From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-7.7 required=3.0 tests=DKIM_ADSP_ALL,DKIM_INVALID, DKIM_SIGNED,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI, SIGNED_OFF_BY,SPF_PASS,USER_AGENT_GIT autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id CC80CC282E1 for ; Wed, 24 Apr 2019 21:54:26 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 937C920811 for ; Wed, 24 Apr 2019 21:54:26 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (1024-bit key) header.d=stbuehler.de header.i=@stbuehler.de header.b="X/OcvXly" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1731840AbfDXVyZ (ORCPT ); Wed, 24 Apr 2019 17:54:25 -0400 Received: from mail.stbuehler.de ([5.9.32.208]:41014 "EHLO mail.stbuehler.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1731763AbfDXVyZ (ORCPT ); Wed, 24 Apr 2019 17:54:25 -0400 Received: from chromobil.fritz.box (unknown [IPv6:2a02:8070:a29c:5000:823f:5dff:fe0f:b5b6]) by mail.stbuehler.de (Postfix) with ESMTPSA id 46327C030BD; Wed, 24 Apr 2019 21:54:23 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple; d=stbuehler.de; s=stbuehler1; t=1556142863; bh=j00YEKIBIYx8iSi1Y8E2AsbiGHpsyQBbnqztzAtf1uE=; h=From:To:Subject:Date:In-Reply-To:References:From; b=X/OcvXlyLV6MDawzUJ6YwlKQ9k84bUI0JnIL3yFch0mWe/Gt2DnZCJgbW5Vgk8/zE K6rNDpIXYp0txCL9Cw1eATrjana3yCnzKjac4Zrnjh6mP4JhwiKuK3d8Y7a5Fgr5Ij ij+SHikoHgGn61q1+fT9RszJTJNxgM/WzqOySGcc= From: =?UTF-8?q?Stefan=20B=C3=BChler?= To: Jens Axboe , linux-block@vger.kernel.org, linux-fsdevel@vger.kernel.org Subject: [PATCH barrier cleanup v1 3/7] io_uring: remove unnecessary barrier before reading cq head Date: Wed, 24 Apr 2019 23:54:18 +0200 Message-Id: <20190424215422.7404-3-source@stbuehler.de> X-Mailer: git-send-email 2.20.1 In-Reply-To: <20190424215422.7404-1-source@stbuehler.de> References: <54496e17-97de-9f9a-9972-c448226bb768@stbuehler.de> <20190424215422.7404-1-source@stbuehler.de> MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Sender: linux-fsdevel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org The memory operations before reading cq head are unrelated and we don't care about their order. Document that the control dependency in combination with READ_ONCE and WRITE_ONCE forms a barrier we need. Signed-off-by: Stefan Bühler --- fs/io_uring.c | 7 +++++-- 1 file changed, 5 insertions(+), 2 deletions(-) diff --git a/fs/io_uring.c b/fs/io_uring.c index 1f4419f38ef1..2c101230df71 100644 --- a/fs/io_uring.c +++ b/fs/io_uring.c @@ -431,8 +431,11 @@ static struct io_uring_cqe *io_get_cqring(struct io_ring_ctx *ctx) unsigned tail; tail = ctx->cached_cq_tail; - /* See comment at the top of the file */ - smp_rmb(); + /* + * writes to the cq entry need to come after reading head; the + * control dependency is enough as we're using WRITE_ONCE to + * fill the cq entry + */ if (tail - READ_ONCE(ring->r.head) == ring->ring_entries) return NULL; -- 2.20.1