From owner-freebsd-hackers@FreeBSD.ORG Sat Apr 1 18:03:12 2006 Return-Path: X-Original-To: hackers@freebsd.org Delivered-To: freebsd-hackers@FreeBSD.ORG Received: from mx1.FreeBSD.org (mx1.freebsd.org [216.136.204.125]) by hub.freebsd.org (Postfix) with ESMTP id D096E16A424 for ; Sat, 1 Apr 2006 18:03:12 +0000 (UTC) (envelope-from dan@dan.emsphone.com) Received: from dan.emsphone.com (dan.emsphone.com [199.67.51.101]) by mx1.FreeBSD.org (Postfix) with ESMTP id 48D9C43D5D for ; Sat, 1 Apr 2006 18:03:12 +0000 (GMT) (envelope-from dan@dan.emsphone.com) Received: (from dan@localhost) by dan.emsphone.com (8.13.1/8.13.4) id k31I3Bp2041675; Sat, 1 Apr 2006 12:03:11 -0600 (CST) (envelope-from dan) Date: Sat, 1 Apr 2006 12:03:11 -0600 From: Dan Nelson To: =?utf-8?Q?V=C3=A1clav?= Haisman Message-ID: <20060401180311.GA83967@dan.emsphone.com> References: <442E9354.5010700@sh.cvut.cz> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <442E9354.5010700@sh.cvut.cz> X-OS: FreeBSD 5.5-PRERELEASE X-message-flag: Outlook Error User-Agent: Mutt/1.5.11 Cc: hackers@freebsd.org Subject: Re: kqueue oddity X-BeenThere: freebsd-hackers@freebsd.org X-Mailman-Version: 2.1.5 Precedence: list List-Id: Technical Discussions relating to FreeBSD List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Sat, 01 Apr 2006 18:03:12 -0000 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