| Message ID | 4F9257F4.2070505@huawei.com |
|---|---|
| State | RFC, archived |
| Delegated to: | David Miller |
| Headers | show |
On 04/21/2012 03:47 AM, Li Zefan wrote: > 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); > } I'll see if I can work something out. -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
--- 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); }