_scan_state {
+	struct kqueue	*kqs_kq;		/* kqueue of this scan */
+	struct knote	 kqs_start;		/* start marker */
+	struct knote	 kqs_end;		/* end marker */
+	int		 kqs_nevent;		/* number of events collected */
+	int		 kqs_queued;		/* if set, end marker is
+						 * in queue */
+};
+
 struct proc;
 
 extern const struct filterops sig_filtops;
@@ -200,6 +209,8 @@ extern void	knote_fdclose(struct proc *p, int fd);
 extern void	knote_processexit(struct proc *);
 extern int	kqueue_register(struct kqueue *kq,
 		    struct kevent *kev, struct proc *p);
+extern void	kqueue_scan_setup(struct kqueue_scan_state *, struct kqueue *);
+extern void	kqueue_scan_finish(struct kqueue_scan_state *);
 extern int	filt_seltrue(struct knote *kn, long hint);
 extern int	seltrue_kqfilter(dev_t, struct knote *);
 extern void	klist_insert(struct klist *, struct knote *);