From patchwork Thu Jul 1 12:26:42 2021 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Yunsheng Lin X-Patchwork-Id: 469270 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=-16.8 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI, 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 CCCAEC11F69 for ; Thu, 1 Jul 2021 12:27:24 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id B3E0161407 for ; Thu, 1 Jul 2021 12:27:24 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S236447AbhGAM3x (ORCPT ); Thu, 1 Jul 2021 08:29:53 -0400 Received: from szxga03-in.huawei.com ([45.249.212.189]:9334 "EHLO szxga03-in.huawei.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S236423AbhGAM3w (ORCPT ); Thu, 1 Jul 2021 08:29:52 -0400 Received: from dggemv704-chm.china.huawei.com (unknown [172.30.72.54]) by szxga03-in.huawei.com (SkyGuard) with ESMTP id 4GFy6F2RBTz74M0; Thu, 1 Jul 2021 20:23:01 +0800 (CST) Received: from dggpemm500005.china.huawei.com (7.185.36.74) by dggemv704-chm.china.huawei.com (10.3.19.47) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.2176.2; Thu, 1 Jul 2021 20:27:14 +0800 Received: from localhost.localdomain (10.69.192.56) by dggpemm500005.china.huawei.com (7.185.36.74) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.2176.2; Thu, 1 Jul 2021 20:27:14 +0800 From: Yunsheng Lin To: , , , CC: , , , , , , , , Subject: [PATCH net-next v3 3/3] ptr_ring: add barrier to ensure the visiblity of r->queue[] Date: Thu, 1 Jul 2021 20:26:42 +0800 Message-ID: <1625142402-64945-4-git-send-email-linyunsheng@huawei.com> X-Mailer: git-send-email 2.7.4 In-Reply-To: <1625142402-64945-1-git-send-email-linyunsheng@huawei.com> References: <1625142402-64945-1-git-send-email-linyunsheng@huawei.com> MIME-Version: 1.0 X-Originating-IP: [10.69.192.56] X-ClientProxiedBy: dggems701-chm.china.huawei.com (10.3.19.178) To dggpemm500005.china.huawei.com (7.185.36.74) X-CFilter-Loop: Reflected Precedence: bulk List-ID: X-Mailing-List: linux-kselftest@vger.kernel.org After r->consumer_head is updated in __ptr_ring_discard_one(), r->queue[r->consumer_head] is already cleared in the previous round of __ptr_ring_discard_one(). But there is no guarantee other thread will see the r->queue[r->consumer_head] being NULL because there is no explicit barrier between r->queue[] clearing and r->consumer_head updating. So add two explicit barrier to make sure r->queue[] cleared in __ptr_ring_discard_one() to be visible to other cpu, mainly to make sure the cpu calling the __ptr_ring_empty() will see the correct r->queue[r->consumer_head]. Hopefully the previous and this patch have ensured the correct visibility of r->queue[], so update the comment accordingly about __ptr_ring_empty(). Tested using the "perf stat -r 1000 ./ptr_ring_test -s 1000 -m 1 -N 100000000", comparing the elapsed time: arch unpatched patched improvement arm64 1.888224 sec 1.893673 sec -0.2% X86 2.5422 sec 2.5587 sec -0.6% Reported-by: Michael S. Tsirkin Signed-off-by: Yunsheng Lin --- include/linux/ptr_ring.h | 29 +++++++++++++++++++---------- 1 file changed, 19 insertions(+), 10 deletions(-) diff --git a/include/linux/ptr_ring.h b/include/linux/ptr_ring.h index db9c282..d78aab8 100644 --- a/include/linux/ptr_ring.h +++ b/include/linux/ptr_ring.h @@ -178,15 +178,11 @@ static inline void *__ptr_ring_peek(struct ptr_ring *r) * * NB: This is only safe to call if ring is never resized. * - * However, if some other CPU consumes ring entries at the same time, the value - * returned is not guaranteed to be correct. - * - * In this case - to avoid incorrectly detecting the ring - * as empty - the CPU consuming the ring entries is responsible - * for either consuming all ring entries until the ring is empty, - * or synchronizing with some other CPU and causing it to - * re-test __ptr_ring_empty and/or consume the ring enteries - * after the synchronization point. + * caller might need to use the smp_rmb() to pair with smp_wmb() + * or smp_store_release() in __ptr_ring_discard_one() and smp_wmb() + * in __ptr_ring_produce() to ensure correct ordering between + * __ptr_ring_empty() checking and subsequent operation after + * __ptr_ring_empty() checking. * * Note: callers invoking this in a loop must use a compiler barrier, * for example cpu_relax(). @@ -274,7 +270,12 @@ static inline void __ptr_ring_discard_one(struct ptr_ring *r) if (unlikely(consumer_head >= r->size)) { r->consumer_tail = 0; - WRITE_ONCE(r->consumer_head, 0); + + /* Make sure r->queue[0] ~ r->queue[r->consumer_tail] + * cleared in previous __ptr_ring_discard_one() is + * visible to other cpu. + */ + smp_store_release(&r->consumer_head, 0); } else { r->consumer_tail = consumer_head; WRITE_ONCE(r->consumer_head, consumer_head); @@ -288,6 +289,14 @@ static inline void __ptr_ring_discard_one(struct ptr_ring *r) while (likely(--consumer_head >= tail)) r->queue[consumer_head] = NULL; + if (unlikely(!r->consumer_head)) { + /* Make sure r->queue[r->consumer_tail] ~ + * r->queue[r->size - 1] cleared above is visible to + * other cpu. + */ + smp_wmb(); + } + return; }