From patchwork Tue Nov 30 11:44:17 2021 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Marco Elver X-Patchwork-Id: 12646953 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 E3F27C433EF for ; Tue, 30 Nov 2021 11:45:33 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S241066AbhK3Lsv (ORCPT ); Tue, 30 Nov 2021 06:48:51 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:44966 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S240986AbhK3Lsp (ORCPT ); Tue, 30 Nov 2021 06:48:45 -0500 Received: from mail-wm1-x349.google.com (mail-wm1-x349.google.com [IPv6:2a00:1450:4864:20::349]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 7B6BCC061748 for ; Tue, 30 Nov 2021 03:45:26 -0800 (PST) Received: by mail-wm1-x349.google.com with SMTP id 144-20020a1c0496000000b003305ac0e03aso13597293wme.8 for ; Tue, 30 Nov 2021 03:45:26 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20210112; h=date:in-reply-to:message-id:mime-version:references:subject:from:to :cc; bh=rG1xNdPiM+b/19SfERFIEiLMABj+g1AA81VAk56ezfY=; b=P81cETHjfxAPwcXjxLEZoAKIClsMw0V0wD96HhwxcWsW+J35pjN4VZotJ0h51Pevyb G+wsHCsj6PaNbXpEzOb+mwsLQcl2U089MyHL5XWXa0CuIbGa89sbqWRlYWYT+qYvMhrF OQdjMifbE8i9BvuU784f2inF6Z6ZkIaBuprMWhQC/UYSsgI3wtWUrwHOwIa/Leqtefqq sJWWJfRoQx4l/22qMAsurBMJur+PevDjruCtLeW8hjawcRo+ERNcdhJWQv9EgGpMpM5H QtYhGhCW2Is32Dsg/YCLDxGCqA2Rh3TbFJUS8FeoHPV7CTCKP6M5fnXqNhK7KJioPWII h1CQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=x-gm-message-state:date:in-reply-to:message-id:mime-version :references:subject:from:to:cc; bh=rG1xNdPiM+b/19SfERFIEiLMABj+g1AA81VAk56ezfY=; b=LVxCR8lUtqxbHV2taCd1FWUGS4twnrU+X149jLMd/skx6bw4HDi2pC/R8NQIhtAJEq j++Z/zzB2AIp4zCz+al56VuZJEngqoAHd96INGZvX5OPhug2herik5vrPeqDlm13OAH8 aKdXb4kStQOB04A08N4Ezmf+4E4zey0KSG+Z/wX9LlZdtu/MZ/+zXdtKMeI+rgW/gJB1 7prDSFuk2rFTtlV9uxMoer0WQ1ez7E3j+pwwLB84AqHrlP1XIL3Re4xvfWi9MmUqped/ jwv5+2oJ4qUewion/zNhjK4sxOi1KHoIUzoHCReqNUIlyl1fDUDASjjmonrAeWgw3PO9 7Ctw== X-Gm-Message-State: AOAM532Nrm43XqRvQJMjequk602UAclIjzTi8iwm58TTP0rMFeeY33FA 9bntpONHVSNU6mFA3vd7UEpBOQtUeg== X-Google-Smtp-Source: ABdhPJxbkloraurf0oEulH3ag6+7yY6BzRbHK2TeZCzdaSdJW7S6qqAtdeXCdcM3HaCBmlZIxfVSP6YzOA== X-Received: from elver.muc.corp.google.com ([2a00:79e0:15:13:86b7:11e9:7797:99f0]) (user=elver job=sendgmr) by 2002:adf:f0c8:: with SMTP id x8mr41133135wro.290.1638272725050; Tue, 30 Nov 2021 03:45:25 -0800 (PST) Date: Tue, 30 Nov 2021 12:44:17 +0100 In-Reply-To: <20211130114433.2580590-1-elver@google.com> Message-Id: <20211130114433.2580590-10-elver@google.com> Mime-Version: 1.0 References: <20211130114433.2580590-1-elver@google.com> X-Mailer: git-send-email 2.34.0.rc2.393.gf8c9666880-goog Subject: [PATCH v3 09/25] kcsan: Document modeling of weak memory From: Marco Elver To: elver@google.com, "Paul E. McKenney" Cc: Alexander Potapenko , Boqun Feng , Borislav Petkov , Dmitry Vyukov , Ingo Molnar , Mark Rutland , Peter Zijlstra , Thomas Gleixner , Waiman Long , Will Deacon , kasan-dev@googlegroups.com, linux-arch@vger.kernel.org, linux-doc@vger.kernel.org, linux-kbuild@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, llvm@lists.linux.dev, x86@kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kbuild@vger.kernel.org Document how KCSAN models a subset of weak memory and the subset of missing memory barriers it can detect as a result. Signed-off-by: Marco Elver --- v2: * Note the reason that address or control dependencies do not require special handling. --- Documentation/dev-tools/kcsan.rst | 76 +++++++++++++++++++++++++------ 1 file changed, 63 insertions(+), 13 deletions(-) diff --git a/Documentation/dev-tools/kcsan.rst b/Documentation/dev-tools/kcsan.rst index 7db43c7c09b8..3ae866dcc924 100644 --- a/Documentation/dev-tools/kcsan.rst +++ b/Documentation/dev-tools/kcsan.rst @@ -204,17 +204,17 @@ Ultimately this allows to determine the possible executions of concurrent code, and if that code is free from data races. KCSAN is aware of *marked atomic operations* (``READ_ONCE``, ``WRITE_ONCE``, -``atomic_*``, etc.), but is oblivious of any ordering guarantees and simply -assumes that memory barriers are placed correctly. In other words, KCSAN -assumes that as long as a plain access is not observed to race with another -conflicting access, memory operations are correctly ordered. - -This means that