Move kernel locking inside knote_fdclose() from finishdup() and

fdrelease(). This makes the upper layer of file descriptor closing
free of KERNEL_LOCK() when the process does not use kqueue.

The kernel locking around fdremove() and knote_fdclose() is no longer
needed because kqueue_register() checks if there has been a race with
file descriptor close. Moreover, the locking became ineffective against
these races when filterops callbacks were allowed to sleep.

OK anton@, mpi@

2 files changed, 8 insertions, 15 deletions

diff --git a/sys/kern/kern_descrip.c b/sys/kern/kern_descrip.c
index 9a35e7c1e13..3a55ab9bfd1 100644
--- a/sys/kern/kern_descrip.c
+++ b/sys/kern/kern_descrip.c
@@ -1,4 +1,4 @@
-/*	$OpenBSD: kern_descrip.c,v 1.197 2020/02/01 08:57:27 anton Exp $	*/
+/*	$OpenBSD: kern_descrip.c,v 1.198 2020/02/05 17:03:13 visa Exp $	*/
 /*	$NetBSD: kern_descrip.c,v 1.42 1996/03/30 22:24:38 christos Exp $	*/
 
 /*
@@ -667,9 +667,6 @@ finishdup(struct proc *p, struct file *fp, int old, int new,
 		fd_used(fdp, new);
  	}
 
-	/* Prevent race with kevent. */
-	KERNEL_LOCK();
-
 	/*
 	 * Use `fd_fplock' to synchronize with fd_getfile() so that
 	 * the function no longer creates a new reference to the old file.
@@ -686,8 +683,6 @@ finishdup(struct proc *p, struct file *fp, int old, int new,
 		closef(oldfp, p);
 	}
 
-	KERNEL_UNLOCK();
-
 	return (0);
 }
 
@@ -740,7 +735,6 @@ fdrelease(struct proc *p, int fd)
 {
 	struct filedesc *fdp = p->p_fd;
 	struct file *fp;
-	int error;
 
 	fdpassertlocked(fdp);
 
@@ -749,14 +743,10 @@ fdrelease(struct proc *p, int fd)
 		fdpunlock(fdp);
 		return (EBADF);
 	}
-	/* Prevent race with kevent. */
-	KERNEL_LOCK();
 	fdremove(fdp, fd);
 	knote_fdclose(p, fd);
 	fdpunlock(fdp);
-	error = closef(fp, p);
-	KERNEL_UNLOCK();
-	return (error);
+	return (closef(fp, p));
 }
 
 /*
diff --git a/sys/kern/kern_event.c b/sys/kern/kern_event.c
index b84a674e38b..0e3e48452c8 100644
--- a/sys/kern/kern_event.c
+++ b/sys/kern/kern_event.c
@@ -1,4 +1,4 @@
-/*	$OpenBSD: kern_event.c,v 1.121 2020/01/18 08:59:48 visa Exp $	*/
+/*	$OpenBSD: kern_event.c,v 1.122 2020/02/05 17:03:13 visa Exp $	*/
 
 /*-
  * Copyright (c) 1999,2000,2001 Jonathan Lemon <jlemon@FreeBSD.org>
@@ -1217,8 +1217,6 @@ knote_fdclose(struct proc *p, int fd)
 	struct kqueue *kq;
 	struct klist *list;
 
-	KERNEL_ASSERT_LOCKED();
-
 	/*
 	 * fdplock can be ignored if the file descriptor table is being freed
 	 * because no other thread can access the fdp.
@@ -1226,6 +1224,10 @@ knote_fdclose(struct proc *p, int fd)
 	if (fdp->fd_refcnt != 0)
 		fdpassertlocked(fdp);
 
+	if (LIST_EMPTY(&fdp->fd_kqlist))
+		return;
+
+	KERNEL_LOCK();
 	LIST_FOREACH(kq, &fdp->fd_kqlist, kq_next) {
 		if (fd >= kq->kq_knlistsize)
 			continue;
@@ -1233,6 +1235,7 @@ knote_fdclose(struct proc *p, int fd)
 		list = &kq->kq_knlist[fd];
 		knote_remove(p, list);
 	}
+	KERNEL_UNLOCK();
 }
 
 /*