From patchwork Tue Oct 5 10:58:51 2021 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Marco Elver X-Patchwork-Id: 12536221 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 mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 27040C433F5 for ; Tue, 5 Oct 2021 11:00:37 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 0D45861186 for ; Tue, 5 Oct 2021 11:00:37 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S234558AbhJELC0 (ORCPT ); Tue, 5 Oct 2021 07:02:26 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:52080 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S234460AbhJELBz (ORCPT ); Tue, 5 Oct 2021 07:01:55 -0400 Received: from mail-qt1-x84a.google.com (mail-qt1-x84a.google.com [IPv6:2607:f8b0:4864:20::84a]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id F4224C061786 for ; Tue, 5 Oct 2021 04:00:04 -0700 (PDT) Received: by mail-qt1-x84a.google.com with SMTP id x28-20020ac8701c000000b0029f4b940566so22756710qtm.19 for ; Tue, 05 Oct 2021 04:00:04 -0700 (PDT) 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=VR1v+uOEj5JHWgZG/AjKjsmQRdRMH76n4CGK16N25tc=; b=OOmVz5m9sgHxZ5tpr/f91NKUEC1uc9rMyBTH60E4A2rtnobKjQUKS5BS5vP0IphX4h wZwGLnUaMH/gItLkkn3k+udKE5o/pECwpdVoaNBrXEfthT0ThUaifj4RWj+cabsTGBGf l1ZHm1JhkDyaGdhfJDbvEYYUbwneLAxtPO0KzSK63NWqe69yIu5BETJMFPUjGr/reJ5p 8C70N/BQQzpJJdfVViy3y/O1XL0IgkIsY0a+Rycv8smMQBb9va/r8WGrDJttmTTfKgVt P/LE7k6AHKligqpuKS3WqrmkDoaFPkHsV8/Gm5n/MWjw9huBNrITLIWi2bSp/lTViEvC YuYg== 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=VR1v+uOEj5JHWgZG/AjKjsmQRdRMH76n4CGK16N25tc=; b=6re2icHcQWRcCs7FG673F3y8aEAkpzQzH+PMWNXCPMfy9H3x5+4qH/p9vpjKxHR8Uk 25cSpPYMY/FrMAMomyHn1ofukSC8f3qlkHvLx6kxk+sIEyDH1LLsuIq6vepmXD114MnE 8Q4UQOwTjGH+htOecsEJ6iUOizehwY1Jw+CTNw3miufyJwfXwKJoRE41qFOgcwYL9kwo +Q3U9E1forJHzUPcHyuE+5OtSH6844OR54NYn5zCEfep1qnN6+bgdYY/Vs+MtROaOi8S uLHt4wVf1kEK+gIB5wFpSf7402Garura/x4FOcTiLACUdCnkO2vgQRZRwlDjwFFdJcNY vPFg== X-Gm-Message-State: AOAM530U2IGF2hokC0s8oJejB/NpOJeQQrc+Tz6Wq36W4HtHJUbAhvEf SLrbEvusn1IkfsTx2L3wejfvPMEZaA== X-Google-Smtp-Source: ABdhPJzKD6+pLzVxtocJ8QXUwjt+bDfPEzIrOBgSPHmtyTm0gDoWxM9bjjQ9uNCP0wR03odJaPoCD9G9MA== X-Received: from elver.muc.corp.google.com ([2a00:79e0:15:13:e44f:5054:55f8:fcb8]) (user=elver job=sendgmr) by 2002:a05:6214:1022:: with SMTP id k2mr27293030qvr.53.1633431604158; Tue, 05 Oct 2021 04:00:04 -0700 (PDT) Date: Tue, 5 Oct 2021 12:58:51 +0200 In-Reply-To: <20211005105905.1994700-1-elver@google.com> Message-Id: <20211005105905.1994700-10-elver@google.com> Mime-Version: 1.0 References: <20211005105905.1994700-1-elver@google.com> X-Mailer: git-send-email 2.33.0.800.g4c38ced690-goog Subject: [PATCH -rcu/kcsan 09/23] 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 , Josh Poimboeuf , 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, 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 --- Documentation/dev-tools/kcsan.rst | 72 +++++++++++++++++++++++++------ 1 file changed, 59 insertions(+), 13 deletions(-) diff --git a/Documentation/dev-tools/kcsan.rst b/Documentation/dev-tools/kcsan.rst index 7db43c7c09b8..4fc3773fead9 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 +268,52 @@ 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. 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. Furthermore, with the currently available compiler support, the +implementation is limited to modeling the effects of "buffering" (delaying +accesses), since the runtime cannot "prefetch" accesses. One implication of +this is that acquire operations do not require barrier instrumentation. + Key Properties ~~~~~~~~~~~~~~ @@ -290,8 +336,8 @@ 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