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 878BECFA462 for ; Wed, 23 Oct 2024 19:45:56 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 33F026B0099; Wed, 23 Oct 2024 15:45:54 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 2EFEE6B009B; Wed, 23 Oct 2024 15:45:54 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 1B7386B009E; Wed, 23 Oct 2024 15:45:54 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0016.hostedemail.com [216.40.44.16]) by kanga.kvack.org (Postfix) with ESMTP id F13586B0099 for ; Wed, 23 Oct 2024 15:45:53 -0400 (EDT) Received: from smtpin24.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay08.hostedemail.com (Postfix) with ESMTP id C9A2F140E8E for ; Wed, 23 Oct 2024 19:45:34 +0000 (UTC) X-FDA: 82705896882.24.48A5BD8 Received: from desiato.infradead.org (desiato.infradead.org [90.155.92.199]) by imf30.hostedemail.com (Postfix) with ESMTP id 4B0E98000E for ; Wed, 23 Oct 2024 19:45:18 +0000 (UTC) Authentication-Results: imf30.hostedemail.com; dkim=pass header.d=infradead.org header.s=desiato.20200630 header.b="h/yD9D/8"; spf=none (imf30.hostedemail.com: domain of peterz@infradead.org has no SPF policy when checking 90.155.92.199) smtp.mailfrom=peterz@infradead.org; dmarc=none ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1729712600; 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: in-reply-to:in-reply-to:references:references:dkim-signature; bh=pnofjtWFRW1a3Ex4iamW6Sy5pqigg3kUWm7+B1kI9+Q=; b=Ls/bUZpgkBxRW+7D0N0krX+BQ2AT1tzm6KpsIQuuXTs82cXL3orggwvzByoOrZvEV+c8Dv 3cpU3MQGIXwa7KTa9bzAjtbVxeqOABdOdKNu9Swrnd/8A/DCeKmTFmyXhkfppZqIHildvf i6asQT0o273T0ZtEWfKVwpGB6aVu3XQ= ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1729712600; a=rsa-sha256; cv=none; b=mzUgGOMvPX4c55aktBhKGMHaL+yzGw5mhPwpJhV/1+ZQRKj7wJmn20fsWFNK4wjJEdhRHM RFrImq67owCqZFzfnY0tPt3rHoBe5aGKxQPJvowgU/ATS7DX/RI04sWdSfP8LbeM/0Fzr4 QJ4Mx6EsciY0MrnGDAvnsP0kb6k0cvI= ARC-Authentication-Results: i=1; imf30.hostedemail.com; dkim=pass header.d=infradead.org header.s=desiato.20200630 header.b="h/yD9D/8"; spf=none (imf30.hostedemail.com: domain of peterz@infradead.org has no SPF policy when checking 90.155.92.199) smtp.mailfrom=peterz@infradead.org; dmarc=none DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=desiato.20200630; 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; bh=pnofjtWFRW1a3Ex4iamW6Sy5pqigg3kUWm7+B1kI9+Q=; b=h/yD9D/860kV5yyshWWgMIMGPr 5Y/QxMgy8Qx7w2cTYX6kRQhdlB3KqN+MY4cNRvt94WqfJu2NixJ9fxTo1FARn0AAqDH8Vuz6Y9hDy yCCJe/z8FtwFDDV/NMRQtAnkEH9L7b9ZtlOem2aYepMOLYuKN1/PuTkAA4FZsxMpz0mCfVgCAuGbS +/bRgSqshAfX0KzU0kSNf8eN2HeVbC4k3Oic4VlOhr/Lol8SLWQ/Omq7d2xnGf5jC9cbiwoOSffm1 MN/J8bsc2HGzEjpkaTJRCjv0ggYmOAfUtDFpWrw7IXY8d755vXZRD4V+VugCbNw/JRk7466sntgBS o71bzRkw==; Received: from j130084.upc-j.chello.nl ([24.132.130.84] helo=noisy.programming.kicks-ass.net) by desiato.infradead.org with esmtpsa (Exim 4.98 #2 (Red Hat Linux)) id 1t3hIe-00000008XNO-3Dyc; Wed, 23 Oct 2024 19:45:46 +0000 Received: by noisy.programming.kicks-ass.net (Postfix, from userid 1000) id D8E61301171; Wed, 23 Oct 2024 21:45:43 +0200 (CEST) Date: Wed, 23 Oct 2024 21:45:43 +0200 From: Peter Zijlstra To: Linus Torvalds Cc: "Christoph Lameter (Ampere)" , Will Deacon , Thomas Gleixner , Catalin Marinas , Ingo Molnar , Waiman Long , Boqun Feng , linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-arch@vger.kernel.org Subject: Re: [PATCH v3] Avoid memory barrier in read_seqcount() through load acquire Message-ID: <20241023194543.GD11151@noisy.programming.kicks-ass.net> References: <20240912-seq_optimize-v3-1-8ee25e04dffa@gentwo.org> <20240917071246.GA27290@willie-the-truck> <4b546151-d5e1-22a3-a6d5-167a82c5724d@gentwo.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspamd-Queue-Id: 4B0E98000E X-Stat-Signature: p4nczqyq1sbtzj1bqz351g7kmhftow4n X-Rspamd-Server: rspam09 X-Rspam-User: X-HE-Tag: 1729712718-332175 X-HE-Meta: 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 FG2c3JIY W6aC5QOwip9So1H46EOqAalnnwDYnQ/KokDpYY01OrqfF6ybPK8RpVU7aF2MJiqLNULN9ylvAH5hXpvX4T32JmgyfK8/XguG3p1HOEOLZjB8dtp8OLa0Dzr3QfWKSQ8mGi4TQIsor6LBuIPrXkYJJ+aonyH556c2ZEgonEyX06pg/Eut4mc6ti1ZdtFcWzx8PKZguxzgYLLCbdL/6K/CTTg8t+QZrHpAbbJFbgJAoFgbZUYBB4XloesOTnVkQiTYlnTg4Cnh1MWEKyLZSAIG6a8g6X6qFTsd6DTBtsJfkfblvqR9qh45Juheo2QEjj2JUnu/3FXYYt2N0jy9Ofrz3/7sOjXnmZtYqyM6Yj+8kUZkvzqQeb6iACgahq9I1GMbD1+gEWgCkPEWQeuY7vv6GBbCn1X+DRPOFa9n4ZTBjFjrohi5DaGn7RLvc/w== 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 Mon, Sep 23, 2024 at 09:28:31AM -0700, Linus Torvalds wrote: > On Wed, 18 Sept 2024 at 08:22, Linus Torvalds > wrote: > > > > On Wed, 18 Sept 2024 at 13:15, Christoph Lameter (Ampere) wrote: > > > > > > Other arches do not have acquire / release and will create additional > > > barriers in the fallback implementation of smp_load_acquire. So it needs > > > to be an arch config option. > > > > Actually, I looked at a few cases, and it doesn't really seem to be true. > > Bah. I ended up just committing the minimal version of this all. I > gave Christoph credit for the commit, because I stole his commit > message, and he did most of the work, I just ended up going "simplify, > simplify, simplify". > > I doubt anybody will notice, and smp_load_acquire() is the future. Any > architecture that does badly on it just doesn't matter (and, as > mentioned, I don't think they even exist - "smp_rmb()" is generally at > least as expensive). Do we want to do the complementing patch and make write_seqcount_end() use smp_store_release() ? I think at least ARM (the 32bit thing) has wmb but uses mb for store_release. But I also think I don't really care about that.