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=-5.3 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE, SPF_PASS,USER_AGENT_SANE_1 autolearn=no 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 621DFC4707F for ; Thu, 27 May 2021 07:21:22 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 46274613D4 for ; Thu, 27 May 2021 07:21:22 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S234661AbhE0HWw (ORCPT ); Thu, 27 May 2021 03:22:52 -0400 Received: from szxga02-in.huawei.com ([45.249.212.188]:2497 "EHLO szxga02-in.huawei.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233918AbhE0HWq (ORCPT ); Thu, 27 May 2021 03:22:46 -0400 Received: from dggeml710-chm.china.huawei.com (unknown [172.30.72.54]) by szxga02-in.huawei.com (SkyGuard) with ESMTP id 4FrK133nRNzYnSb; Thu, 27 May 2021 15:18:31 +0800 (CST) Received: from dggpemm500005.china.huawei.com (7.185.36.74) by dggeml710-chm.china.huawei.com (10.3.17.140) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA256_P256) id 15.1.2176.2; Thu, 27 May 2021 15:21:11 +0800 Received: from [127.0.0.1] (10.69.30.204) by dggpemm500005.china.huawei.com (7.185.36.74) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA256) id 15.1.2176.2; Thu, 27 May 2021 15:21:11 +0800 Subject: Re: [PATCH net-next] ptr_ring: make __ptr_ring_empty() checking more reliable To: Jason Wang , , CC: , , , , , , References: <1622032173-11883-1-git-send-email-linyunsheng@huawei.com> <25a6b73d-06ec-fe07-b34c-10fea709e055@huawei.com> <51bc1c38-da20-1090-e3ef-1972f28adfee@redhat.com> From: Yunsheng Lin Message-ID: <938bdb23-4335-845d-129e-db8af2484c27@huawei.com> Date: Thu, 27 May 2021 15:21:10 +0800 User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:52.0) Gecko/20100101 Thunderbird/52.2.0 MIME-Version: 1.0 In-Reply-To: <51bc1c38-da20-1090-e3ef-1972f28adfee@redhat.com> Content-Type: text/plain; charset="utf-8" Content-Language: en-US Content-Transfer-Encoding: 8bit X-Originating-IP: [10.69.30.204] X-ClientProxiedBy: dggeme715-chm.china.huawei.com (10.1.199.111) To dggpemm500005.china.huawei.com (7.185.36.74) X-CFilter-Loop: Reflected Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2021/5/27 14:53, Jason Wang wrote: > > 在 2021/5/27 下午2:07, Yunsheng Lin 写道: >> On 2021/5/27 12:57, Jason Wang wrote: >>> 在 2021/5/26 下午8:29, Yunsheng Lin 写道: >>>> Currently r->queue[] is cleared after r->consumer_head is moved >>>> forward, which makes the __ptr_ring_empty() checking called in >>>> page_pool_refill_alloc_cache() unreliable if the checking is done >>>> after the r->queue clearing and before the consumer_head moving >>>> forward. >>>> >>>> Move the r->queue[] clearing after consumer_head moving forward >>>> to make __ptr_ring_empty() checking more reliable. >>> >>> If I understand this correctly, this can only happens if you run __ptr_ring_empty() in parallel with ptr_ring_discard_one(). >> Yes. >> >>> I think those two needs to be serialized. Or did I miss anything? >> As the below comment in __ptr_ring_discard_one, if the above is true, I >> do not think we need to keep consumer_head valid at all times, right? >> >> >> /* Note: we must keep consumer_head valid at all times for __ptr_ring_empty >> * to work correctly. >> */ > > > I'm not sure I understand. But my point is that you need to synchronize the __ptr_ring_discard_one() and __ptr_empty() as explained in the comment above __ptr_ring_empty(): I am saying if __ptr_ring_empty() and __ptr_ring_discard_one() is always serialized, then it seems that the below commit is unnecessary? 406de7555424 ("ptr_ring: keep consumer_head valid at all times") > > /* > * Test ring empty status without taking any locks. > * > * 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. I am not sure I understand "incorrectly detecting the ring as empty" means, is it because of the data race described in the commit log? Or other data race? I can not think of other data race if consuming and __ptr_ring_empty() is serialized:) I am agreed that __ptr_ring_empty() checking is not totally reliable without taking r->consumer_lock, that is why I use "more reliable" in the title:) > * > * Note: callers invoking this in a loop must use a compiler barrier, > * for example cpu_relax(). > */ > > Thanks > > >