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 Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by smtp.lore.kernel.org (Postfix) with ESMTP id 1BACACF34BC for ; Thu, 3 Oct 2024 18:02:47 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 8011E6B00B5; Thu, 3 Oct 2024 14:02:46 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 7AFDE6B00B6; Thu, 3 Oct 2024 14:02:46 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 650C36B0439; Thu, 3 Oct 2024 14:02:46 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0017.hostedemail.com [216.40.44.17]) by kanga.kvack.org (Postfix) with ESMTP id 3FEE66B00B5 for ; Thu, 3 Oct 2024 14:02:46 -0400 (EDT) Received: from smtpin03.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay10.hostedemail.com (Postfix) with ESMTP id 9B43AC0470 for ; Thu, 3 Oct 2024 18:02:45 +0000 (UTC) X-FDA: 82633061490.03.5817409 Received: from smtpout.efficios.com (smtpout.efficios.com [167.114.26.122]) by imf29.hostedemail.com (Postfix) with ESMTP id E17DB120025 for ; Thu, 3 Oct 2024 18:02:42 +0000 (UTC) Authentication-Results: imf29.hostedemail.com; dkim=pass header.d=efficios.com header.s=smtpout1 header.b=EsuFjsfi; dmarc=pass (policy=none) header.from=efficios.com; spf=pass (imf29.hostedemail.com: domain of mathieu.desnoyers@efficios.com designates 167.114.26.122 as permitted sender) smtp.mailfrom=mathieu.desnoyers@efficios.com ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1727978540; a=rsa-sha256; cv=none; b=vpkxnQvQcLaWmdd2P4L8m8QVhNDa/23fA3EXrnvr9i6OQkwTy7ZOvko4+vs6f/YbpNdoiW eT+y4x6a4ZutAoItWh2aCh2sCJfQC5ZIGdNv4V/zHztQ6Pq0DxMufXBKLrLSPKS1lRn1l9 rjtWNg1ZrwW/kKfsGle2v6VX6V0e++k= ARC-Authentication-Results: i=1; imf29.hostedemail.com; dkim=pass header.d=efficios.com header.s=smtpout1 header.b=EsuFjsfi; dmarc=pass (policy=none) header.from=efficios.com; spf=pass (imf29.hostedemail.com: domain of mathieu.desnoyers@efficios.com designates 167.114.26.122 as permitted sender) smtp.mailfrom=mathieu.desnoyers@efficios.com ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1727978540; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references:dkim-signature; bh=qVBPB+1aPZcmh8WsI5QbpZJdJ0EsHfR2Ehn3f/p7zvs=; b=KmjYvM0J1tqlYot9Pr7ZQR7pUhIhNP3Qh48GzMq8oKeHy4vwpl6+OYXWMa6GNWXAs9txGD HRhxeiFAMCulvBuINvzNIlHHIXbR3xbplrvHdFtsByM3QmRX9O3Ydypwgebd0JLUem+c7H B1jfAwVbrssyd3RflGjA2T/IiY1HFOY= DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=efficios.com; s=smtpout1; t=1727978561; bh=HNpqj2LAI7uBi+EmJxWZgOWPeLGVvAZIHpXsFU6OCzU=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=EsuFjsfi0tJGARLyFENnM/KPllKTKVUFOqXqIvZ96lFuTElWnaRQoZ5ie9s4M5lu2 bkJJ3HEyZTmzyQDPN5IZ7eh4rr3s25Dtq/+QxQyzbcXDGdM8gcFXLlwoszavHm4H+s N1oqU8jfxwGLKL5o6MHY5LAtIrkMY5WoMGX/3PRGoEPi6Te/UXlGtWU1b/93LM6+pr wDvgAFFplTe90z7+N/CrPFT97TQRGYDH3ZtpKY9P/rVoO0g6nflOoBM49lssmWN6q3 M+gvN6ikw279UQ0Hf1Vxuw0wcmWS/Op3Lj3JTn/DcsZeXlxVMSUq4VT78vgjIzjK7g RdiolnQ52ILsQ== Received: from [172.16.0.134] (96-127-217-162.qc.cable.ebox.net [96.127.217.162]) by smtpout.efficios.com (Postfix) with ESMTPSA id 4XKKJx1Q9yz78q; Thu, 3 Oct 2024 14:02:41 -0400 (EDT) Message-ID: Date: Thu, 3 Oct 2024 14:00:40 -0400 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 1/2] compiler.h: Introduce ptr_eq() to preserve address