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 87661C433FF for ; Fri, 2 Aug 2019 17:14:56 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 4E93B20880 for ; Fri, 2 Aug 2019 17:14:56 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1564766096; bh=RJ5F0Z8HeRllTFmoyJ/qdw3Zeuzk1T4D+QOe21ZmDAo=; h=Date:From:To:Cc:Subject:References:In-Reply-To:List-ID:From; b=oTPGynFJgiADT/bqYVljUdikuuzCDatCf/L54wsqjTcK0hYnfbnadU/S56zMRztO7 V9jMjNY5SSzxel+KE99uCapbUXVhjRIlwuhy118IxCZNs1WD5gxloJAGrtTB0iKJRO NyknV+mqdoI4fmodfsyDq23eYZ4EtUxQyH+Qt3NA= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2391596AbfHBROz (ORCPT ); Fri, 2 Aug 2019 13:14:55 -0400 Received: from mx2.suse.de ([195.135.220.15]:37202 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1729364AbfHBROz (ORCPT ); Fri, 2 Aug 2019 13:14:55 -0400 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 E9D3EACC6; Fri, 2 Aug 2019 17:14:53 +0000 (UTC) Date: Fri, 2 Aug 2019 19:14:51 +0200 From: Michal Hocko To: Roman Gushchin Cc: Andrew Morton , "linux-mm@kvack.org" , Johannes Weiner , "linux-kernel@vger.kernel.org" , Kernel Team Subject: Re: [PATCH] mm: memcontrol: switch to rcu protection in drain_all_stock() Message-ID: <20190802171451.GN6461@dhcp22.suse.cz> References: <20190801233513.137917-1-guro@fb.com> <20190802080422.GA6461@dhcp22.suse.cz> <20190802085947.GC6461@dhcp22.suse.cz> <20190802170030.GB28431@tower.DHCP.thefacebook.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190802170030.GB28431@tower.DHCP.thefacebook.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 Fri 02-08-19 17:00:34, Roman Gushchin wrote: > On Fri, Aug 02, 2019 at 10:59:47AM +0200, Michal Hocko wrote: > > On Fri 02-08-19 10:04:22, Michal Hocko wrote: > > > On Thu 01-08-19 16:35:13, Roman Gushchin wrote: > > > > Commit 72f0184c8a00 ("mm, memcg: remove hotplug locking from try_charge") > > > > introduced css_tryget()/css_put() calls in drain_all_stock(), > > > > which are supposed to protect the target memory cgroup from being > > > > released during the mem_cgroup_is_descendant() call. > > > > > > > > However, it's not completely safe. In theory, memcg can go away > > > > between reading stock->cached pointer and calling css_tryget(). > > > > > > I have to remember how is this whole thing supposed to work, it's been > > > some time since I've looked into that. > > > > OK, I guess I remember now and I do not see how the race is possible. > > Stock cache is keeping its memcg alive because it elevates the reference > > counting for each cached charge. And that should keep the whole chain up > > to the root (of draining) alive, no? Or do I miss something, could you > > generate a sequence of events that would lead to use-after-free? > > Right, but it's true when you reading a local percpu stock. > But here we read a remote stock->cached pointer, which can be cleared > by a remote concurrent drain_local_stock() execution. OK, I can see how refill_stock can race with drain_all_stock. I am not sure I see drain_local_stock race because that should be triggered only from drain_all_stock and only one cpu is allowed to do that. Maybe we might have scheduled a work from the previous run? In any case, please document the race in the changelog please. This code is indeed tricky and a comment would help as well. -- Michal Hocko SUSE Labs