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[2a01:7a0:2:106d:700::1]) by mx.google.com with ESMTPS id l14si13774869wmc.80.2019.04.24.12.51.18 for (version=TLS1_2 cipher=AES128-SHA bits=128/128); Wed, 24 Apr 2019 12:51:18 -0700 (PDT) Received-SPF: pass (google.com: best guess record for domain of tglx@linutronix.de designates 2a01:7a0:2:106d:700::1 as permitted sender) client-ip=2a01:7a0:2:106d:700::1; Authentication-Results: mx.google.com; spf=pass (google.com: best guess record for domain of tglx@linutronix.de designates 2a01:7a0:2:106d:700::1 as permitted sender) smtp.mailfrom=tglx@linutronix.de Received: from p5de0b374.dip0.t-ipconnect.de ([93.224.179.116] helo=nanos) by Galois.linutronix.de with esmtpsa (TLS1.2:DHE_RSA_AES_256_CBC_SHA256:256) (Exim 4.80) (envelope-from ) id 1hJNv9-0005Zr-Gt; Wed, 24 Apr 2019 21:51:07 +0200 Date: Wed, 24 Apr 2019 21:51:05 +0200 (CEST) From: Thomas Gleixner To: Peter Zijlstra cc: LKML , Josh Poimboeuf , x86@kernel.org, Andy Lutomirski , Steven Rostedt , Alexander Potapenko , Alexey Dobriyan , Andrew Morton , Pekka Enberg , linux-mm@kvack.org, David Rientjes , Christoph Lameter , Catalin Marinas , Dmitry Vyukov , Andrey Ryabinin , kasan-dev@googlegroups.com, Mike Rapoport , Akinobu Mita , iommu@lists.linux-foundation.org, Robin Murphy , Christoph Hellwig , Marek Szyprowski , Johannes Thumshirn , David Sterba , Chris Mason , Josef Bacik , linux-btrfs@vger.kernel.org, dm-devel@redhat.com, Mike Snitzer , Alasdair Kergon , intel-gfx@lists.freedesktop.org, Joonas Lahtinen , Maarten Lankhorst , dri-devel@lists.freedesktop.org, David Airlie , Jani Nikula , Daniel Vetter , Rodrigo Vivi , linux-arch@vger.kernel.org Subject: Re: [patch V2 18/29] lockdep: Move stack trace logic into check_prev_add() In-Reply-To: <20190424194505.GR11158@hirez.programming.kicks-ass.net> Message-ID: References: <20190418084119.056416939@linutronix.de> <20190418084254.729689921@linutronix.de> <20190424194505.GR11158@hirez.programming.kicks-ass.net> User-Agent: Alpine 2.21 (DEB 202 2017-01-01) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII X-Linutronix-Spam-Score: -1.0 X-Linutronix-Spam-Level: - X-Linutronix-Spam-Status: No , -1.0 points, 5.0 required, ALL_TRUSTED=-1,SHORTCIRCUIT=-0.0001 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Wed, 24 Apr 2019, Peter Zijlstra wrote: > On Thu, Apr 18, 2019 at 10:41:37AM +0200, Thomas Gleixner wrote: > > There is only one caller of check_prev_add() which hands in a zeroed struct > > stack trace and a function pointer to save_stack(). Inside check_prev_add() > > the stack_trace struct is checked for being empty, which is always > > true. Based on that one code path stores a stack trace which is unused. The > > comment there does not make sense either. It's all leftovers from > > historical lockdep code (cross release). > > I was more or less expecting a revert of: > > ce07a9415f26 ("locking/lockdep: Make check_prev_add() able to handle external stack_trace") > > And then I read the comment that went with the "static struct > stack_trace trace" that got removed (in the above commit) and realized > that your patch will consume more stack entries. > > The problem is when the held lock stack in check_prevs_add() has multple > trylock entries on top, in that case we call check_prev_add() multiple > times, and this patch will then save the exact same stack-trace multiple > times, consuming static resources. > > Possibly we should copy what stackdepot does (but we cannot use it > directly because stackdepot uses locks; but possible we can share bits), > but that is a patch for another day I think. > > So while convoluted, perhaps we should retain this code for now. Uurg, what a mess.