From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.2 (2018-09-13) on archive.lwn.net X-Spam-Level: X-Spam-Status: No, score=-13.3 required=5.0 tests=DKIMWL_WL_MED,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,RCVD_IN_DNSWL_HI,SPF_HELO_NONE,SPF_NONE, USER_IN_DEF_DKIM_WL autolearn=unavailable autolearn_force=no version=3.4.2 Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by archive.lwn.net (Postfix) with ESMTP id 666C77E288 for ; Thu, 14 Nov 2019 18:04:51 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727257AbfKNSEn (ORCPT ); Thu, 14 Nov 2019 13:04:43 -0500 Received: from mail-vk1-f201.google.com ([209.85.221.201]:52520 "EHLO mail-vk1-f201.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727468AbfKNSEW (ORCPT ); Thu, 14 Nov 2019 13:04:22 -0500 Received: by mail-vk1-f201.google.com with SMTP id x1so2944766vkc.19 for ; Thu, 14 Nov 2019 10:04:21 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20161025; h=date:in-reply-to:message-id:mime-version:references:subject:from:to :cc; bh=A8ALfglUAafCNqhIvaCjHjdAel0vCMD5ZtmY0/3TtjA=; b=mJoHEAo5pqc+qK89IBDOQOcVkVYj619MJaPZ705KzX7T9U8iHezXczplW4ubbIxVdM 84nMLUV7d4T4EVNSQTS2UgACgz40aa3H48N9mrK02ROhyHQgvmtLZJ+xQoaiCe9PXjVg uLAw3gL1JH5FRJ4tQXR1oHyfqfEt0GvvVH4yzLo+x1znGqmMUJFXYrXTA5TlwSVD+r5x N14ZvbEDJ9qialK2dhrbfOIsBSTOF3zr0Bp3ndHbSaf30aYV2Ri+II3D010YXiH8t0fV vnyCC7WhuaFr7Tvtz7eCjw3hEHqmO8miwf5UG27RLIZJpONke9XpSy5sOlk+sbbzAPFl 6Ijw== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:in-reply-to:message-id:mime-version :references:subject:from:to:cc; bh=A8ALfglUAafCNqhIvaCjHjdAel0vCMD5ZtmY0/3TtjA=; b=bdFxlrr2xK0JmpHznhU3/Xy/IQgFPmvcPjxYU/sL3c4SDcOe9QgWaBk/sX5I09Yb6O hGVD5WrTlTP8Jn6EULDUnY8khV8DCdj1OuuzQ++NwD/sCxuS8c4jmyk5OJCHqnkUpkOo uzGzdBpWvdcPQkZ/PDnGzbpNLB094vkqqGCjXvDwXjmCaaTzCZCV2ApvlPne6IPFkVp3 vqIVqX2zDacBJ91TsWFyjqMY+Y7W2poUSAEL2KiZBfW0h0ohqYY6vpEr7EOACxUtvZJG vhRgAPf+WaV6cylM4Gz8ltaJN4sANLKTOZfk6hQV9bRS8IN9zjUA9jTILp/qU7GKDn5o W4Kw== X-Gm-Message-State: APjAAAWSZQY5W2IJcIQqjFRL1kve4GVQXr0VcsGH7Q78iZB1oQFGhc+x 6kbl5Ure4qy9v0s4uL4Bo84j8/T7Ow== X-Google-Smtp-Source: APXvYqw7JnmHN+nEWhLp7FzaV0lkDhpgCLsWCQoT4EFYL/l9Bdo0fKwoOXqxtktyO21eG3K2d3sQq29XBQ== X-Received: by 2002:ab0:2395:: with SMTP id b21mr6064758uan.122.1573754660635; Thu, 14 Nov 2019 10:04:20 -0800 (PST) Date: Thu, 14 Nov 2019 19:03:00 +0100 In-Reply-To: <20191114180303.66955-1-elver@google.com> Message-Id: <20191114180303.66955-8-elver@google.com> Mime-Version: 1.0 References: <20191114180303.66955-1-elver@google.com> X-Mailer: git-send-email 2.24.0.rc1.363.gb1bccd3e3d-goog Subject: [PATCH v4 07/10] seqlock: Require WRITE_ONCE surrounding raw_seqcount_barrier From: Marco Elver To: elver@google.com Cc: akiyks@gmail.com, stern@rowland.harvard.edu, glider@google.com, parri.andrea@gmail.com, andreyknvl@google.com, luto@kernel.org, ard.biesheuvel@linaro.org, arnd@arndb.de, boqun.feng@gmail.com, bp@alien8.de, dja@axtens.net, dlustig@nvidia.com, dave.hansen@linux.intel.com, dhowells@redhat.com, dvyukov@google.com, hpa@zytor.com, mingo@redhat.com, j.alglave@ucl.ac.uk, joel@joelfernandes.org, corbet@lwn.net, jpoimboe@redhat.com, luc.maranget@inria.fr, mark.rutland@arm.com, npiggin@gmail.com, paulmck@kernel.org, peterz@infradead.org, tglx@linutronix.de, will@kernel.org, edumazet@google.com, kasan-dev@googlegroups.com, linux-arch@vger.kernel.org, linux-doc@vger.kernel.org, linux-efi@vger.kernel.org, linux-kbuild@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, x86@kernel.org Content-Type: text/plain; charset="UTF-8" Sender: linux-doc-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-doc@vger.kernel.org This patch proposes to require marked atomic accesses surrounding raw_write_seqcount_barrier. We reason that otherwise there is no way to guarantee propagation nor atomicity of writes before/after the barrier [1]. For example, consider the compiler tears stores either before or after the barrier; in this case, readers may observe a partial value, and because readers are unaware that writes are going on (writes are not in a seq-writer critical section), will complete the seq-reader critical section while having observed some partial state. [1] https://lwn.net/Articles/793253/ This came up when designing and implementing KCSAN, because KCSAN would flag these accesses as data-races. After careful analysis, our reasoning as above led us to conclude that the best thing to do is to propose an amendment to the raw_seqcount_barrier usage. Signed-off-by: Marco Elver Acked-by: Paul E. McKenney --- v3: * Add missing comment that was in preceding seqlock patch. --- include/linux/seqlock.h | 11 +++++++++-- 1 file changed, 9 insertions(+), 2 deletions(-) diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h index 61232bc223fd..f52c91be8939 100644 --- a/include/linux/seqlock.h +++ b/include/linux/seqlock.h @@ -265,6 +265,13 @@ static inline void raw_write_seqcount_end(seqcount_t *s) * usual consistency guarantee. It is one wmb cheaper, because we can * collapse the two back-to-back wmb()s. * + * Note that, writes surrounding the barrier should be declared atomic (e.g. + * via WRITE_ONCE): a) to ensure the writes become visible to other threads + * atomically, avoiding compiler optimizations; b) to document which writes are + * meant to propagate to the reader critical section. This is necessary because + * neither writes before and after the barrier are enclosed in a seq-writer + * critical section that would ensure readers are aware of ongoing writes. + * * seqcount_t seq; * bool X = true, Y = false; * @@ -284,11 +291,11 @@ static inline void raw_write_seqcount_end(seqcount_t *s) * * void write(void) * { - * Y = true; + * WRITE_ONCE(Y, true); * * raw_write_seqcount_barrier(seq); * - * X = false; + * WRITE_ONCE(X, false); * } */ static inline void raw_write_seqcount_barrier(seqcount_t *s) -- 2.24.0.rc1.363.gb1bccd3e3d-goog