From patchwork Thu Aug 3 03:24:06 2023 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Joel Fernandes X-Patchwork-Id: 13339208 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 vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 43FEDC001DF for ; Thu, 3 Aug 2023 03:24:26 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233244AbjHCDYY (ORCPT ); Wed, 2 Aug 2023 23:24:24 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:50304 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233159AbjHCDYR (ORCPT ); Wed, 2 Aug 2023 23:24:17 -0400 Received: from mail-io1-xd35.google.com (mail-io1-xd35.google.com [IPv6:2607:f8b0:4864:20::d35]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id E355A2733 for ; Wed, 2 Aug 2023 20:24:13 -0700 (PDT) Received: by mail-io1-xd35.google.com with SMTP id ca18e2360f4ac-7835c92c373so41698539f.0 for ; Wed, 02 Aug 2023 20:24:13 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=joelfernandes.org; s=google; t=1691033053; x=1691637853; h=content-transfer-encoding:mime-version:message-id:date:subject:cc :to:from:from:to:cc:subject:date:message-id:reply-to; bh=ri+AV8Kf+G5whax+bk9HTGfLE0fpaHCunlBX3LcI+MY=; b=SSROfGyqV9e3u74mTKmoa9swo8SFey1Lh0VpFbL1JE70n1rclCj8tC30xw4OMGN2vB +IlNzwUN30w4Nw4O0wOw4zYh/3Z5xy5EI6IPgTWrmkRvfYDNsjzL6uSm3a0h4mlH2GOM 10aW6NWacR4dbgGPHJmVb5bJhYKUm3LEhZML0= X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20221208; t=1691033053; x=1691637853; h=content-transfer-encoding:mime-version:message-id:date:subject:cc :to:from:x-gm-message-state:from:to:cc:subject:date:message-id :reply-to; bh=ri+AV8Kf+G5whax+bk9HTGfLE0fpaHCunlBX3LcI+MY=; b=jpyFJTfzPnSN1+CnYDFFvTvwMlCX4Gn5nMLr+P99pf+gAALXmP+HWif0xPdrPQB4RC dO1NBh78M5Fh8mcW9rk+ISF0gNJnESSRIVmye63vtG7VLSbxXsgIR/t3w2IVvV7To/Q8 3JpaowgVYjb5zd1UXOgUXxKWd5iOi0udIrWsZqYyeC7FrXcRzIZadKJze6yEA9O8FVaY KRH7UdTDu0uxd6R+moBi538HW4KYuTnZl2i3FMgsabS5H7zRUQM/HryRRqrVg3op2UfI LBWOkktC648qy6NlN1uje4ItSA7qm7y5HsHbB0kZKznIx1y5jsDF9PAaRQjyHzAp5QeF hbSg== X-Gm-Message-State: ABy/qLYYP0IcxDDp+cVNWv+BdUdqV/icQiYx8B3TNs64EaP3amZbieKi y7At5BT9o6bfsurmW/Ha27+7xw== X-Google-Smtp-Source: APBJJlHbI5KfWmA5QewNsyrWaZR6V3ElyXs1v2enBL4DptJM5Dt3K9W5sbejVgaJWYiVHbEr+1qIJQ== X-Received: by 2002:a05:6602:2559:b0:785:cac9:2d49 with SMTP id cg25-20020a056602255900b00785cac92d49mr14841576iob.1.1691033053197; Wed, 02 Aug 2023 20:24:13 -0700 (PDT) Received: from joelboxx5.c.googlers.com.com (156.190.123.34.bc.googleusercontent.com. [34.123.190.156]) by smtp.gmail.com with ESMTPSA id a14-20020a02ac0e000000b0042b67b12363sm4535176jao.37.2023.08.02.20.24.11 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 02 Aug 2023 20:24:12 -0700 (PDT) From: "Joel Fernandes (Google)" To: linux-kernel@vger.kernel.org, rcu@vger.kernel.org Cc: "Joel Fernandes (Google)" , Will Deacon , "Paul E. McKenney" , Frederic Weisbecker , Neeraj Upadhyay , Josh Triplett , Boqun Feng , Steven Rostedt , Mathieu Desnoyers , Lai Jiangshan , Zqiang , Jonathan Corbet Subject: [PATCH 1/2] docs: rcu: Add cautionary note on plain-accesses to requirements Date: Thu, 3 Aug 2023 03:24:06 +0000 Message-ID: <20230803032408.2514989-1-joel@joelfernandes.org> X-Mailer: git-send-email 2.41.0.585.gd2178a4bd4-goog MIME-Version: 1.0 Precedence: bulk List-ID: X-Mailing-List: rcu@vger.kernel.org Add a detailed note to explain the potential side effects of plain-accessing the gp pointer using a plain load, without using the rcu_dereference() macros; which might trip neighboring code that does