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]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 36876D66B87 for ; Thu, 18 Dec 2025 01:45:41 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id F29556B0092; Wed, 17 Dec 2025 20:45:38 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id EA8E96B0089; Wed, 17 Dec 2025 20:45:38 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id C82B96B008C; Wed, 17 Dec 2025 20:45:38 -0500 (EST) 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 991E16B0088 for ; Wed, 17 Dec 2025 20:45:38 -0500 (EST) Received: from smtpin22.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay01.hostedemail.com (Postfix) with ESMTP id 1A6E960E01 for ; Thu, 18 Dec 2025 01:45:38 +0000 (UTC) X-FDA: 84230899956.22.CB231EB Received: from smtpout.efficios.com (smtpout.efficios.com [158.69.130.18]) by imf10.hostedemail.com (Postfix) with ESMTP id 65E48C0008 for ; Thu, 18 Dec 2025 01:45:36 +0000 (UTC) Authentication-Results: imf10.hostedemail.com; dkim=pass header.d=efficios.com header.s=smtpout1 header.b=fpefWz2O; dmarc=pass (policy=none) header.from=efficios.com; spf=pass (imf10.hostedemail.com: domain of mathieu.desnoyers@efficios.com designates 158.69.130.18 as permitted sender) smtp.mailfrom=mathieu.desnoyers@efficios.com ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1766022336; a=rsa-sha256; cv=none; b=cy5C+ODtAh0YgMuXbt2N3YizJNH0CUz4OcLkCbxbZTSyN8/IO/pBL41QCFenHxhdALyJX6 bSByRvtbt9Uzw0ZIZLllXtgN6U5GexzVlvpQyij2YS1XfC1LHWGIbGuseIfil9X7dmZr6i ZcKNMuzqgibOPbZuwBJcg8ATQgNFKTs= ARC-Authentication-Results: i=1; imf10.hostedemail.com; dkim=pass header.d=efficios.com header.s=smtpout1 header.b=fpefWz2O; dmarc=pass (policy=none) header.from=efficios.com; spf=pass (imf10.hostedemail.com: domain of mathieu.desnoyers@efficios.com designates 158.69.130.18 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=1766022336; 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=eSepe1r3J5AkA/ggHcrTZYYdvp9Ci1dmASMa/AW6Syo=; b=bK4q1FPLu3ujXC7WbVsxIk0B+SXFHx2yz7KQgl53FgsHlecm11ulm0SoPuLg/MXttf8IgO U3Hgkga3cFHmGRnKddZCoENwMVK52d19BH7zKcSEpGdCgR/tyi/RbO/Jn9bf3LvAV5lfIJ GD2xYbiIj/9j3zP9WantbzmnOwYMI9Q= DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=efficios.com; s=smtpout1; t=1766022335; bh=eSepe1r3J5AkA/ggHcrTZYYdvp9Ci1dmASMa/AW6Syo=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=fpefWz2OOWYhr17qhvXdJUHCSZlnkKeCwwFJI/K2lhYX9vBN17F0aiYUo7GSpMVxO ppO5B8VBOtTk7DNbQJ5QyFNUCsRHRxKbglk2Xy6mbDY77Xtby95tMV7tlnGqc4K+Oi qdJ6j9RZmslbWmQBmcu17X5PYsn/YKzDY6P4wXKgOLuZOiSqqx/sryvCAJLYsQPh+T hPJua80/8rB8rElwDvyNPo67qMQ6mhbVVJ/OlUcPjhclzwdporTHPm8jgMW1AaMU6l YS5rUv9aN/dSNW+MUNZixj6eYpamFotey5l0QVhzmJRafWDa99MNPsC7gnZ3xgmxeG Vn2WW1vgksAMA== Received: from thinkos.internal.efficios.com (unknown [IPv6:2606:6d00:100:4000:a253:d09e:90e7:323f]) by smtpout.efficios.com (Postfix) with ESMTPSA id 4dWtlz3Z4MzbxM; Wed, 17 Dec 2025 20:45:35 -0500 (EST) From: Mathieu Desnoyers To: Boqun Feng , Joel Fernandes , "Paul E. McKenney" Cc: linux-kernel@vger.kernel.org, Mathieu Desnoyers , Nicholas Piggin , Michael Ellerman , Greg Kroah-Hartman , Sebastian Andrzej Siewior , Will Deacon , Peter Zijlstra , Alan Stern , John Stultz , Neeraj Upadhyay , Linus Torvalds , Andrew Morton , Frederic Weisbecker , 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 v4 1/4] compiler.h: Introduce ptr_eq() to preserve address dependency Date: Wed, 17 Dec 2025 20:45:28 -0500 Message-Id: <20251218014531.3793471-2-mathieu.desnoyers@efficios.com> X-Mailer: git-send-email 2.39.5 In-Reply-To: <20251218014531.3793471-1-mathieu.desnoyers@efficios.com> References: <20251218014531.3793471-1-mathieu.desnoyers@efficios.com> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Rspamd-Queue-Id: 65E48C0008 X-Stat-Signature: 5bx48ux6tqa7s8qdo1rs6d4cfqxg3b4d X-Rspam-User: X-Rspamd-Server: rspam06 X-HE-Tag: 1766022336-915386 X-HE-Meta: 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 GQh4WcLu 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 9MO59o+J Z4vxGddBg1BKsTcotzvRIjtid+YinLgvzFr13t36EQ6/sL+tLZ6eTV0aroiXo1HrmIym6pH3vMCdSowk7FxhcMi/14gqjsPBEvQrHn3iMmV1gLaU7vaykp1tqWDRWZRJ/8MQY+NOi5Sqk5CT4iCT8VxzijbtRX6vWTns+yD198Pk8GqqRsXTvYmR7kcPvqPKeEyuBBcgnaM= 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 5b45ea7dff3e..c5ca3b54c112 100644 --- a/include/linux/compiler.h +++ b/include/linux/compiler.h @@ -163,6 +163,69 @@ 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__) /** -- 2.39.5