From patchwork Fri Sep 30 21:30:53 2016 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Patchwork-Submitter: =?utf-8?q?Alex_Benn=C3=A9e?= X-Patchwork-Id: 677241 Return-Path: X-Original-To: incoming@patchwork.ozlabs.org Delivered-To: patchwork-incoming@bilbo.ozlabs.org Received: from lists.gnu.org (lists.gnu.org [IPv6:2001:4830:134:3::11]) (using TLSv1 with cipher AES256-SHA (256/256 bits)) (No client certificate requested) by ozlabs.org (Postfix) with ESMTPS id 3sm4dq1PC6z9ryv for ; Sat, 1 Oct 2016 07:41:51 +1000 (AEST) Authentication-Results: ozlabs.org; dkim=fail reason="signature verification failed" (1024-bit key; unprotected) header.d=linaro.org header.i=@linaro.org header.b=R4YIqEs3; dkim-atps=neutral Received: from localhost ([::1]:47080 helo=lists.gnu.org) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1bq5Yx-0007nw-Lk for incoming@patchwork.ozlabs.org; Fri, 30 Sep 2016 17:41:47 -0400 Received: from eggs.gnu.org ([2001:4830:134:3::10]:49625) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1bq5Ou-0006Rc-DG for qemu-devel@nongnu.org; Fri, 30 Sep 2016 17:31:25 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1bq5Or-0006VG-0p for qemu-devel@nongnu.org; Fri, 30 Sep 2016 17:31:23 -0400 Received: from mail-wm0-x22f.google.com ([2a00:1450:400c:c09::22f]:37268) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1bq5Oq-0006Uu-Qg for qemu-devel@nongnu.org; Fri, 30 Sep 2016 17:31:20 -0400 Received: by mail-wm0-x22f.google.com with SMTP id b80so66254640wme.0 for ; Fri, 30 Sep 2016 14:31:20 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linaro.org; s=google; h=from:to:cc:subject:date:message-id:in-reply-to:references :mime-version:content-transfer-encoding; bh=jLt7HYWLH/IQnnOOljJswAOPr+bNIhupxEGvaKck7Gg=; b=R4YIqEs3IeiD8IMqrscxoHAqehbepZGkiPN3ToCGFf32dsIPuXIFmuKiGA4Dj0Jfwy 10IITxh6YTVH50SizBPaijPbVFsC7ypkNoOsTch80G2XTVKYw70HT6XndT1h1Kmzo2Cf 1FI1huQfceZGKCz6At1d6a3FmUDCX2J7DcrTI= X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20130820; h=x-gm-message-state:from:to:cc:subject:date:message-id:in-reply-to :references:mime-version:content-transfer-encoding; bh=jLt7HYWLH/IQnnOOljJswAOPr+bNIhupxEGvaKck7Gg=; b=c6BjLIk90FhMDh5QR7gsVUgcrTVpNu04TvNj0JGkXCPvLH8Vq3VhAZcZok/r92wPz8 9gvHKO4k3+Io8drlyMhuCjrv8IZt0i5IsSVM1amVuCSzv8skOvF1yrQxE+RG+WqK0upZ vqHsWlV20As1RWwPjR2p8gsG+BfSsDm2XOUKCkieUbOu7GERZfh6MeUjfdMnlwqtz0XY 0qGJ/RoaqtG/CodA2JW9I2CZLMWmMFntepEaLOT+2cvVCUVIwTeQLtxx1J93RMf/Tps/ xkxjcVsjuMO7mtv08Apo7yuJupgld416YUuqS2ZCqwi8garWTMdnyFffELURcUtUn0Co ynTg== X-Gm-Message-State: AA6/9Rk2s0GnLwpGUd50mu0SVAz6dJdSS3pbbUqYKYO5AC8c93/GQi3u8UKGJSZNba3DjlgB X-Received: by 10.28.109.86 with SMTP id i83mr5547105wmc.104.1475271080119; Fri, 30 Sep 2016 14:31:20 -0700 (PDT) Received: from zen.linaro.local ([81.128.185.34]) by smtp.gmail.com with ESMTPSA id r142sm1485360wme.10.2016.09.30.14.31.17 (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Fri, 30 Sep 2016 14:31:19 -0700 (PDT) Received: from zen.linaroharston (localhost [127.0.0.1]) by zen.linaro.local (Postfix) with ESMTP id 97CF03E02FE; Fri, 30 Sep 2016 22:31:17 +0100 (BST) From: =?UTF-8?q?Alex=20Benn=C3=A9e?= To: qemu-devel@nongnu.org, pbonzini@redhat.com Date: Fri, 30 Sep 2016 22:30:53 +0100 Message-Id: <20160930213106.20186-3-alex.bennee@linaro.org> X-Mailer: git-send-email 2.9.3 In-Reply-To: <20160930213106.20186-1-alex.bennee@linaro.org> References: <20160930213106.20186-1-alex.bennee@linaro.org> MIME-Version: 1.0 X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] X-Received-From: 2a00:1450:400c:c09::22f Subject: [Qemu-devel] [PATCH v3 02/15] atomic.h: comment on use of atomic_read/set X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.21 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: mttcg@listserver.greensocs.com, peter.maydell@linaro.org, claudio.fontana@huawei.com, nikunj@linux.vnet.ibm.com, jan.kiszka@siemens.com, mark.burton@greensocs.com, a.rigo@virtualopensystems.com, cota@braap.org, serge.fdrv@gmail.com, bobby.prani@gmail.com, rth@twiddle.net, =?UTF-8?q?Alex=20Benn=C3=A9e?= , fred.konrad@greensocs.com Errors-To: qemu-devel-bounces+incoming=patchwork.ozlabs.org@nongnu.org Sender: "Qemu-devel" Add some notes on the use of the relaxed atomic access helpers and their importance for defined behaviour in C11's multi-threaded memory model. Signed-off-by: Alex Bennée --- include/qemu/atomic.h | 6 ++++++ 1 file changed, 6 insertions(+) diff --git a/include/qemu/atomic.h b/include/qemu/atomic.h index c493f89..c4f6950 100644 --- a/include/qemu/atomic.h +++ b/include/qemu/atomic.h @@ -92,6 +92,12 @@ /* Weak atomic operations prevent the compiler moving other * loads/stores past the atomic operation load/store. However there is * no explicit memory barrier for the processor. + * + * The C11 memory model says that variables that are accessed from + * different threads should at least be done with __ATOMIC_RELAXED + * primitives or the result is undefined. Generally this has little to + * no effect on the generated code but not using the atomic primitives + * will get flagged by sanitizers as a violation. */ #define atomic_read(ptr) \ ({ \