From patchwork Mon Nov 4 14:27:42 2019 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Marco Elver X-Patchwork-Id: 11225943 Return-Path: Received: from mail.kernel.org (pdx-korg-mail-1.web.codeaurora.org [172.30.200.123]) by pdx-korg-patchwork-2.web.codeaurora.org (Postfix) with ESMTP id DD5711515 for ; Mon, 4 Nov 2019 14:29:32 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id BBA75222C9 for ; Mon, 4 Nov 2019 14:29:32 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b="FhPDq8Y8" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729062AbfKDO3O (ORCPT ); Mon, 4 Nov 2019 09:29:14 -0500 Received: from mail-vs1-f73.google.com ([209.85.217.73]:51430 "EHLO mail-vs1-f73.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1729009AbfKDO3L (ORCPT ); Mon, 4 Nov 2019 09:29:11 -0500 Received: by mail-vs1-f73.google.com with SMTP id l3so2777777vsd.18 for ; Mon, 04 Nov 2019 06:29:09 -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=sQU0jAVefneuTdQpIgbUF6GdSN4TcMOiQRxRVI16Vls=; b=FhPDq8Y8jgh3GaqH19Ri5l4th6SIgrfB5k6uYp6Qe1zCUALU5Q1h777JkPDXGoqtF+ rUVB4HT5dCMdMd/+svGvi1KkuFsbojwRCgfaJkdh2DPMBB53CrCa1TeF/tRilzh/U2Vv rC8CcHxh3PrEaKH9Qc4nO5dN6oULNuw2D+ebZe/2C78pc5pykjiO0Bg4leEG7yFIFNxI lT1Onq8IrfB98Hhn323WbkEF/3JHPJUZwZzMucC6nWXt4j1r7WLeMRYJuLt6zhN4vW9Q 0cZ7MGwIvE1QsZGodu2uWGo2OqoLsx5v5yIqO4x8fyNCQ7jT/uIEts0ZzNEU0DdB7OYH Ef0A== 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=sQU0jAVefneuTdQpIgbUF6GdSN4TcMOiQRxRVI16Vls=; b=may1Y6aiEm6tMPlrqqbYY1ODtPCHTixfNm+xjhN5ZGhvcNdFydiEqdA1csji7R3UKq TE43zNyphA73muETTwBz3tz8RvcJNKtEgf0psPR4YOeRhx/15a0rIV/r/lOIpR02Jhkp MP4VwJLNBUmTt8Cfm0bDin9XDNhSA7xuVVwqlUu2iUWgC01myqYtqjwuRmFL00PRqqAi 9SojoTkEpxMUTDnTj3xW0EhRHbtO2i0+Nnpti0pu72Ug/sUt0IHsszTItuLExHy4dvjc JvvdDEfK5Uv0Vr8SLjYpckPA/GCwbVuEUC8hnz8fMe5ydEVqoxk/bfbX8teZjlKPFcMr Aq5Q== X-Gm-Message-State: APjAAAWCqT44LJX6dgIrnosX3/zF1a54qcssvaLJovG4O20s6Qo/nihh 1oRmR+iBCw9kCac7/VOJoGPca44aDg== X-Google-Smtp-Source: APXvYqxZ17ci8UfBwQliZ986Aa6FaGmsNO4/Ie/63JBd/SxBBgXy0wxlK6hBzj3ifIPGT1Jnqi5jP2CEFw== X-Received: by 2002:a05:6122:2c7:: with SMTP id k7mr10982783vki.97.1572877749135; Mon, 04 Nov 2019 06:29:09 -0800 (PST) Date: Mon, 4 Nov 2019 15:27:42 +0100 In-Reply-To: <20191104142745.14722-1-elver@google.com> Message-Id: <20191104142745.14722-7-elver@google.com> Mime-Version: 1.0 References: <20191104142745.14722-1-elver@google.com> X-Mailer: git-send-email 2.24.0.rc1.363.gb1bccd3e3d-goog Subject: [PATCH v3 6/9] 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, 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 Sender: linux-kbuild-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kbuild@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 --- 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)