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=-2.5 required=3.0 tests=MAILING_LIST_MULTI, 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 15E3EC43215 for ; Fri, 15 Nov 2019 17:40:36 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id DA5312073A for ; Fri, 15 Nov 2019 17:40:35 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org DA5312073A Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=kernel.org Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id 624EE6B0005; Fri, 15 Nov 2019 12:40:35 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id 5AE9C6B0006; Fri, 15 Nov 2019 12:40:35 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 49CD56B0007; Fri, 15 Nov 2019 12:40:35 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0107.hostedemail.com [216.40.44.107]) by kanga.kvack.org (Postfix) with ESMTP id 395AB6B0005 for ; Fri, 15 Nov 2019 12:40:35 -0500 (EST) Received: from smtpin10.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay03.hostedemail.com (Postfix) with SMTP id 0712E82499A8 for ; Fri, 15 Nov 2019 17:40:35 +0000 (UTC) X-FDA: 76159226430.10.boys19_5c57eb0c4b602 X-HE-Tag: boys19_5c57eb0c4b602 X-Filterd-Recvd-Size: 2639 Received: from mx1.suse.de (mx2.suse.de [195.135.220.15]) by imf03.hostedemail.com (Postfix) with ESMTP for ; Fri, 15 Nov 2019 17:40:34 +0000 (UTC) X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx1.suse.de (Postfix) with ESMTP id 24C8069CD8; Fri, 15 Nov 2019 17:40:33 +0000 (UTC) Date: Fri, 15 Nov 2019 18:40:31 +0100 From: Michal Hocko To: Tejun Heo Cc: Roman Gushchin , Michal =?iso-8859-1?Q?Koutn=FD?= , "linux-mm@kvack.org" , Andrew Morton , Johannes Weiner , "linux-kernel@vger.kernel.org" , Kernel Team , "stable@vger.kernel.org" Subject: Re: [PATCH 1/2] mm: memcg: switch to css_tryget() in get_mem_cgroup_from_mm() Message-ID: <20191115174031.GA15216@dhcp22.suse.cz> References: <20191106225131.3543616-1-guro@fb.com> <20191113162934.GF19372@blackbody.suse.cz> <20191113170823.GA12464@castle.DHCP.thefacebook.com> <20191114191657.GN20866@dhcp22.suse.cz> <20191114192018.GJ4163745@devbig004.ftw2.facebook.com> <20191114193340.GA24848@dhcp22.suse.cz> <20191114193736.GL4163745@devbig004.ftw2.facebook.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20191114193736.GL4163745@devbig004.ftw2.facebook.com> User-Agent: Mutt/1.10.1 (2018-07-13) X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Thu 14-11-19 11:37:36, Tejun Heo wrote: > Hello, > > On Thu, Nov 14, 2019 at 08:33:40PM +0100, Michal Hocko wrote: > > > It is useful for controlling admissions of new userspace visible uses > > > - e.g. a tracepoint shouldn't be allowed to be attached to a cgroup > > > which has already been deleted. > > > > I am not sure I understand. Roman says that the cgroup can get offline > > right after the function returns. How is "already deleted" different > > from "just deleted"? I thought that the state is preserved at least > > while the rcu lock is held but my memory is dim here. > > It's the same difference as between "opening a file and deleting it" > and "deleting a file and opening it". I am sorry but I do not follow. How can css_tryget_online provide the same semantic when the css can go offline right after the tryget call returns so it is effectivelly undistinguishable from the case when the css was already online before the call was made. Or is my understanding of what Roman's said earlier in the thread? -- Michal Hocko SUSE Labs