From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-1.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 34B09C43441 for ; Mon, 12 Nov 2018 22:56:13 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id DBCFA223DD for ; Mon, 12 Nov 2018 22:56:12 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org DBCFA223DD Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727330AbeKMIv0 (ORCPT ); Tue, 13 Nov 2018 03:51:26 -0500 Received: from mx1.redhat.com ([209.132.183.28]:38792 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725973AbeKMIv0 (ORCPT ); Tue, 13 Nov 2018 03:51:26 -0500 Received: from smtp.corp.redhat.com (int-mx04.intmail.prod.int.phx2.redhat.com [10.5.11.14]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 9A17981F0C; Mon, 12 Nov 2018 22:56:09 +0000 (UTC) Received: from llong.remote.csb (ovpn-117-67.phx2.redhat.com [10.3.117.67]) by smtp.corp.redhat.com (Postfix) with ESMTP id 0A6565D968; Mon, 12 Nov 2018 22:56:07 +0000 (UTC) Subject: Re: [RFC PATCH 00/12] locking/lockdep: Add a new class of terminal locks To: Josh Poimboeuf , Ingo Molnar Cc: Peter Zijlstra , Ingo Molnar , Will Deacon , Thomas Gleixner , linux-kernel@vger.kernel.org, kasan-dev@googlegroups.com, linux-mm@kvack.org, Petr Mladek , Sergey Senozhatsky , Andrey Ryabinin , Tejun Heo , Andrew Morton , Linus Torvalds References: <1541709268-3766-1-git-send-email-longman@redhat.com> <20181109080412.GC86700@gmail.com> <20181110141045.GD3339@worktop.programming.kicks-ass.net> <20181112051033.GA123204@gmail.com> <20181112055324.f7div2ahx5emkbbe@treble> <20181112063050.GB61749@gmail.com> <20181112222250.h37hkrj6warqewkd@treble> From: Waiman Long Openpgp: preference=signencrypt Autocrypt: addr=longman@redhat.com; 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Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.9.1 MIME-Version: 1.0 In-Reply-To: <20181112222250.h37hkrj6warqewkd@treble> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Content-Language: en-US X-Scanned-By: MIMEDefang 2.79 on 10.5.11.14 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.27]); Mon, 12 Nov 2018 22:56:10 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 11/12/2018 02:22 PM, Josh Poimboeuf wrote: > On Mon, Nov 12, 2018 at 07:30:50AM +0100, Ingo Molnar wrote: >> * Josh Poimboeuf wrote: >> >>> On Mon, Nov 12, 2018 at 06:10:33AM +0100, Ingo Molnar wrote: >>>> * Waiman Long wrote: >>>> >>>>> On 11/10/2018 09:10 AM, Peter Zijlstra wrote: >>>>>> On Fri, Nov 09, 2018 at 09:04:12AM +0100, Ingo Molnar wrote: >>>>>>> BTW., if you are interested in more radical approaches to optimize >>>>>>> lockdep, we could also add a static checker via objtool driven call graph >>>>>>> analysis, and mark those locks terminal that we can prove are terminal. >>>>>>> >>>>>>> This would require the unified call graph of the kernel image and of all >>>>>>> modules to be examined in a final pass, but that's within the principal >>>>>>> scope of objtool. (This 'final pass' could also be done during bootup, at >>>>>>> least in initial versions.) >>>>>> Something like this is needed for objtool LTO support as well. I just >>>>>> dread the build time 'regressions' this will introduce :/ >>>>>> >>>>>> The final link pass is already by far the most expensive part (as >>>>>> measured in wall-time) of building a kernel, adding more work there >>>>>> would really suck :/ >>>>> I think the idea is to make objtool have the capability to do that. It >>>>> doesn't mean we need to turn it on by default in every build. >>>> Yeah. >>>> >>>> Also note that much of the objtool legwork would be on a per file basis >>>> which is reasonably parallelized already. On x86 it's also already done >>>> for every ORC build i.e. every distro build and the incremental overhead >>>> from also extracting locking dependencies should be reasonably small. >>>> >>>> The final search of the global graph would be serialized but still >>>> reasonably fast as these are all 'class' level dependencies which are >>>> much less numerous than runtime dependencies. >>>> >>>> I.e. I think we are talking about tens of thousands of dependencies, not >>>> tens of millions. >>>> >>>> At least in theory. ;-) >>> Generating a unified call graph sounds very expensive (and very far >>> beyond what objtool can do today). >> Well, objtool already goes through the instruction stream and recognizes >> function calls - so it can in effect generate a stream of "function x >> called by function y" data, correct? > Yeah, though it would be quite simple to get the same data with a simple > awk script at link time. > >>> Also, what about function pointers? >> So maybe it's possible to enumerate all potential values for function >> pointers with a reasonably simple compiler plugin and work from there? > I think this would be somewhere between very difficult and impossible to > do properly. I can't even imagine how this would be implemented in a > compiler plugin. But I'd love to be proven wrong on that. I would say we have to assume for the worst when a function pointer is being called while holding a lock unless we are able to find out all its possible targets. Cheers, Longman