From patchwork Sat Sep 28 13:51:28 2024 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Mathieu Desnoyers X-Patchwork-Id: 13814686 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 4376FCF6493 for ; Sat, 28 Sep 2024 13:53:56 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id CFEFD6B0200; Sat, 28 Sep 2024 09:53:55 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id CAE636B0201; Sat, 28 Sep 2024 09:53:55 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id B28CD6B0202; Sat, 28 Sep 2024 09:53:55 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0015.hostedemail.com [216.40.44.15]) by kanga.kvack.org (Postfix) with ESMTP id 927B56B0200 for ; Sat, 28 Sep 2024 09:53:55 -0400 (EDT) Received: from smtpin06.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay08.hostedemail.com (Postfix) with ESMTP id 39B49141304 for ; Sat, 28 Sep 2024 13:53:55 +0000 (UTC) X-FDA: 82614290430.06.DE59309 Received: from smtpout.efficios.com (smtpout.efficios.com [167.114.26.122]) by imf13.hostedemail.com (Postfix) with ESMTP id A93C120004 for ; Sat, 28 Sep 2024 13:53:53 +0000 (UTC) Authentication-Results: imf13.hostedemail.com; dkim=pass header.d=efficios.com header.s=smtpout1 header.b=VZ2bjJPy; dmarc=pass (policy=none) header.from=efficios.com; spf=pass (imf13.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=1727531542; a=rsa-sha256; cv=none; b=xjICSwJ5cuyrnFDU1XqFWNySFGvS2ZOL0bswi9R/0HuU+vob29zlIQQ0IScGSnHpLb/anx n3W0FcVEFoN7S3e1NcK5qSe4IuSv7u56De82ieF69RvEFiP//PiPKG4uYMD7Qi3zjfnQtj uOMEMrUWJ9UAXH9tbU3CN8o/lbM67AE= ARC-Authentication-Results: i=1; imf13.hostedemail.com; dkim=pass header.d=efficios.com header.s=smtpout1 header.b=VZ2bjJPy; dmarc=pass (policy=none) header.from=efficios.com; spf=pass (imf13.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=1727531542; 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-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references:dkim-signature; bh=0pavNFWu7yQ4JbL6+99UgN2a2SA7TP1/9zropkrdk0o=; b=fNuS+/NtJMeqXXRhnXTXgmPuGv5LCm0PfdZwIFRC+uUHsb4Yq8WJXfALKxm7wNxFvflzXj 8ZdSXwrr7LwkuifH+a9xPTfCPEByF7N3IFtuQnApJqeexQDzmmoNJMY0w6qoTWMh0ih5qM 0YaQGj0e7NqNltrkJF3EnjFtu8avhbo= DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=efficios.com; s=smtpout1; t=1727531632; bh=adr64DJrpL036YYqX3VlBhGw9oL6SMunripgcF3S5mk=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=VZ2bjJPyzV8LOr/fUK3q5+I26yAxJvJ8sorbWZ4MKmbGYZTIJ/fIr7Hyx30CAKpxO WpIBkNQNlW+jolCn9hc284TT8OGzd/5YzG2DKaCz5NhwzMJGPWiHb3T3Q2ElKSsVQk iY5gtCZDD8laZdfbhHUFc1K4XKjgTuoGqs8JP62v3fuje2aC3KS1EKrX9G6HQmkKQ9 afMfgD+0hWceM6vNAUQGUEgDUjAWBhXzNeWOA2qANQmQZadycqwMlcCHN1lkspEiZU i4ggfozc1u0PYWQlkV8coab3oTkaE8OMtg/HAVLx7XKlwkGOVU3RwUet1Tk7XSLeES KNMf+4kwaXmuQ== Received: from thinkos.internal.efficios.com (unknown [IPv6:2606:6d00:100:4000:cacb:9855:de1f:ded2]) by smtpout.efficios.com (Postfix) with ESMTPSA id 4XG8282zDgz1bsD; Sat, 28 Sep 2024 09:53:52 -0400 (EDT) From: Mathieu Desnoyers To: Linus Torvalds Cc: linux-kernel@vger.kernel.org, Mathieu Desnoyers , Greg Kroah-Hartman , Sebastian Andrzej Siewior , "Paul E. McKenney" , Will Deacon , Peter Zijlstra , Boqun Feng , Alan Stern , 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 , Jonas Oberhauser , rcu@vger.kernel.org, linux-mm@kvack.org, lkmm@lists.linux.dev Subject: [PATCH 2/2] Documentation: RCU: Refer to ptr_eq() Date: Sat, 28 Sep 2024 09:51:28 -0400 Message-Id: <20240928135128.991110-3-mathieu.desnoyers@efficios.com> X-Mailer: git-send-email 2.39.2 In-Reply-To: <20240928135128.991110-1-mathieu.desnoyers@efficios.com> References: <20240928135128.991110-1-mathieu.desnoyers@efficios.com> MIME-Version: 1.0 X-Rspamd-Queue-Id: A93C120004 X-Rspam-User: X-Rspamd-Server: rspam05 X-Stat-Signature: yy1su3yp3foit11t6ha9oaqrxo8c1fo7 X-HE-Tag: 1727531633-972203 X-HE-Meta: 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 KPL/bjxj 