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 Received: from lists.ozlabs.org (lists.ozlabs.org [112.213.38.117]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id ED247C54EBC for ; Thu, 12 Jan 2023 03:58:38 +0000 (UTC) Received: from boromir.ozlabs.org (localhost [IPv6:::1]) by lists.ozlabs.org (Postfix) with ESMTP id 4NsrPn3TPjz3cdW for ; Thu, 12 Jan 2023 14:58:37 +1100 (AEDT) Authentication-Results: lists.ozlabs.org; spf=pass (sender SPF authorized) smtp.mailfrom=linux.alibaba.com (client-ip=115.124.30.1; helo=out30-1.freemail.mail.aliyun.com; envelope-from=jefflexu@linux.alibaba.com; receiver=) Received: from out30-1.freemail.mail.aliyun.com (out30-1.freemail.mail.aliyun.com [115.124.30.1]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by lists.ozlabs.org (Postfix) with ESMTPS id 4NsrPh70SDz3c3m for ; Thu, 12 Jan 2023 14:58:31 +1100 (AEDT) X-Alimail-AntiSpam: AC=PASS;BC=-1|-1;BR=01201311R251e4;CH=green;DM=||false|;DS=||;FP=0|-1|-1|-1|0|-1|-1|-1;HT=ay29a033018045170;MF=jefflexu@linux.alibaba.com;NM=1;PH=DS;RN=7;SR=0;TI=SMTPD_---0VZPF1qd_1673495907; Received: from 30.221.131.229(mailfrom:jefflexu@linux.alibaba.com fp:SMTPD_---0VZPF1qd_1673495907) by smtp.aliyun-inc.com; Thu, 12 Jan 2023 11:58:28 +0800 Message-ID: <6acd0aea-2e7b-e30e-214f-81f4c3766ead@linux.alibaba.com> Date: Thu, 12 Jan 2023 11:58:27 +0800 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:102.0) Gecko/20100101 Thunderbird/102.4.0 Subject: Re: [PATCH v2 1/2] fscache: Use wait_on_bit() to wait for the freeing of relinquished volume Content-Language: en-US To: David Howells , Hou Tao References: <20221226103309.953112-2-houtao@huaweicloud.com> <20221226103309.953112-1-houtao@huaweicloud.com> <2431838.1673453170@warthog.procyon.org.uk> From: Jingbo Xu In-Reply-To: <2431838.1673453170@warthog.procyon.org.uk> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-BeenThere: linux-erofs@lists.ozlabs.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Development of Linux EROFS file system List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: linux-erofs@lists.ozlabs.org, linux-cachefs@redhat.com, Jeff Layton , linux-kernel@vger.kernel.org, houtao1@huawei.com Errors-To: linux-erofs-bounces+linux-erofs=archiver.kernel.org@lists.ozlabs.org Sender: "Linux-erofs" On 1/12/23 12:06 AM, David Howells wrote: > Hou Tao wrote: > >> clear_bit(FSCACHE_VOLUME_ACQUIRE_PENDING, &cursor->flags); >> + /* >> + * Paired with barrier in wait_on_bit(). Check >> + * wake_up_bit() and waitqueue_active() for details. >> + */ >> + smp_mb__after_atomic(); >> wake_up_bit(&cursor->flags, FSCACHE_VOLUME_ACQUIRE_PENDING); > > What two values are you applying a partial ordering to? Yeah Hou Tao has explained that a full barrier is needed here to avoid the potential reordering at the waker side. As I was also researching on this these days, I'd like to share my thought on this, hopefully if it could give some insight :) Without the barrier at the waker side, it may suffer from the following race: ``` CPU0 - waker CPU1 - waiter if (waitqueue_active(wq_head)) <-- find no wq_entry in wq_head list wake_up(wq_head); for (;;) { prepare_to_wait(...); # add wq_entry into wq_head list if (@cond) <-- @cond is false break; schedule(); <-- wq_entry still in wq_head list, wait for next wakeup } finish_wait(&wq_head, &wait); @cond = true; ``` in which case the waiter misses the wakeup for one time. -- Thanks, Jingbo