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=-13.1 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,INCLUDES_PATCH,MAILING_LIST_MULTI, SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS,USER_AGENT_GIT autolearn=unavailable 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 6DC24C433E8 for ; Fri, 10 Jul 2020 16:52:48 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 4C32C207BB for ; Fri, 10 Jul 2020 16:52:48 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1594399968; bh=ioP+zE3QcZYWtrbP8JNe5R3MHrpHsHsJaawhzRfRFq4=; h=From:To:Cc:Subject:Date:In-Reply-To:References:List-ID:From; b=T7SIFVN2iIIFDnUm47MLSIFVRlLHkvi4tcwfMsM9B1ChnlqVgwOJ5RgwLqD0CtEoj B1+MYAjPVEnjITCr/wlh94y8Mn4yj9YjcCYS6hPmjQn/c1JpLoHefckzwcrGHk8EU+ PXQyDa7Ckov10fX9yFROwI37JzLiTH0IYKhUx6Oc= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728448AbgGJQwq (ORCPT ); Fri, 10 Jul 2020 12:52:46 -0400 Received: from mail.kernel.org ([198.145.29.99]:59660 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728415AbgGJQwk (ORCPT ); Fri, 10 Jul 2020 12:52:40 -0400 Received: from localhost.localdomain (236.31.169.217.in-addr.arpa [217.169.31.236]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 9355D206F4; Fri, 10 Jul 2020 16:52:36 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1594399960; bh=ioP+zE3QcZYWtrbP8JNe5R3MHrpHsHsJaawhzRfRFq4=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=olt02LKeAL91nPxgaC1qQAJZnrzmwBteJROoi562TZOoKlwf2GsHdueJYh9wQvHS3 ta9HVOXq+Jp+Nl1VoxyEu4NOLAeCoB6OaLnJ5Iop/QE1Nmg6joEiitmdW4N8HXMUXb PbP/PPuekBo452bRN9Tvaf8qj1qEbhU6Ap3dq+mo= From: Will Deacon To: linux-kernel@vger.kernel.org Cc: Will Deacon , Joel Fernandes , Sami Tolvanen , Nick Desaulniers , Kees Cook , Marco Elver , "Paul E. McKenney" , Matt Turner , Ivan Kokshaysky , Richard Henderson , Peter Zijlstra , Alan Stern , "Michael S. Tsirkin" , Jason Wang , Arnd Bergmann , Boqun Feng , Catalin Marinas , Mark Rutland , linux-arm-kernel@lists.infradead.org, linux-alpha@vger.kernel.org, virtualization@lists.linux-foundation.org, kernel-team@android.com Subject: [PATCH v3 07/19] vhost: Remove redundant use of read_barrier_depends() barrier Date: Fri, 10 Jul 2020 17:51:51 +0100 Message-Id: <20200710165203.31284-8-will@kernel.org> X-Mailer: git-send-email 2.20.1 In-Reply-To: <20200710165203.31284-1-will@kernel.org> References: <20200710165203.31284-1-will@kernel.org> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Since commit 76ebbe78f739 ("locking/barriers: Add implicit smp_read_barrier_depends() to READ_ONCE()"), there is no need to use smp_read_barrier_depends() outside of the Alpha architecture code. Unfortunately, there is precisely _one_ user in the vhost code, and there isn't an obvious READ_ONCE() access making the barrier redundant. However, on closer inspection (thanks, Jason), it appears that vring synchronisation between the producer and consumer occurs via the 'avail_idx' field, which is followed up by an rmb() in vhost_get_vq_desc(), making the read_barrier_depends() redundant on Alpha. Jason says: | I'm also confused about the barrier here, basically in driver side | we did: | | 1) allocate pages | 2) store pages in indirect->addr | 3) smp_wmb() | 4) increase the avail idx (somehow a tail pointer of vring) | | in vhost we did: | | 1) read avail idx | 2) smp_rmb() | 3) read indirect->addr | 4) read from indirect->addr | | It looks to me even the data dependency barrier is not necessary | since we have rmb() which is sufficient for us to the correct | indirect->addr and driver are not expected to do any writing to | indirect->addr after avail idx is increased Remove the redundant barrier invocation. Suggested-by: Jason Wang Acked-by: Paul E. McKenney Signed-off-by: Will Deacon --- drivers/vhost/vhost.c | 5 ----- 1 file changed, 5 deletions(-) diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c index d7b8df3edffc..74d135ee7e26 100644 --- a/drivers/vhost/vhost.c +++ b/drivers/vhost/vhost.c @@ -2092,11 +2092,6 @@ static int get_indirect(struct vhost_virtqueue *vq, return ret; } iov_iter_init(&from, READ, vq->indirect, ret, len); - - /* We will use the result as an address to read from, so most - * architectures only need a compiler barrier here. */ - read_barrier_depends(); - count = len / sizeof desc; /* Buffers are chained via a 16 bit next field, so * we can have at most 2^16 of these. */ -- 2.27.0.383.g050319c2ae-goog