Re: [PATCH v5 3/3] cgroups: make procs file writable

[Li Zefan <lizf-BthXqXjhjHXQFUHtdCDX3A@public.gmane.org>]

David Rientjes wrote:
> On Thu, 30 Dec 2010, Li Zefan wrote:
> 
>> That's what we did for cpu masks :). See commit
>> 2341d1b6598c7146d64a5050b53a72a5a819617f.
>>
>> I made a patchset to remove on stack cpu masks.
>>
>> What I meant is we don't have to allocate nodemasks in cpuset_sprintf_memlist().
>> This is sufficient:
>>
>> diff --git a/kernel/cpuset.c b/kernel/cpuset.c
>> index 4349935..a159612 100644
>> --- a/kernel/cpuset.c
>> +++ b/kernel/cpuset.c
>> @@ -1620,20 +1620,12 @@ static int cpuset_sprintf_cpulist(char *page, struct cpu
>>
>>  static int cpuset_sprintf_memlist(char *page, struct cpuset *cs)
>>  {
>> -       NODEMASK_ALLOC(nodemask_t, mask, GFP_KERNEL);
>>         int retval;
>>
>> -       if (mask == NULL)
>> -               return -ENOMEM;
>> -
>>         mutex_lock(&callback_mutex);
>> -       *mask = cs->mems_allowed;
>> +       retval = nodelist_scnprintf(page, PAGE_SIZE, cs->mems_allowed);
>>         mutex_unlock(&callback_mutex);
>>
>> -       retval = nodelist_scnprintf(page, PAGE_SIZE, *mask);
>> -
>> -       NODEMASK_FREE(mask);
>> -
>>         return retval;
>>  }
>>
> 
> This needs to be done with cgroup_lock() instead of callback_mutex since 
> the post_clone() callback will store to cs->mems_allowed on 
> cgroup_clone().
> 

Then cpuset_post_clone() breaks the lock rule:

 * A task must hold both mutexes to modify cpusets...
 ...
 * If a task is only holding callback_mutex, then it has read-only
 * access to cpusets.

But that's Ok, because cgroup_clone() is called during the creation of
the new cgroup, so no one can access the cpuset at that time.