From mboxrd@z Thu Jan 1 00:00:00 1970 From: Li Zefan Subject: Re: [PATCH 2/3] don't take cgroup_mutex in destroy() Date: Sat, 21 Apr 2012 14:47:16 +0800 Message-ID: <4F9257F4.2070505@huawei.com> References: <1334875758-20939-1-git-send-email-glommer@parallels.com> <1334875758-20939-3-git-send-email-glommer@parallels.com> <20120419225704.GE10553@google.com> <4F917AEB.7080404@parallels.com> Mime-Version: 1.0 Content-Transfer-Encoding: 7BIT Return-path: In-reply-to: <4F917AEB.7080404-bzQdu9zFT3WakBO8gow8eQ@public.gmane.org> Sender: cgroups-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org List-ID: Content-Type: text/plain; charset="us-ascii" To: Glauber Costa Cc: Tejun Heo , netdev-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, cgroups-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, kamezawa.hiroyu-+CUm20s59erQFUHtdCDX3A@public.gmane.org, David Miller , devel-GEFAQzZX7r8dnm+yROfE0A@public.gmane.org, Vivek Goyal Glauber Costa wrote: > On 04/19/2012 07:57 PM, Tejun Heo wrote: >> On Thu, Apr 19, 2012 at 07:49:17PM -0300, Glauber Costa wrote: >>> Most of the destroy functions are only doing very simple things >>> like freeing memory. >>> >>> The ones who goes through lists and such, already use its own >>> locking for those. >>> >>> * The cgroup itself won't go away until we free it, (after destroy) >>> * The parent won't go away because we hold a reference count >>> * There are no more tasks in the cgroup, and the cgroup is declared >>> dead (cgroup_is_removed() == true) >>> >>> For the blk-cgroup and the cpusets, I got the impression that the mutex >>> is still necessary. >>> >>> For those, I grabbed it from within the destroy function itself. >>> >>> If the maintainer for those subsystems consider it safe to remove >>> it, we can discuss it separately. >> >> I really don't like cgroup_lock() usage spreading more. It's >> something which should be contained in cgroup.c proper. I looked at >> the existing users a while ago and they seemed to be compensating >> deficencies in API, so, if at all possible, let's not spread the >> disease. > > Well, I can dig deeper and see if they are really needed. I don't know cpusets and blkcg *that* well, that's why I took them there, hoping that someone could enlighten me, maybe they aren't really needed even now. > > I agree with the compensating: As I mentioned, most of them are already taking other kinds of lock to protect their structures, which is the right thing to do. > > There were only two or three spots in cpusets and blkcg where I wasn't that sure that we could drop the lock... What do you say about that ? > . We can drop cgroup_mutex for cpusets with changes like this: (Note: as I'm not able to get the latest code at this momment, this patch is based on 3.0.) There are several places reading number_of_cpusets, but no one holds cgroup_mutex, except the one in generate_sched_domains(). With this patch, both cpuset_create() and generate_sched_domains() are still holding cgroup_mutex, so it's safe. --- linux-kernel/kernel/cpuset.c.orig 2012-04-21 01:55:57.000000000 -0400 +++ linux-kernel/kernel/cpuset.c 2012-04-21 02:30:53.000000000 -0400 @@ -1876,7 +1876,9 @@ static struct cgroup_subsys_state *cpuse cs->relax_domain_level = -1; cs->parent = parent; + mutex_lock(&callback_mutex); number_of_cpusets++; + mutex_unlock(&callback_mutex); return &cs->css ; } @@ -1890,10 +1892,18 @@ static void cpuset_destroy(struct cgroup { struct cpuset *cs = cgroup_cs(cont); - if (is_sched_load_balance(cs)) + if (is_sched_load_balance(cs)) { + /* + * This cpuset is under destruction, so no one else can + * modify it, so it's safe to call update_flag() without + * cgroup_lock. + */ update_flag(CS_SCHED_LOAD_BALANCE, cs, 0); + } + mutex_lock(&callback_mutex); number_of_cpusets--; + mutex_lock(&callback_mutex); free_cpumask_var(cs->cpus_allowed); kfree(cs); }