From mboxrd@z Thu Jan 1 00:00:00 1970 From: Wei Yang Subject: Re: [RFC PATCH] mm: thp: grab the lock before manipulation defer list Date: Thu, 9 Jan 2020 11:18:21 +0800 Message-ID: <20200109031821.GA5206@richard> References: <20200103143407.1089-1-richardw.yang@linux.intel.com> <20200106102345.GE12699@dhcp22.suse.cz> <20200107012241.GA15341@richard> <20200107083808.GC32178@dhcp22.suse.cz> <20200108003543.GA13943@richard> <20200108094041.GQ32178@dhcp22.suse.cz> Reply-To: Wei Yang Mime-Version: 1.0 Return-path: Content-Disposition: inline In-Reply-To: <20200108094041.GQ32178@dhcp22.suse.cz> Sender: linux-kernel-owner@vger.kernel.org List-ID: Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: Michal Hocko Cc: Wei Yang , hannes@cmpxchg.org, vdavydov.dev@gmail.com, akpm@linux-foundation.org, kirill.shutemov@linux.intel.com, cgroups@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, yang.shi@linux.alibaba.com On Wed, Jan 08, 2020 at 10:40:41AM +0100, Michal Hocko wrote: >On Wed 08-01-20 08:35:43, Wei Yang wrote: >> On Tue, Jan 07, 2020 at 09:38:08AM +0100, Michal Hocko wrote: >> >On Tue 07-01-20 09:22:41, Wei Yang wrote: >> >> On Mon, Jan 06, 2020 at 11:23:45AM +0100, Michal Hocko wrote: >> >> >On Fri 03-01-20 22:34:07, Wei Yang wrote: >> >> >> As all the other places, we grab the lock before manipulate the defer list. >> >> >> Current implementation may face a race condition. >> >> > >> >> >Please always make sure to describe the effect of the change. Why a racy >> >> >list_empty check matters? >> >> > >> >> >> >> Hmm... access the list without proper lock leads to many bad behaviors. >> > >> >My point is that the changelog should describe that bad behavior. >> > >> >> For example, if we grab the lock after checking list_empty, the page may >> >> already be removed from list in split_huge_page_list. And then list_del_init >> >> would trigger bug. >> > >> >And how does list_empty check under the lock guarantee that the page is >> >on the deferred list? >> >> Just one confusion, is this kind of description basic concept of concurrent >> programming? How detail level we need to describe the effect? > >When I write changelogs for patches like this I usually describe, what >is the potential race - e.g. > CPU1 CPU2 > path1 path2 > check lock > operation2 > unlock > lock > # check might not hold anymore > operation1 > unlock > >and what is the effect of the race - e.g. a crash, data corruption, >pointless attempt for operation1 which fails with user visible effect >etc. Hi, Michal, here is my attempt for an example. Hope this one looks good to you. For example, the potential race would be: CPU1 CPU2 mem_cgroup_move_account split_huge_page_to_list !list_empty lock !list_empty list_del unlock lock # !list_empty might not hold anymore list_del_init unlock When this sequence happens, the list_del_init() in mem_cgroup_move_account() would crash since the page is already been removed by list_del in split_huge_page_to_list(). -- Wei Yang Help you, Help me