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 A2982CF8854 for ; Fri, 4 Oct 2024 18:29:46 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 016B16B02CF; Fri, 4 Oct 2024 14:29:46 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id F0A176B02D0; Fri, 4 Oct 2024 14:29:45 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id DA8406B02D2; Fri, 4 Oct 2024 14:29:45 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0014.hostedemail.com [216.40.44.14]) by kanga.kvack.org (Postfix) with ESMTP id B43046B02CF for ; Fri, 4 Oct 2024 14:29:45 -0400 (EDT) Received: from smtpin07.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay03.hostedemail.com (Postfix) with ESMTP id 42094A1AEC for ; Fri, 4 Oct 2024 18:29:45 +0000 (UTC) X-FDA: 82636758330.07.9379667 Received: from smtpout.efficios.com (smtpout.efficios.com [167.114.26.122]) by imf16.hostedemail.com (Postfix) with ESMTP id AB84918000D for ; Fri, 4 Oct 2024 18:29:43 +0000 (UTC) Authentication-Results: imf16.hostedemail.com; dkim=pass header.d=efficios.com header.s=smtpout1 header.b=GY4zUt2T; dmarc=pass (policy=none) header.from=efficios.com; spf=pass (imf16.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=1728066560; a=rsa-sha256; cv=none; b=A3bvjF/ouwKQQSiZPp5ZqMwTaF8CXYD+uCIEOx/GaMB07X2KP00qiSSaPEMFxJ+5J135bR SNtEf+ozdmkZ3rQ1xGdI4NEkIXYOb8jTjPJGSd2CwjmYGR91hTzly8nnQH8ZbzgBc9kUWr yfckfxzbSFHrHiegfzcM6yqaV0BoWXY= ARC-Authentication-Results: i=1; imf16.hostedemail.com; dkim=pass header.d=efficios.com header.s=smtpout1 header.b=GY4zUt2T; dmarc=pass (policy=none) header.from=efficios.com; spf=pass (imf16.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=1728066560; 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=3kXqX9kwZUqLQ5vuGBV+0fBOaaNpCVSZ1lREQGq+YPw=; b=qQDDnknRg0xNULG/FJg+5yipOxpb6Bl3w/TzsSg/gPo6AqdXacipgSQzRDPzEgg78bfFvl 5fKWvn/2culiYLX+UPa/dH8KHSGOqmcZN4ZeV/i0YkBGxrbXO2WwqY8pBsjlmPXf3jaOxh /cMQIPSQfEQYSjgln2RT6LUy+grKrdo= DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=efficios.com; s=smtpout1; t=1728066582; bh=ruHajFuAP+9RCH6285NJzkwm+MUgE21IicKMAn8zEWg=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=GY4zUt2TAPM5YfusXMih3fV3IYkWKP/euMtVv+/PxLXdHpNsXgxya95dn+7c+pICP TpoTwpgWojwhrqN47JfH1uuLyYp0vMw/moy7QhNJmUmXEes6GkKZwj8YBIPPdOzGc4 BvWFvWnu7uUKktoRhbiXX6SvCscRhca9DEuWp4GSkYprAWVw2nrADESHEJMN6SrU8i TofjfkvbUk0C+uiUrkxROZTRRmYYz4GrhJ8xBdy7djHqB1AYp8Sg8DOZz1eEQm+PvP FjTvIgbhj27mqs4K8KUeht0xuEIFzqFejkGpvNzMDsIJ13HCSkFoEr7TK7LMgFeW/I DaiWIjTVWM/Dw== Received: from thinkos.internal.efficios.com (96-127-217-162.qc.cable.ebox.net [96.127.217.162]) by smtpout.efficios.com (Postfix) with ESMTPSA id 4XKxsf36hmzN8Q; Fri, 4 Oct 2024 14:29:42 -0400 (EDT) From: Mathieu Desnoyers To: Boqun Feng Cc: linux-kernel@vger.kernel.org, Mathieu Desnoyers , Linus Torvalds , Andrew Morton , Peter Zijlstra , Nicholas Piggin , Michael Ellerman , Greg Kroah-Hartman , Sebastian Andrzej Siewior , "Paul E. McKenney" , Will Deacon , 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 , Jonas Oberhauser , rcu@vger.kernel.org, linux-mm@kvack.org, lkmm@lists.linux.dev, Gary Guo , Nikita Popov , llvm@lists.linux.dev Subject: [RFC PATCH v2 1/4] compiler.h: Introduce ptr_eq() to preserve address dependency Date: Fri, 4 Oct 2024 14:27:31 -0400 Message-Id: <20241004182734.1761555-2-mathieu.desnoyers@efficios.com> X-Mailer: git-send-email 2.39.2 In-Reply-To: <20241004182734.1761555-1-mathieu.desnoyers@efficios.com> References: <20241004182734.1761555-1-mathieu.desnoyers@efficios.com> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Rspam-User: X-Stat-Signature: 16egqff1coqrw8pis8whzrg3tpqh8fx4 X-Rspamd-Queue-Id: AB84918000D X-Rspamd-Server: rspam02 X-HE-Tag: 1728066583-72942 X-HE-Meta: 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 e7i0L/i2 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 E92WJhq1 p2682ByIuhWTvZVKEHXVskVYRzg8pMIHWdZLEAQyaviSNOnMbYjr6wVCD1OZXdu7Qtq5B0Xpdgw87inzkcj2EcPKctkFq5eubm3UTzw02FM6SLvyoqdl0kXQxWh60CLoCiCLcFPd330f8qsEE8LHXXu8WsSbqlVgHKZkCdMjO8PsaSfEahoPYw== 