From owner-freebsd-dtrace@FreeBSD.ORG Sat Jul 27 07:12:45 2013 Return-Path: Delivered-To: freebsd-dtrace@FreeBSD.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2001:1900:2254:206a::19:1]) (using TLSv1 with cipher ADH-AES256-SHA (256/256 bits)) (No client certificate requested) by hub.freebsd.org (Postfix) with ESMTP id E3E5CA2B; Sat, 27 Jul 2013 07:12:45 +0000 (UTC) (envelope-from avg@FreeBSD.org) Received: from citadel.icyb.net.ua (citadel.icyb.net.ua [212.40.38.140]) by mx1.freebsd.org (Postfix) with ESMTP id 015E32A71; Sat, 27 Jul 2013 07:12:44 +0000 (UTC) Received: from porto.starpoint.kiev.ua (porto-e.starpoint.kiev.ua [212.40.38.100]) by citadel.icyb.net.ua (8.8.8p3/ICyb-2.3exp) with ESMTP id KAA05365; Sat, 27 Jul 2013 10:12:42 +0300 (EEST) (envelope-from avg@FreeBSD.org) Received: from localhost ([127.0.0.1]) by porto.starpoint.kiev.ua with esmtp (Exim 4.34 (FreeBSD)) id 1V2ygD-000NBb-P6; Sat, 27 Jul 2013 10:12:41 +0300 Message-ID: <51F372C8.1000107@FreeBSD.org> Date: Sat, 27 Jul 2013 10:12:08 +0300 From: Andriy Gapon User-Agent: Mozilla/5.0 (X11; FreeBSD amd64; rv:17.0) Gecko/20130708 Thunderbird/17.0.7 MIME-Version: 1.0 To: Mark Johnston Subject: Re: [RFC] reworking FreeBSD's SDT implementation References: <20130703041023.GA82673@raichu> <20130711024500.GA67976@raichu> <20130711210215.GB7506@gmail.com> <20130713234200.GA40803@raichu> <20130714075634.GC2832@gmail.com> <20130722022811.GA14288@raichu> <51F14150.7000509@FreeBSD.org> <20130727022656.GB67227@raichu> In-Reply-To: <20130727022656.GB67227@raichu> X-Enigmail-Version: 1.5.1 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Cc: freebsd-dtrace@FreeBSD.org X-BeenThere: freebsd-dtrace@freebsd.org X-Mailman-Version: 2.1.14 Precedence: list List-Id: "A discussion list for developers working on DTrace in FreeBSD." List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Sat, 27 Jul 2013 07:12:46 -0000 on 27/07/2013 05:26 Mark Johnston said the following: > On Thu, Jul 25, 2013 at 06:16:32PM +0300, Andriy Gapon wrote: >> on 22/07/2013 05:28 markj@freebsd.org said the following: >>> http://people.freebsd.org/~markj/patches/sdt-module-info/20130721-sdt-module-info.diff >> >> Mark, >> >> this is a minor suggestion only partially related to your patch. >> I think that it would be nice if module loading and unloading events were posted >> via EVENTHANDLER(9) mechanism. Then instead of introducing yet more DTrace >> related hooks in the kernel code, DTrace modules could just subscribe to those >> events. Also, those events could be potentially useful to other consumers >> beyond DTrace. >> What do you think? > > Hm, now that I look at this, I'm not sure if it can work. The unload > hooks need to be able to veto a module unload in the case that one of > its probes is enabled. This is done by checking whether lf->nenabled > 0, > and it needs to be done with the dtrace lock held to prevent races. > > I've done this by having the unload hooks return a non-zero value if > there are probes enabled, but EVENTHANDLER(9) doesn't give me a way to > look at a handler's return value. Do you see a way to get around this? Hmm, I didn't think about this problem in advance... Having looked around I think that it should be possible to handle this situation in a way similar to watchdog_list. watchdog(9) documents how that works. Of course, all handlers will have to be careful to not override error if it's already set. -- Andriy Gapon