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=-2.6 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,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 D4BD6C432C3 for ; Fri, 15 Nov 2019 17:40:37 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id A3D0020733 for ; Fri, 15 Nov 2019 17:40:37 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1573839637; bh=Y06CUqOEhV1JcVfIPatnO0zNjcYH4kvhsK9z7jrPrAw=; h=Date:From:To:Cc:Subject:References:In-Reply-To:List-ID:From; b=EuON5d8mzR5ZsDTDHryK8yweaGrSMoMZTl116QmPeJS1tRQTxitHeeWRGoC+GzwPu D1mkqxTzM0nDmbMEkJ5zRBDOMe3eINE17kKo4XDK41JkwfLNaDiqE+9TtjUNmiRjCP TQVhS2iLtL6GyXgaDcc41MC7BuG+xsHuBuMr2nkk= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727020AbfKORkg (ORCPT ); Fri, 15 Nov 2019 12:40:36 -0500 Received: from mx2.suse.de ([195.135.220.15]:57502 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726920AbfKORke (ORCPT ); Fri, 15 Nov 2019 12:40:34 -0500 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) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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