Introduce klistops

This patch extends struct klist with a callback descriptor and
an argument. The main purpose of this is to let the kqueue subsystem
assert when a klist should be locked, and operate the klist lock
in klist_invalidate().

Access to a knote list of a kqueue-monitored object has to be
serialized somehow. Because the object often has a lock for protecting
its state, and because the object often acquires this lock at the latest
in its f_event callback function, it makes sense to use this lock also
for the knote lists. The existing uses of NOTE_SUBMIT already show
a pattern that is likely to become more prevalent.

There could be an embedded lock in klist. However, such a lock would be
redundant in many cases. The code cannot rely on a single lock type
(mutex, rwlock, something else) because the needs of monitored objects
vary. In addition, an embedded lock would introduce new lock order
constraints. Note that the patch does not rule out use of dedicated
klist locks.

The patch introduces a way to associate lock operations with a klist.
The caller can provide a custom implementation, or use a ready-made
interface with a mutex or rwlock.

For compatibility with old code, the new code falls back to using the
kernel lock if no specific klist initialization has been done. The
existing code already relies on implicit initialization of klist.

Sadly, this change increases the size of struct klist. dlg@ thinks this
is not fatal, though.

OK mpi@

1 files changed, 17 insertions, 1 deletions

diff --git a/sys/sys/event.h b/sys/sys/event.h
index 916ddac75a6..c8160f4cc55 100644
--- a/sys/sys/event.h
+++ b/sys/sys/event.h
@@ -1,4 +1,4 @@
-/*	$OpenBSD: event.h,v 1.50 2020/12/18 16:10:58 visa Exp $	*/
+/*	$OpenBSD: event.h,v 1.51 2020/12/20 12:54:05 visa Exp $	*/
 
 /*-
  * Copyright (c) 1999,2000,2001 Jonathan Lemon <jlemon@FreeBSD.org>
@@ -127,11 +127,15 @@ struct kevent {
  * programs which pull in <sys/proc.h> or <sys/selinfo.h>.
  */
 #include <sys/queue.h>
+
+struct klistops;
 struct knote;
 SLIST_HEAD(knlist, knote);
 
 struct klist {
 	struct knlist		 kl_list;
+	const struct klistops	*kl_ops;
+	void			*kl_arg;
 };
 
 #ifdef _KERNEL
@@ -200,6 +204,12 @@ struct knote {
 #define kn_fp		kn_ptr.p_fp
 };
 
+struct klistops {
+	void	(*klo_assertlk)(void *);
+	int	(*klo_lock)(void *);
+	void	(*klo_unlock)(void *, int);
+};
+
 struct kqueue_scan_state {
 	struct kqueue	*kqs_kq;		/* kqueue of this scan */
 	struct knote	 kqs_start;		/* start marker */
@@ -209,7 +219,9 @@ struct kqueue_scan_state {
 						 * in queue */
 };
 
+struct mutex;
 struct proc;
+struct rwlock;
 struct timespec;
 
 extern const struct filterops sig_filtops;
@@ -229,6 +241,10 @@ extern void	kqueue_scan_finish(struct kqueue_scan_state *);
 extern void	kqueue_purge(struct proc *, struct kqueue *);
 extern int	filt_seltrue(struct knote *kn, long hint);
 extern int	seltrue_kqfilter(dev_t, struct knote *);
+extern void	klist_init(struct klist *, const struct klistops *, void *);
+extern void	klist_init_mutex(struct klist *, struct mutex *);
+extern void	klist_init_rwlock(struct klist *, struct rwlock *);
+extern void	klist_free(struct klist *);
 extern void	klist_insert(struct klist *, struct knote *);
 extern void	klist_remove(struct klist *, struct knote *);
 extern int	klist_empty(struct klist *);