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=-2.2 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,FREEMAIL_FORGED_FROMDOMAIN,FREEMAIL_FROM, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 autolearn=no 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 49C9BC5DF60 for ; Thu, 7 Nov 2019 06:08:23 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 094A721882 for ; Thu, 7 Nov 2019 06:08:22 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=gmail.com header.i=@gmail.com header.b="Wj0bs5MG" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 094A721882 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=gmail.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id 7E6566B0007; Thu, 7 Nov 2019 01:08:22 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id 797026B0008; Thu, 7 Nov 2019 01:08:22 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 6AF6B6B000A; Thu, 7 Nov 2019 01:08:22 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0047.hostedemail.com [216.40.44.47]) by kanga.kvack.org (Postfix) with ESMTP id 51DD66B0007 for ; Thu, 7 Nov 2019 01:08:22 -0500 (EST) Received: from smtpin22.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay02.hostedemail.com (Postfix) with SMTP id CA8C22DFA for ; Thu, 7 Nov 2019 06:08:21 +0000 (UTC) X-FDA: 76128451602.22.base08_8329c31b4ac24 X-HE-Tag: base08_8329c31b4ac24 X-Filterd-Recvd-Size: 5802 Received: from mail-pf1-f194.google.com (mail-pf1-f194.google.com [209.85.210.194]) by imf48.hostedemail.com (Postfix) with ESMTP for ; Thu, 7 Nov 2019 06:08:21 +0000 (UTC) Received: by mail-pf1-f194.google.com with SMTP id p24so1691521pfn.4 for ; Wed, 06 Nov 2019 22:08:21 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to:user-agent; bh=eToy4w56eW6HWTLeixUVeOOwuR/Is81CE8VEHg0yHXg=; b=Wj0bs5MGw8Vb7joYFopit9Dg0AiXHYnw6Ui69Tv5ACxuoGVCw+nDfkurHFAtMCDUKH Rlxf+Ukj0Q12gkV7i5SY5ZtWdDVR8/A7LnmWQovuSlC8RjAqypFAeJEjYlpzDUZfH1bL M3JpmeLQGFXe48eK9I4WJJPh4GDhnAlmDbIrK6u+OQW4IXpkGS2AtXPYs0AOorz+0Lcs ptv4B8z2RKlZZk7vzDb1fgFF3BYfL64tGqNFXcW/h3tVaL+KrYvXSS9PKHKUHxonFQ9P F4qfuXC0xeCil9iS9ZCfyCwVDRx4t6KFBP5X6eOsJcbupwq2ccXJAmcqS6dpSfYxsTa9 cvrg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to:user-agent; bh=eToy4w56eW6HWTLeixUVeOOwuR/Is81CE8VEHg0yHXg=; b=H6DzlhKH1p55IynOYq6CNBa6wGh6mCe9OBworpbX7JFjLRSPN2u2Wb4eXpxTW5tosw vUzfVS95l5F2Z7wpZw3rHXILIQmmiQkfzqumuU9Gf4DFMXSaO9ZAndhYhLI2FVgcDNfF GkWUJbc4LqqEL+h8nJFsb6+zGyaBYDIg3hgawXKkGvR9v5MuBzKFmPjElxopO1JXMj7e fMVVMZmyiAvrNHYDdQKyg9o2fL3qU1mg1JCJzXynMqeuFQXZ89jLx60dUagmHr9HEQyp vulwe7TFCFj6DHcBUBoUZ8rUbAGpolJMcq5zwV4lY34kTGryZgJZMDZfWRtLJjfgoK/0 PJgA== X-Gm-Message-State: APjAAAV48kuBMs5d/GpMJy/3LHEoU8l/kit3+5BDlqUftYzs/XSsLzf1 gNL+M2AgHRhYeTN4BGfxbCA= X-Google-Smtp-Source: APXvYqztPRw33aDfXM1t3vNcTIKTpnzVcKa7AUnAShk0qeT8gqbWdTPSqU3eHFt2MDmXPrHLDGYaig== X-Received: by 2002:a17:90a:c789:: with SMTP id gn9mr2586902pjb.99.1573106900156; Wed, 06 Nov 2019 22:08:20 -0800 (PST) Received: from localhost ([2401:fa00:8f:203:250d:e71d:5a0a:9afe]) by smtp.gmail.com with ESMTPSA id q11sm976428pgq.71.2019.11.06.22.08.18 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 06 Nov 2019 22:08:19 -0800 (PST) Date: Thu, 7 Nov 2019 15:08:16 +0900 From: Sergey Senozhatsky To: Alexander Potapenko Cc: Sergey Senozhatsky , Vegard Nossum , Dmitry Vyukov , Linux Memory Management List , Al Viro , Andrew Morton , Andrey Ryabinin , Andy Lutomirski , Ard Biesheuvel , Arnd Bergmann , Christoph Hellwig , Dmitry Torokhov , Eric Dumazet , Eric Van Hensbergen , Greg Kroah-Hartman , Harry Wentland , Herbert Xu , Ingo Molnar , Jens Axboe , "Martin K . Petersen" , Martin Schwidefsky , "Michael S. Tsirkin" , Michal Simek , Petr Mladek , Sergey Senozhatsky , Steven Rostedt , Takashi Iwai , Theodore Ts'o , Thomas Gleixner , Wolfram Sang , Vasily Gorbik , Ilya Leoshkevich , Mark Rutland , Matthew Wilcox , Randy Dunlap , Andrey Konovalov , Marco Elver Subject: Re: [PATCH RFC v2 02/25] stackdepot: prevent Clang from optimizing away stackdepot_memcmp() Message-ID: <20191107060816.GA93084@google.com> References: <20191030142237.249532-1-glider@google.com> <20191030142237.249532-3-glider@google.com> <20191101055033.GA226263@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: 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 (19/11/06 12:43), Alexander Potapenko wrote: > > On (19/10/30 15:22), glider@google.com wrote: > > > @@ -163,6 +163,11 @@ int stackdepot_memcmp(const unsigned long *u1, const unsigned long *u2, > > > unsigned int n) > > > { > > > for ( ; n-- ; u1++, u2++) { > > > + /* > > > + * Prevent Clang from replacing this function with a bcmp() > > > + * call. > > > + */ > > > + barrier(); > > > if (*u1 != *u2) > > > return 1; > > > } > > > > Would 'volatile' do the trick? > It does. I can replace the barrier with a volatile if you think that's better. > However this'll add a checkpatch warning, as volatiles are discouraged > for synchronization (although in this case there's no synchronization) Yeah, 'volatile' in this case will do what it sort of meant to do - prevent compiler optimizations. So, like you said, it's not a synchronization issue and we don't 'volatile' data structures. Do you need to do barrier() on every iteration? Does clang behave if you do one barrier() instead of 'n' barrier()-s? -ss