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=-10.1 required=3.0 tests=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=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 F0D5BFA372C for ; Fri, 8 Nov 2019 17:01:46 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id C27FC20869 for ; Fri, 8 Nov 2019 17:01:46 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1573232506; bh=VVtq+ZHdbjrlTwd/SCnm4uvq7L7ueujETZx9zl4iAGc=; h=From:To:Cc:Subject:Date:In-Reply-To:References:List-ID:From; b=z5IKb6mREesLLlMkDglEm0N2BEiSOSAJ3Am8cQQDQqpv0wLAcmehKgfHiRQaNeacl CDuANPjoBYcnNnCWdcoWLtT+Wlz1i1cxrcr6BR/RLtmRYlFjOA//My3YzA6smhIobQ uc4t6HObVb400srFskGfnmbZXEzQ0LuGAl4dm03w= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730510AbfKHRBp (ORCPT ); Fri, 8 Nov 2019 12:01:45 -0500 Received: from mail.kernel.org ([198.145.29.99]:45508 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728734AbfKHRBn (ORCPT ); Fri, 8 Nov 2019 12:01:43 -0500 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 78A5F21924; Fri, 8 Nov 2019 17:01:39 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1573232502; bh=VVtq+ZHdbjrlTwd/SCnm4uvq7L7ueujETZx9zl4iAGc=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=YUa1yoEc+Oh4+7c5Af2VchosNGv2bBcCTGJQqem5RTN2ec+m0vFv2pfV/XmoM+JOY bdqQi841G7zyMtYyjENHqVGoXCClsC/RUl4ZOihf2FqGe55wW8edq0ymGNm63qG2vm SP+GQB8maD3cEt16jtgShBi8H4QwIgsDxLQnWXVY= From: Will Deacon To: linux-kernel@vger.kernel.org Cc: Will Deacon , Yunjae Lee , SeongJae Park , "Paul E. McKenney" , Josh Triplett , Matt Turner , Ivan Kokshaysky , Richard Henderson , Peter Zijlstra , Alan Stern , Michael Ellerman , "Michael S. Tsirkin" , Jason Wang , Arnd Bergmann , Joe Perches , Boqun Feng , linux-alpha@vger.kernel.org, virtualization@lists.linux-foundation.org Subject: [PATCH 04/13] vhost: Remove redundant use of read_barrier_depends() barrier Date: Fri, 8 Nov 2019 17:01:11 +0000 Message-Id: <20191108170120.22331-5-will@kernel.org> X-Mailer: git-send-email 2.20.1 In-Reply-To: <20191108170120.22331-1-will@kernel.org> References: <20191108170120.22331-1-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 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 36ca2cf419bf..865bc91b783c 100644 --- a/drivers/vhost/vhost.c +++ b/drivers/vhost/vhost.c @@ -2128,11 +2128,6 @@ 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. */ -- 2.24.0.rc1.363.gb1bccd3e3d-goog