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 Jghwe/Re Z0UpW6Q7Uycunw8wEGk64MeKdLkparo0DwGZEPqgkOVgaFqE501ZEp4L0LHhssLo5pZEkIO3DRNz1HUJEJsbS4lP9jpf448KSgHljkc6tQePPzpA2g4+zNxf1DPaK+I5AM5ScEJDuMqwIEsefXdqvQ== 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: Refer to ptr_eq() in the rcu_dereference() documentation. ptr_eq() is a mechanism that preserves address dependencies when comparing pointers, and should be favored when comparing a pointer obtained from rcu_dereference() against another pointer. Signed-off-by: Mathieu Desnoyers Cc: Greg Kroah-Hartman Cc: Sebastian Andrzej Siewior Cc: "Paul E. McKenney" Cc: Will Deacon Cc: Peter Zijlstra Cc: Boqun Feng Cc: Alan Stern Cc: John Stultz Cc: Neeraj Upadhyay Cc: Linus Torvalds Cc: Boqun Feng Cc: Frederic Weisbecker Cc: Joel Fernandes Cc: Josh Triplett Cc: Uladzislau Rezki Cc: Steven Rostedt Cc: Lai Jiangshan Cc: Zqiang Cc: Ingo Molnar Cc: Waiman Long Cc: Mark Rutland Cc: Thomas Gleixner Cc: Vlastimil Babka Cc: maged.michael@gmail.com Cc: Mateusz Guzik Cc: Gary Guo Cc: Jonas Oberhauser Cc: rcu@vger.kernel.org Cc: linux-mm@kvack.org Cc: lkmm@lists.linux.dev --- Documentation/RCU/rcu_dereference.rst | 34 +++++++++++++++++++++++---- 1 file changed, 29 insertions(+), 5 deletions(-) diff --git a/Documentation/RCU/rcu_dereference.rst b/Documentation/RCU/rcu_dereference.rst index 2524dcdadde2..c36b8d1721f6 100644 --- a/Documentation/RCU/rcu_dereference.rst +++ b/Documentation/RCU/rcu_dereference.rst @@ -104,11 +104,13 @@ readers working properly: after such branches, but can speculate loads, which can again result in misordering bugs. -- Be very careful about comparing pointers obtained from - rcu_dereference() against non-NULL values. As Linus Torvalds - explained, if the two pointers are equal, the compiler could - substitute the pointer you are comparing against for the pointer - obtained from rcu_dereference(). For example:: +- Use relational operators which preserve address dependencies + (such as "ptr_eq()") to compare pointers obtained from + rcu_dereference() against non-NULL values or against pointers + obtained from prior loads. As Linus Torvalds explained, if the + two pointers are equal, the compiler could substitute the + pointer you are comparing against for the pointer obtained from + rcu_dereference(). For example:: p = rcu_dereference(gp); if (p == &default_struct) @@ -125,6 +127,23 @@ readers working properly: On ARM and Power hardware, the load from "default_struct.a" can now be speculated, such that it might happen before the rcu_dereference(). This could result in bugs due to misordering. + Performing the comparison with "ptr_eq()" ensures the compiler + does not perform such transformation. + + If the comparison is against a pointer obtained from prior + loads, the compiler is allowed to use either register for the + following accesses, which loses the address dependency and + allows weakly-ordered architectures such as ARM and PowerPC + to speculate the address-dependent load before rcu_dereference(). + For example:: + + p1 = READ_ONCE(gp); + p2 = rcu_dereference(gp); + if (p1 == p2) + do_default(p2->a); + + Performing the comparison with "ptr_eq()" ensures the compiler + preserves the address dependencies. However, comparisons are OK in the following cases: @@ -204,6 +223,11 @@ readers working properly: comparison will provide exactly the information that the compiler needs to deduce the value of the pointer. + When in doubt, use relational operators that preserve address + dependencies (such as "ptr_eq()") to compare pointers obtained + from rcu_dereference() against non-NULL values or against + pointers obtained from prior loads. + - Disable any value-speculation optimizations that your compiler might provide, especially if you are making use of feedback-based optimizations that take data collected from prior runs. Such