Date: Sat, 1 Apr 2006 12:03:11 -0600 From: Dan Nelson <dnelson@allantgroup.com> To: =?utf-8?Q?V=C3=A1clav?= Haisman <V.Haisman@sh.cvut.cz> Cc: hackers@freebsd.org Subject: Re: kqueue oddity Message-ID: <20060401180311.GA83967@dan.emsphone.com> In-Reply-To: <442E9354.5010700@sh.cvut.cz> References: <442E9354.5010700@sh.cvut.cz>
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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@allantgroup.com
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