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[46.139.12.213]) by smtp.gmail.com with ESMTPSA id w18-v6sm37639365wrn.66.2018.11.11.22.30.52 (version=TLS1_2 cipher=ECDHE-RSA-CHACHA20-POLY1305 bits=256/256); Sun, 11 Nov 2018 22:30:52 -0800 (PST) Date: Mon, 12 Nov 2018 07:30:50 +0100 From: Ingo Molnar To: Josh Poimboeuf Cc: Waiman Long , 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 Subject: Re: [RFC PATCH 00/12] locking/lockdep: Add a new class of terminal locks Message-ID: <20181112063050.GB61749@gmail.com> 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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20181112055324.f7div2ahx5emkbbe@treble> User-Agent: Mutt/1.9.4 (2018-02-28) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org * 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? > 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? One complication would be function pointers encoded as opaque data types... > BTW there's another kernel static analysis tool which attempts to > create such a call graph already: smatch. It's not included in the kernel tree though and I'd expect tight coupling (or at least lock-step improvements) between tooling and lockdep here. Thanks, Ingo