dependency To: David Laight , 'Alan Stern' Cc: Jonas Oberhauser , Linus Torvalds , "linux-kernel@vger.kernel.org" , Greg Kroah-Hartman , Sebastian Andrzej Siewior , "Paul E. McKenney" , Will Deacon , Peter Zijlstra , Boqun Feng , John Stultz , Neeraj Upadhyay , Frederic Weisbecker , Joel Fernandes , Josh Triplett , Uladzislau Rezki , Steven Rostedt , Lai Jiangshan , Zqiang , Ingo Molnar , Waiman Long , Mark Rutland , Thomas Gleixner , Vlastimil Babka , "maged.michael@gmail.com" , Mateusz Guzik , Gary Guo , "rcu@vger.kernel.org" , "linux-mm@kvack.org" , "lkmm@lists.linux.dev" References: <02c63e79-ec8c-4d6a-9fcf-75f0e67ea242@rowland.harvard.edu> <9539c551-5c91-42db-8ac1-cff1d6d7c293@huaweicloud.com> <2cdda043-1ad9-40cf-a157-0c16a0ffb046@rowland.harvard.edu> <5d7d8a59-57f5-4125-95bb-fda9c193b9cf@huaweicloud.com> <82e97ad5-17ad-418d-8791-22297acc7af4@rowland.harvard.edu> <2b1caba3-48fa-43b9-bd44-cf60b9a141d7@rowland.harvard.edu> <22638e2fe1274eb0834fa3e43b44184e@AcuMS.aculab.com> <68dc00b3-1ca1-42bc-8f1e-78ace10e4d64@rowland.harvard.edu> <498f11de5a024f0ca0b70aba4e28b17b@AcuMS.aculab.com> From: Mathieu Desnoyers Content-Language: en-US In-Reply-To: <498f11de5a024f0ca0b70aba4e28b17b@AcuMS.aculab.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspam-User: X-Stat-Signature: wwdb8muhowszxy8d1wtaj3hg8c3fuawf X-Rspamd-Queue-Id: E17DB120025 X-Rspamd-Server: rspam02 X-HE-Tag: 1727978562-877873 X-HE-Meta: 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 qeuPJiA4 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 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: List-Subscribe: List-Unsubscribe: On 2024-10-03 19:07, David Laight wrote: > ... >> What _does_ work however are the following two approaches: >> >> 1) Perform the equality check on the original variables, creating >> new versions (with OPTIMIZER_HIDE_VAR) of both variables for the >> rest of their use, therefore making sure the pointer dereference >> are not derived from versions of the variables which were compared >> with another pointer. (as suggested by Boqun) > > If that is > a1 = a; OPTIMISER_HIDE_VAR(a1); > b1 = b; OPTIMISER_HIDE_BAR(b1); > if (a != b} > return; > // code using a1 and b1 > then can't the compiler first flip it to: > if (a != b) > return; > a1 = a; OPTIMISER_HIDE_VAR(a1); > b1 = b; OPTIMISER_HIDE_VAR(b1); > and then replace the last line with: > b1 = a; OPTIMISER_HIDE_VAR(b1); > which isn't intended at all. Good point, so I suspect Boqun's ADDRESS_EQ() suggestion did not work: https://lore.kernel.org/lkml/ZvX12_1mK8983cXm@boqun-archlinux/ > > > OTOH if you do: > a1 = a; OPTIMISER_HIDE_VAR(a1); > b1 = b; OPTIMISER_HIDE_VAR(b1); > if (a1 != b1) > return; > // code using a and b > (which I think is) This is in line with Linus' suggestion, which is the approach I retained. > >> 2) Perform the equality check on the versions resulting of hiding >> both variables, making sure those versions of the variables are >> not dereferenced afterwards. (as suggested by Linus) > > then the compiler can't possibly reverse the asm blocks. Indeed. Thanks, Mathieu > > David > > - > Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK > Registration No: 1397386 (Wales) -- Mathieu Desnoyers EfficiOS Inc. https://www.efficios.com