Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932267AbWIZUK2 (ORCPT ); Tue, 26 Sep 2006 16:10:28 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S932268AbWIZUK2 (ORCPT ); Tue, 26 Sep 2006 16:10:28 -0400 Received: from tomts25.bellnexxia.net ([209.226.175.188]:24198 "EHLO tomts25-srv.bellnexxia.net") by vger.kernel.org with ESMTP id S932267AbWIZUK1 (ORCPT ); Tue, 26 Sep 2006 16:10:27 -0400 Date: Tue, 26 Sep 2006 16:05:14 -0400 From: Mathieu Desnoyers To: "Frank Ch. Eigler" Cc: Jeremy Fitzhardinge , Martin Bligh , Masami Hiramatsu , prasanna@in.ibm.com, Andrew Morton , Ingo Molnar , Paul Mundt , linux-kernel , Jes Sorensen , Tom Zanussi , Richard J Moore , Michel Dagenais , Christoph Hellwig , Greg Kroah-Hartman , Thomas Gleixner , William Cohen , ltt-dev@shafik.org, systemtap@sources.redhat.com, Alan Cox , Karim Yaghmour , Pavel Machek , Joe Perches , "Randy.Dunlap" , "Jose R. Santos" Subject: Re: [PATCH] Linux Kernel Markers 0.13 for 2.6.17 Message-ID: <20060926200514.GB12532@Krystal> References: <45187146.8040302@goop.org> <20060926002551.GA18276@Krystal> <20060926004535.GA2978@Krystal> <45187C0E.1080601@goop.org> <20060926025924.GA27366@Krystal> <4518B4A0.6070509@goop.org> <20060926180414.GA10497@Krystal> <4519781D.9040503@goop.org> <20060926190849.GA2280@Krystal> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Content-Disposition: inline In-Reply-To: X-Editor: vi X-Info: http://krystal.dyndns.org:8080 X-Operating-System: Linux/2.4.32-grsec (i686) X-Uptime: 15:57:49 up 34 days, 17:06, 4 users, load average: 0.10, 0.21, 0.27 User-Agent: Mutt/1.5.13 (2006-08-11) Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2230 Lines: 61 * Frank Ch. Eigler (fche@redhat.com) wrote: > Mathieu Desnoyers writes: > > > [...] > > > Yep, that looks reasonable. Though you could just directly test a > > > per-marker enable flag, rather than using "condition"... > > [...] > > I am not sure I understand your suggestion correctly.. do you mean having > > a per-marker flag that would be loaded and tested at every marker site ? > > I gather that one reason for working so hard with the inline assembly > is a race condition problem with the plain STAP_MARK style of marker > disconnection: > > if (pointer) (*pointer)(args ...); > > Granted, but this problem could almost certainly be dealt with simpler > than that. How about a compxchg or other atomic-fetch of the static > pointer with a local variable? That should solve the worry of an > (*NULL) call. > I don't really see how cmpxchg might be needed here. Atomic fetch of a static variable is how I will do it in my next version for the non optimized case : volatile static var = 0; if(var) { preempt disable call preempt_enable_no_resched } But, still, in the optimized case, the if(var) will depend on an immediate value, therefore saving the memory read. > If we then become concerned with a valid pointer become obsolete (the > probe handler function wanting to unload), we might be able to use > some RCU-type deferral mechanism and/or preempt controls to ensure > that this does not happen. > This is exactly why the preemption is disabled around the call. However, RCU must always _see_ a coherent version of the structure in memory. Calling an empty function, disabling preemption around the call and calling synchronize_sched() before deleting the removed function looks very much like a RCU-style protection (actually, that's what it is). Mathieu OpenPGP public key: http://krystal.dyndns.org:8080/key/compudj.gpg Key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68 - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/