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: Compiler CSE and SSA GVN optimizations can cause the address dependency of addresses returned by rcu_dereference to be lost when comparing those pointers with either constants or previously loaded pointers. Introduce ptr_eq() to compare two addresses while preserving the address dependencies for later use of the address. It should be used when comparing an address returned by rcu_dereference(). This is needed to prevent the compiler CSE and SSA GVN optimizations from using @a (or @b) in places where the source refers to @b (or @a) based on the fact that after the comparison, the two are known to be equal, which does not preserve address dependencies and allows the following misordering speculations: - If @b is a constant, the compiler can issue the loads which depend on @a before loading @a. - If @b is a register populated by a prior load, weakly-ordered CPUs can speculate loads which depend on @a before loading @a. The same logic applies with @a and @b swapped. Suggested-by: Linus Torvalds Suggested-by: Boqun Feng Signed-off-by: Mathieu Desnoyers Reviewed-by: Boqun Feng Reviewed-by: Joel Fernandes (Google) Tested-by: Joel Fernandes (Google) Acked-by: "Paul E. McKenney" Acked-by: Alan Stern 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 Cc: Nikita Popov Cc: llvm@lists.linux.dev --- Changes since v0: - Include feedback from Alan Stern. --- include/linux/compiler.h | 63 ++++++++++++++++++++++++++++++++++++++++ 1 file changed, 63 insertions(+) diff --git a/include/linux/compiler.h b/include/linux/compiler.h index 2df665fa2964..75a378ae7af1 100644 --- a/include/linux/compiler.h +++ b/include/linux/compiler.h @@ -186,6 +186,69 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val, __asm__ ("" : "=r" (var) : "0" (var)) #endif +/* + * Compare two addresses while preserving the address dependencies for + * later use of the address. It should be used when comparing an address + * returned by rcu_dereference(). + * + * This is needed to prevent the compiler CSE and SSA GVN optimizations + * from using @a (or @b) in places where the source refers to @b (or @a) + * based on the fact that after the comparison, the two are known to be + * equal, which does not preserve address dependencies and allows the + * following misordering speculations: + * + * - If @b is a constant, the compiler can issue the loads which depend + * on @a before loading @a. + * - If @b is a register populated by a prior load, weakly-ordered + * CPUs can speculate loads which depend on @a before loading @a. + * + * The same logic applies with @a and @b swapped. + * + * Return value: true if pointers are equal, false otherwise. + * + * The compiler barrier() is ineffective at fixing this issue. It does + * not prevent the compiler CSE from losing the address dependency: + * + * int fct_2_volatile_barriers(void) + * { + * int *a, *b; + * + * do { + * a = READ_ONCE(p); + * asm volatile ("" : : : "memory"); + * b = READ_ONCE(p); + * } while (a != b); + * asm volatile ("" : : : "memory"); <-- barrier() + * return *b; + * } + * + * With gcc 14.2 (arm64): + * + * fct_2_volatile_barriers: + * adrp x0, .LANCHOR0 + * add x0, x0, :lo12:.LANCHOR0 + * .L2: + * ldr x1, [x0] <-- x1 populated by first load. + * ldr x2, [x0] + * cmp x1, x2 + * bne .L2 + * ldr w0, [x1] <-- x1 is used for access which should depend on b. + * ret + * + * On weakly-ordered architectures, this lets CPU speculation use the + * result from the first load to speculate "ldr w0, [x1]" before + * "ldr x2, [x0]". + * Based on the RCU documentation, the control dependency does not + * prevent the CPU from speculating loads. + */ +static __always_inline +int ptr_eq(const volatile void *a, const volatile void *b) +{ + OPTIMIZER_HIDE_VAR(a); + OPTIMIZER_HIDE_VAR(b); + return a == b; +} + #define __UNIQUE_ID(prefix) __PASTE(__PASTE(__UNIQUE_ID_, prefix), __COUNTER__) /** -- 2.39.2