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 vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 188A9C77B73 for ; Wed, 24 May 2023 04:22:55 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232537AbjEXEWx (ORCPT ); Wed, 24 May 2023 00:22:53 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:38582 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229540AbjEXEWw (ORCPT ); Wed, 24 May 2023 00:22:52 -0400 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id BDF15FC for ; Tue, 23 May 2023 21:22:11 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1684902131; 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=8RThbuZCrca9ja7PWjTCEifSQrfusG35C3xzRTql4Rg=; b=cYNb0sAiu/W6R/ezsJOfoKzDhh0X6MACnGi3jNBPawLquYmEKzuQglKuJLI1ipnDP//yu2 vQ6jZoIftNOKDum5JTEkAvlHuYxy+YHW3jg+L1SSEQUGTx4+yQ5vum5yp1Z6m4ByYdqlaa e3AosYOaBk21sqAe9VIJITh787uhq7g= 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-659-F5la8j4wM1GiyPo0V-Mo0A-1; Wed, 24 May 2023 00:22:07 -0400 X-MC-Unique: F5la8j4wM1GiyPo0V-Mo0A-1 Received: from smtp.corp.redhat.com (int-mx06.intmail.prod.int.rdu2.redhat.com [10.11.54.6]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id E5A52811E85; Wed, 24 May 2023 04:22:06 +0000 (UTC) Received: from ovpn-8-17.pek2.redhat.com (ovpn-8-17.pek2.redhat.com [10.72.8.17]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 8B0532166B25; Wed, 24 May 2023 04:21:59 +0000 (UTC) Date: Wed, 24 May 2023 12:21:53 +0800 From: Ming Lei To: Waiman Long Cc: Yosry Ahmed , 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, ming.lei@redhat.com Subject: Re: [PATCH] blk-cgroup: Flush stats before releasing blkcg_gq Message-ID: References: <20230524011935.719659-1-ming.lei@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: X-Scanned-By: MIMEDefang 3.1 on 10.11.54.6 Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org On Wed, May 24, 2023 at 12:10:55AM -0400, Waiman Long wrote: > > 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 blkg_release() does guarantee that no new stat associated with this blkg can be added any more, so what is the use-after-free? > 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 Yeah, but is it one big deal? The percpu list should be flushed just in one of blkg's release handler. > blkcg. I still prefer to do it once in blkcg_destroy_blkgs(). I am going to Problem is that new stat still may be added to this list after percpu_ref_kill() returns. To be honest, I really hate to grab/put blkg refcnt for adding/consuming state, and this way is too fragile. Thanks, Ming