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[2607:7c80:54:e::133]) by mx.google.com with ESMTPS id v9si18274905pgr.167.2019.04.24.12.45.22 for (version=TLS1_2 cipher=ECDHE-RSA-CHACHA20-POLY1305 bits=256/256); Wed, 24 Apr 2019 12:45:22 -0700 (PDT) Received-SPF: pass (google.com: best guess record for domain of peterz@infradead.org designates 2607:7c80:54:e::133 as permitted sender) client-ip=2607:7c80:54:e::133; Authentication-Results: mx.google.com; dkim=pass header.i=@infradead.org header.s=bombadil.20170209 header.b=jnXyb2lO; spf=pass (google.com: best guess record for domain of peterz@infradead.org designates 2607:7c80:54:e::133 as permitted sender) smtp.mailfrom=peterz@infradead.org DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=bombadil.20170209; h=In-Reply-To:Content-Type:MIME-Version :References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Id: List-Help:List-Unsubscribe:List-Subscribe:List-Post:List-Owner:List-Archive; bh=nXNCzgXPp/MC/y/PJgs7pmMKbFnjx8a6N8WFYhwtNcs=; b=jnXyb2lOVttIJoTmHH09wDdgf n8NwsMb9Q8EZ9BhZYTUEmA/PHtyPiQZOmqBu9mXa6yS7FNimIsZTp3th5+3stcuYygHbNmLLYqBGT QahsATylTqjWyQwmQWDXx0qU89AA6/OXLd2NCoPGpUOZ+2qhJ4st3eHGbksMYs7v8Vz1EtlVHBU/V 0d2uWTvtE1ut2NEV0rW9PoEWWmPOPkZ6B4Ll/v8YqCWjQwzFWcsIDvwui3aaYqTf72W1WXM1j/0xM JpCvyP3P7MY/egBVMNbkcye19upsSYlZcw08/CLq5k4oepPII8zRfmSr6JNXtPbV5jb8VqzdiXZp7 XpWmu0naw==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=hirez.programming.kicks-ass.net) by bombadil.infradead.org with esmtpsa (Exim 4.90_1 #2 (Red Hat Linux)) id 1hJNpM-0005aH-OA; Wed, 24 Apr 2019 19:45:08 +0000 Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id E5F80203E8871; Wed, 24 Apr 2019 21:45:05 +0200 (CEST) Date: Wed, 24 Apr 2019 21:45:05 +0200 From: Peter Zijlstra To: Thomas Gleixner 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() Message-ID: <20190424194505.GR11158@hirez.programming.kicks-ass.net> References: <20190418084119.056416939@linutronix.de> <20190418084254.729689921@linutronix.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190418084254.729689921@linutronix.de> User-Agent: Mutt/1.10.1 (2018-07-13) 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 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.