[Devel] Re: [PATCH 2/3] don't take cgroup_mutex in destroy()

[Glauber Costa]

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.