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 4B591C43215 for ; Fri, 15 Nov 2019 18:03:07 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 1D9412073C for ; Fri, 15 Nov 2019 18:03:07 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 1D9412073C 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 A828D6B0006; Fri, 15 Nov 2019 13:03:06 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id A33CC6B0008; Fri, 15 Nov 2019 13:03:06 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 8FBD76B000A; Fri, 15 Nov 2019 13:03:06 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0034.hostedemail.com [216.40.44.34]) by kanga.kvack.org (Postfix) with ESMTP id 840A26B0006 for ; Fri, 15 Nov 2019 13:03:06 -0500 (EST) Received: from smtpin01.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay05.hostedemail.com (Postfix) with SMTP id 207B4181AEF1F for ; Fri, 15 Nov 2019 18:03:06 +0000 (UTC) X-FDA: 76159283172.01.tub73_8f6eb0a7e463c X-HE-Tag: tub73_8f6eb0a7e463c X-Filterd-Recvd-Size: 3145 Received: from mx1.suse.de (mx2.suse.de [195.135.220.15]) by imf15.hostedemail.com (Postfix) with ESMTP for ; Fri, 15 Nov 2019 18:03:05 +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 46E8EB9DB; Fri, 15 Nov 2019 18:03:04 +0000 (UTC) Date: Fri, 15 Nov 2019 19:03:03 +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: <20191115180303.GC15216@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> <20191115174721.GB15216@dhcp22.suse.cz> <20191115174844.GR4163745@devbig004.ftw2.facebook.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20191115174844.GR4163745@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 Fri 15-11-19 09:48:44, Tejun Heo wrote: > On Fri, Nov 15, 2019 at 06:47:21PM +0100, Michal Hocko wrote: > > 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. > > It's about not giving new reference when the object is known to be > delted to the user. This part is clear to me. The failure just says it is already too late to do anything. I just still struggle why the success is telling me much more when the state might change before I can do anything on the object. I could see a usefulness if I've had a guarantee that the object stays online while I hold a $FOO lock but if there is nothing like that then we are just having already too late or potentially too late. Anyway it's been a hard week and the brain is just going for the weekend so I just might be dense now. > Can you please think more about how file deletions work? I will try that with a fresh brain next week. -- Michal Hocko SUSE Labs