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 kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by smtp.lore.kernel.org (Postfix) with ESMTP id AA85FC7EE23 for ; Wed, 24 May 2023 04:11:06 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 1201D900002; Wed, 24 May 2023 00:11:06 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 0A9246B0075; Wed, 24 May 2023 00:11:06 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id E8C0F900002; Wed, 24 May 2023 00:11:05 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0012.hostedemail.com [216.40.44.12]) by kanga.kvack.org (Postfix) with ESMTP id D55D36B0074 for ; Wed, 24 May 2023 00:11:05 -0400 (EDT) Received: from smtpin09.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay09.hostedemail.com (Postfix) with ESMTP id AD9218037A for ; Wed, 24 May 2023 04:11:05 +0000 (UTC) X-FDA: 80823823290.09.7C95668 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) by imf14.hostedemail.com (Postfix) with ESMTP id CD0B810000E for ; Wed, 24 May 2023 04:11:03 +0000 (UTC) Authentication-Results: imf14.hostedemail.com; dkim=pass header.d=redhat.com header.s=mimecast20190719 header.b=PVKQCS0j; dmarc=pass (policy=none) header.from=redhat.com; spf=pass (imf14.hostedemail.com: domain of longman@redhat.com designates 170.10.129.124 as permitted sender) smtp.mailfrom=longman@redhat.com ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1684901463; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references:dkim-signature; bh=+jjXC8nEHxswkc61Y0uyyoGWYpO1Pad3VdwaQeUusRw=; b=yaibmrDvDNOxtNvylDwppFosDLZH87WKgKnlH6Hnzz4vY3LZegQabG0/iPrrYyBNCpaUwZ Rj60/dzgphXhklYqaRdohi39B331R/VkQpjhlstGHUJm8R4gYG3/fPXD0LIOeoWISZz9VA k1cn2iyLWgRCLAcvL6GnBhDv6S2NWIc= ARC-Authentication-Results: i=1; imf14.hostedemail.com; dkim=pass header.d=redhat.com header.s=mimecast20190719 header.b=PVKQCS0j; dmarc=pass (policy=none) header.from=redhat.com; spf=pass (imf14.hostedemail.com: domain of longman@redhat.com designates 170.10.129.124 as permitted sender) smtp.mailfrom=longman@redhat.com ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1684901463; a=rsa-sha256; cv=none; b=KXwMkYug1cvVDJ6vCZgMmsRmlVon6pU066KqDAzVhlKhcd3aGSD4J0EAf8dCiMb1fUskd3 hts3LmI6sijy/cLu0MFKIodP0YnA7rZAMaOYq+aDBlIsBsMs2EipI69NFzVlW1SgYlwgYB 5wSpeO61xhrtKS/pvw+Q30el8dBPL4Y= DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1684901462; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=+jjXC8nEHxswkc61Y0uyyoGWYpO1Pad3VdwaQeUusRw=; b=PVKQCS0jk9aeUdxvw/YSIqkJ6Iqtg19Ri4s/YLxMf86HblrwGh4gbB4U5yTXBYRLwrnKf/ 2JOsoyphqENfSPkR3zobDxXNXQecoBuy0OADjfiXbEB5ngaL3A/skSLsJ6RBU88gYexmmY I2mC4n3gCElj8bpcsG6WnVX056iBpX0= Received: from mimecast-mx02.redhat.com (mimecast-mx02.redhat.com [66.187.233.88]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-665-UCW2gKqMPgaKCCBtYR6dqA-1; Wed, 24 May 2023 00:10:57 -0400 X-MC-Unique: UCW2gKqMPgaKCCBtYR6dqA-1 Received: from smtp.corp.redhat.com (int-mx07.intmail.prod.int.rdu2.redhat.com [10.11.54.7]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 7A416101A52C; Wed, 24 May 2023 04:10:56 +0000 (UTC) Received: from [10.22.8.64] (unknown [10.22.8.64]) by smtp.corp.redhat.com (Postfix) with ESMTP id CFB23140E95D; Wed, 24 May 2023 04:10:55 +0000 (UTC) Message-ID: Date: Wed, 24 May 2023 00:10:55 -0400 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.7.1 Subject: Re: [PATCH] blk-cgroup: Flush stats before releasing blkcg_gq Content-Language: en-US To: Ming Lei , Yosry Ahmed Cc: Linux-MM , Michal Hocko , Shakeel Butt , Johannes Weiner , Roman Gushchin , Muchun Song , Jens Axboe , linux-block@vger.kernel.org, cgroups@vger.kernel.org, Tejun Heo , mkoutny@suse.com References: <20230524011935.719659-1-ming.lei@redhat.com> From: Waiman Long In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Scanned-By: MIMEDefang 3.1 on 10.11.54.7 X-Rspamd-Queue-Id: CD0B810000E X-Rspam-User: X-Rspamd-Server: rspam04 X-Stat-Signature: agp6kggetumqmit56oyei88fswh8chxj X-HE-Tag: 1684901463-103707 X-HE-Meta: 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 IpHrlgaa ZDbBmsZbqdWpbKNr0FNZe9VUQarRCagzKC/DPprNbA+cK0IxgcuQ0Gdq4h/f/+oAhxBV4h5zdhkcrwHsMLhVvhm9s9GoXE3YL6dNdb/elBw5DvTBg2f14hzEE7aunpigOrnzvyhHD28IagWwtzAMhrc1M+ADxQfwR3cQmcEzj5NPkgmHAgy/JwvUp5zMEvVfz536WTRvce62INWpMXQbIvbAhhVxJjBTSr2JsyWCWlQ/ZgxQ= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000014, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On 5/23/23 22:37, Ming Lei wrote: > Hi Yosry, > > On Tue, May 23, 2023 at 07:06:38PM -0700, Yosry Ahmed wrote: >> Hi Ming, >> >> On Tue, May 23, 2023 at 6:21 PM Ming Lei wrote: >>> As noted by Michal, the blkg_iostat_set's in the lockless list >>> hold reference to blkg's to protect against their removal. Those >>> blkg's hold reference to blkcg. When a cgroup is being destroyed, >>> cgroup_rstat_flush() is only called at css_release_work_fn() which >>> is called when the blkcg reference count reaches 0. This circular >>> dependency will prevent blkcg and some blkgs from being freed after >>> they are made offline. >> I am not at all familiar with blkcg, but does calling >> cgroup_rstat_flush() in offline_css() fix the problem? > Except for offline, this list needs to be flushed after the associated disk > is deleted. > >> or can items be >> added to the lockless list(s) after the blkcg is offlined? > Yeah. > > percpu_ref_*get(&blkg->refcnt) still can succeed after the percpu refcnt > is killed in blkg_destroy() which is called from both offline css and > removing disk. As suggested by Tejun, we can use percpu_ref_tryget(&blkg->refcnt) to make sure that we can only take a reference when the blkg is online. I think it is a bit safer to take a percpu refcnt to avoid use after free. My other concern about your patch is that the per cpu list iterations will be done multiple times when a blkcg is destroyed if many blkgs are attached to the blkcg. I still prefer to do it once in blkcg_destroy_blkgs(). I am going to post an updated version tomorrow after some more testings. Cheers, Longman