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 D9F51C33C99 for ; Fri, 15 Nov 2019 17:47:25 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id AC3ED2073A for ; Fri, 15 Nov 2019 17:47:25 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1573840045; bh=rCRRtesiccdYDCXwaVE6TpZ6yZ1Ah/Dk3pZgJXH7spE=; h=Date:From:To:Cc:Subject:References:In-Reply-To:List-ID:From; b=OJmb2HuM/c0lwsQD97YvzOzZBDAXFb8CagMisaDT5lCmHFZl6J7bRKMxzQaYlxuph 6SRo/+/QeINRC/oX8mOwuFslHo3+gIUR/VzIlfDv1tu46BZbgT9zqvW/eymrEW/WNU ojutS1Z70VYRx4BUL5xZgEN07+wCBWLEu610meyE= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726988AbfKORrY (ORCPT ); Fri, 15 Nov 2019 12:47:24 -0500 Received: from mx2.suse.de ([195.135.220.15]:34730 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1725848AbfKORrY (ORCPT ); Fri, 15 Nov 2019 12:47:24 -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 956CEAFF1; Fri, 15 Nov 2019 17:47:22 +0000 (UTC) Date: Fri, 15 Nov 2019 18:47:21 +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: <20191115174721.GB15216@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> <20191115174031.GA15216@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20191115174031.GA15216@dhcp22.suse.cz> 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 Fri 15-11-19 18:40:31, Michal Hocko wrote: > 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. s@online@offline@ And reading after myself it turned out to sound differently than I meant. What I wanted to say really is, what is the difference that css_tryget_online really guarantee when the css might go offline right after the call suceeds so more specifically what is the difference between if (css_tryget()) { if (online) DO_SOMETHING } and if (css_tryget_online()) { DO_SOMETHING } both of them are racy and do not provide any guarantee wrt. online state. -- Michal Hocko SUSE Labs