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 C1B1DC43141 for ; Fri, 15 Nov 2019 17:47:25 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 9285120733 for ; Fri, 15 Nov 2019 17:47:25 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 9285120733 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 117E86B0005; Fri, 15 Nov 2019 12:47:25 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id 0C8716B0006; Fri, 15 Nov 2019 12:47:25 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 005336B0007; Fri, 15 Nov 2019 12:47:24 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0252.hostedemail.com [216.40.44.252]) by kanga.kvack.org (Postfix) with ESMTP id E84A36B0005 for ; Fri, 15 Nov 2019 12:47:24 -0500 (EST) Received: from smtpin09.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay01.hostedemail.com (Postfix) with SMTP id AF245180AD80F for ; Fri, 15 Nov 2019 17:47:24 +0000 (UTC) X-FDA: 76159243608.09.dog47_672fdf17121b X-HE-Tag: dog47_672fdf17121b X-Filterd-Recvd-Size: 3180 Received: from mx1.suse.de (mx2.suse.de [195.135.220.15]) by imf03.hostedemail.com (Postfix) with ESMTP for ; Fri, 15 Nov 2019 17:47:24 +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 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) 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 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