use rcu_dereference(). I haven't verified this with a compiler, but this is what I gather from the below link using Will's experience with READ_ONCE(). Link: https://lore.kernel.org/all/20230728124412.GA21303@willie-the-truck/ Cc: Will Deacon Signed-off-by: Joel Fernandes (Google) --- .../RCU/Design/Requirements/Requirements.rst | 32 +++++++++++++++++++ 1 file changed, 32 insertions(+) diff --git a/Documentation/RCU/Design/Requirements/Requirements.rst b/Documentation/RCU/Design/Requirements/Requirements.rst index f3b605285a87..e0b896d3fb9b 100644 --- a/Documentation/RCU/Design/Requirements/Requirements.rst +++ b/Documentation/RCU/Design/Requirements/Requirements.rst @@ -376,6 +376,38 @@ mechanism, most commonly locking or reference counting .. |high-quality implementation of C11 memory_order_consume [PDF]| replace:: high-quality implementation of C11 ``memory_order_consume`` [PDF] .. _high-quality implementation of C11 memory_order_consume [PDF]: http://www.rdrop.com/users/paulmck/RCU/consume.2015.07.13a.pdf +Note that, there can be strange side effects (due to compiler optimizations) if +``gp`` is ever accessed using a plain load (i.e. without ``READ_ONCE()`` or +``rcu_dereference()``) potentially hurting any succeeding +``rcu_dereference()``. For example, consider the code: + + :: + + 1 bool do_something_gp(void) + 2 { + 3 void *tmp; + 4 rcu_read_lock(); + 5 tmp = gp; // Plain-load of GP. + 6 printk("Point gp = %p\n", tmp); + 7 + 8 p = rcu_dereference(gp); + 9 if (p) { + 10 do_something(p->a, p->b); + 11 rcu_read_unlock(); + 12 return true; + 13 } + 14 rcu_read_unlock(); + 15 return false; + 16 } + +The behavior of plain accesses involved in a data race is non-deterministic in +the face of compiler optimizations. Since accesses to the ``gp`` pointer is +by-design a data race, the compiler could trip this code by caching the value +of ``gp`` into a register in line 5, and then using the value of the register +to satisfy the load in line 10. Thus it is important to never mix +plain accesses of a memory location with rcu_dereference() of the same memory +location, in code involved in a data race. + In short, updaters use rcu_assign_pointer() and readers use rcu_dereference(), and these two RCU API elements work together to ensure that readers have a consistent view of newly added data elements. From patchwork Thu Aug 3 03:24:07 2023 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Joel Fernandes X-Patchwork-Id: 13339209 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 vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 9AB74C04A6A for ; Thu, 3 Aug 2023 03:24:28 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233185AbjHCDY0 (ORCPT ); Wed, 2 Aug 2023 23:24:26 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:50264 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230292AbjHCDYR (ORCPT ); Wed, 2 Aug 2023 23:24:17 -0400 Received: from mail-yb1-xb2d.google.com (mail-yb1-xb2d.google.com [IPv6:2607:f8b0:4864:20::b2d]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id A42BC2D68 for ; Wed, 2 Aug 2023 20:24:15 -0700 (PDT) Received: by mail-yb1-xb2d.google.com with SMTP id 3f1490d57ef6-c2cf4e61bc6so483255276.3 for ; Wed, 02 Aug 2023 20:24:15 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=joelfernandes.org; s=google; t=1691033055; x=1691637855; h=content-transfer-encoding:mime-version:references:in-reply-to :message-id:date:subject:cc:to:from:from:to:cc:subject:date :message-id:reply-to; bh=VMHqujoTuT0RsHzRdVhgkyPvYcb4Kgnj2rbEthA3RMM=; b=jmOMCg3aY1NdBWQ0E/xfB3w00lqf6WPH+O3dT/+EXyF79Cpvo08Pb1daWoPjBSPZT6 bqocWqfWMdrhrDRwxYzLemMLfNLaTPpNFlqm6gyTOU4N5ITwyrd3vSnmCCD0lcK8XGzR 5WD5DZc70Rq0qy8ulzg0neW+ZH1ryoSqFeF8A= X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20221208; t=1691033055; x=1691637855; h=content-transfer-encoding:mime-version:references:in-reply-to :message-id:date:subject:cc:to:from:x-gm-message-state:from:to:cc :subject:date:message-id:reply-to; bh=VMHqujoTuT0RsHzRdVhgkyPvYcb4Kgnj2rbEthA3RMM=; b=CfDRGskctsR5sA3cgbS5wKEp/L6tfrk91B4jcZXOISqc5mC2CagdQn4CNoLi5ip+0m Jb6OPE3SPc8M+OAAPkSSFBiOrR1LcJHTQOqxKD20jUkrSWY+xKOvGBvkp+RtsXH6B/VG 1dQao6ogKaTPbi+5MF/9Cu0PGhcIDHYD0pYgD1vcRrm8dGcZqbl8TNnMMHy071Ky+iXh HlJlUwefEwjqnD13PLr2dKDXh+9E2LHIx4KgC9wpKyaMXIMRKq1JIKVwVT9e0/yHtonJ vjsNb8QQ0/1JnyNpcKSr0xz6trI2pLGwQhANEoB8SRWah8kNqwWX1+7AQCLPn4RPQt5v sEtA== X-Gm-Message-State: ABy/qLaludxlS1F/HTOz8YiUA2J2/XNABL7wI8XW8/E7l5nG/tbeM83L kLqBXXsNJlsqf3vPwLa+tjqTdQ== X-Google-Smtp-Source: APBJJlH4GRD9/jRuKtgSZb05B9hoqbfTLgpOFvSbSSnZQlEtSIPmV07jQp3NDVVXj0pcInCGMYURSg== X-Received: by 2002:a25:d401:0:b0:d15:a265:4c43 with SMTP id m1-20020a25d401000000b00d15a2654c43mr18622880ybf.61.1691033054767; Wed, 02 Aug 2023 20:24:14 -0700 (PDT) Received: from joelboxx5.c.googlers.com.com (156.190.123.34.bc.googleusercontent.com. [34.123.190.156]) by smtp.gmail.com with ESMTPSA id a14-20020a02ac0e000000b0042b67b12363sm4535176jao.37.2023.08.02.20.24.13 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 02 Aug 2023 20:24:13 -0700 (PDT) From: "Joel Fernandes (Google)" To: linux-kernel@vger.kernel.org, rcu@vger.kernel.org Cc: "Joel Fernandes (Google)" , Alan Stern , Andrea Parri , Will Deacon , Peter Zijlstra , Boqun Feng , Nicholas Piggin , David Howells , Jade Alglave , Luc Maranget , "Paul E. McKenney" , Akira Yokosawa , Daniel Lustig , Jonathan Corbet Subject: [PATCH 2/2] docs: memory-barriers: Add note on plain-accesses to address-dependency barriers Date: Thu, 3 Aug 2023 03:24:07 +0000 Message-ID: <20230803032408.2514989-2-joel@joelfernandes.org> X-Mailer: git-send-email 2.41.0.585.gd2178a4bd4-goog In-Reply-To: <20230803032408.2514989-1-joel@joelfernandes.org> References: <20230803032408.2514989-1-joel@joelfernandes.org> MIME-Version: 1.0 Precedence: bulk List-ID: X-Mailing-List: rcu@vger.kernel.org The compiler has the ability to cause misordering by destroying address-dependency barriers if comparison operations are used. Add a note about this to memory-barriers.txt and point to rcu-dereference.rst for more information. Signed-off-by: Joel Fernandes (Google) Signed-off-by: Joel Fernandes (Google) --- Documentation/memory-barriers.txt | 5 +++++ 1 file changed, 5 insertions(+) diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt index 06e14efd8662..acc8ec5ce563 100644 --- a/Documentation/memory-barriers.txt +++ b/Documentation/memory-barriers.txt @@ -435,6 +435,11 @@ Memory barriers come in four basic varieties: variables such as READ_ONCE() and rcu_dereference() provide implicit address-dependency barriers. + [!] Note that address dependency barriers can be destroyed by comparison + of a pointer obtained by a marked accessor such as READ_ONCE() or + rcu_dereference() with some value. For an example of this, see + rcu_dereference.rst (part where the comparison of pointers is discussed). + (3) Read (or load) memory barriers. A read barrier is an address-dependency barrier plus a guarantee that all