kqueue oddity

[Dan Nelson]

In the last episode (Apr 01), Vclav Haisman said:
> Hi,
> I have a little problem with kqueue/kevent. I am getting somewhat
> strange events from kevent(). The struct kevent's contents looks like this:
> 
> kevent {.ident =  1, .filter = 0xfffffffe, .flags = 0x0001, 
>   .fflags = 0x00000000, .data = 0x000084c6}
> 
> .ident == 1 is stdout.
> .filter == 0x fffffffe is -2 is EVFILT_WRITE which makes sense for stdout.
> .fflags is empty, nothing out of ordinary for EVFILT_WRITE.
> And here comes the oddity: .flags == 0x0001 means EV_ADD which totally
> doesn't make sense to me and neither does according to man kqueue. The
> code that exhibits this behaviour is io_loop_handler_run() in
> ioloop-kqueue.c of Dovecot IMAP server.

It's a kqueue bug, but a minor one.  The problem is that the same
"flags" field is used to pass "actions" from the client, and return
status from the kernel.  When you call kqueue with EV_ADD, the kernel
never clears the flags when creating the internal object, so it's
always visible when events fire.  This can get annoying when trussing
kqueue-using programs because very single event has the EV_ADD flag :)
Apply the following diff to have the kernel strip EV_ADD:
 
Index: kern_event.c
===================================================================
RCS file: /home/ncvs/src/sys/kern/kern_event.c,v
retrieving revision 1.79.2.9
diff -u -p -r1.79.2.9 kern_event.c
--- kern_event.c	29 Jun 2005 18:08:31 -0000	1.79.2.9
+++ kern_event.c	7 Jul 2005 18:51:29 -0000
@@ -864,6 +864,7 @@ findkn:
 			kev->fflags = 0;
 			kev->data = 0;
 			kn->kn_kevent = *kev;
+			kn->kn_kevent.flags &= ~EV_ADD; 
 			kn->kn_status = KN_INFLUX|KN_DETACHED;
 
 			error = knote_attach(kn, kq);


-- 
	Dan Nelson
	dnelson at allantgroup.com