KCSAN will not report *potential* data races due to missing -memory ordering. Developers should therefore carefully consider the required -memory ordering requirements that remain unchecked. If, however, missing -memory ordering (that is observable with a particular compiler and -architecture) leads to an observable data race (e.g. entering a critical -section erroneously), KCSAN would report the resulting data race. +``atomic_*``, etc.), and a subset of ordering guarantees implied by memory +barriers. With ``CONFIG_KCSAN_WEAK_MEMORY=y``, KCSAN models load or store +buffering, and can detect missing ``smp_mb()``, ``smp_wmb()``, ``smp_rmb()``, +``smp_store_release()``, and all ``atomic_*`` operations with equivalent +implied barriers. + +Note, KCSAN will not report all data races due to missing memory ordering, +specifically where a memory barrier would be required to prohibit subsequent +memory operation from reordering before the barrier. Developers should +therefore carefully consider the required memory ordering requirements that +remain unchecked. Race Detection Beyond Data Races -------------------------------- @@ -268,6 +268,56 @@ marked operations, if all accesses to a variable that is accessed concurrently are properly marked, KCSAN will never trigger a watchpoint and therefore never report the accesses. +Modeling Weak Memory +~~~~~~~~~~~~~~~~~~~~ + +KCSAN's approach to detecting data races due to missing memory barriers is +based on modeling access reordering (with ``CONFIG_KCSAN_WEAK_MEMORY=y``). +Each plain memory access for which a watchpoint is set up, is also selected for +simulated reordering within the scope of its function (at most 1 in-flight +access). + +Once an access has been selected for reordering, it is checked along every +other access until the end of the function scope. If an appropriate memory +barrier is encountered, the access will no longer be considered for simulated +reordering. + +When the result of a memory operation should be ordered by a barrier, KCSAN can +then detect data races where the conflict only occurs as a result of a missing +barrier. Consider the example:: + + int x, flag; + void T1(void) + { + x = 1; // data race! + WRITE_ONCE(flag, 1); // correct: smp_store_release(&flag, 1) + } + void T2(void) + { + while (!READ_ONCE(flag)); // correct: smp_load_acquire(&flag) + ... = x; // data race! + } + +When weak memory modeling is enabled, KCSAN can consider ``x`` in ``T1`` for +simulated reordering. After the write of ``flag``, ``x`` is again checked for +concurrent accesses: because ``T2`` is able to proceed after the write of +``flag``, a data race is detected. With the correct barriers in place, ``x`` +would not be considered for reordering after the proper release of ``flag``, +and no data race would be detected. + +Deliberate trade-offs in complexity but also practical limitations mean only a +subset of data races due to missing memory barriers can be detected. With +currently available compiler support, the implementation is limited to modeling +the effects of "buffering" (delaying accesses), since the runtime cannot +"prefetch" accesses. Also recall that watchpoints are only set up for plain +accesses, and the only access type for which KCSAN simulates reordering. This +means reordering of marked accesses is not modeled. + +A consequence of the above is that acquire operations do not require barrier +instrumentation (no prefetching). Furthermore, marked accesses introducing +address or control dependencies do not require special handling (the marked +access cannot be reordered, later dependent accesses cannot be prefetched). + Key Properties ~~~~~~~~~~~~~~ @@ -290,8 +340,8 @@ Key Properties 4. **Detects Racy Writes from Devices:** Due to checking data values upon setting up watchpoints, racy writes from devices can also be detected. -5. **Memory Ordering:** KCSAN is *not* explicitly aware of the LKMM's ordering - rules; this may result in missed data races (false negatives). +5. **Memory Ordering:** KCSAN is aware of only a subset of LKMM ordering rules; + this may result in missed data races (false negatives). 6. **Analysis Accuracy:** For observed executions, due to using a sampling strategy, the analysis is *unsound* (false negatives possible), but aims to