Re: [PATCH] fix bad behavior in use_hierarchy file

[Glauber Costa <glommer@parallels.com>]

On 06/25/2012 04:08 PM, Michal Hocko wrote:
> On Mon 25-06-12 13:21:01, Glauber Costa wrote:
>> I have an application that does the following:
>>
>> * copy the state of all controllers attached to a hierarchy
>> * replicate it as a child of the current level.
>>
>> I would expect writes to the files to mostly succeed, since they
>> are inheriting sane values from parents.
>>
>> But that is not the case for use_hierarchy. If it is set to 0, we
>> succeed ok. If we're set to 1, the value of the file is automatically
>> set to 1 in the children, but if userspace tries to write the
>> very same 1, it will fail. That same situation happens if we
>> set use_hierarchy, create a child, and then try to write 1 again.
>>
>> Now, there is no reason whatsoever for failing to write a value
>> that is already there. It doesn't even match the comments, that
>> states:
>>
>>   /* If parent's use_hierarchy is set, we can't make any modifications
>>    * in the child subtrees...
>>
>> since we are not changing anything.
>>
>> The following patch tests the new value against the one we're storing,
>> and automatically return 0 if we're not proposing a change.
>
> Fair enough.
>
>>
>> Signed-off-by: Glauber Costa <glommer@parallels.com>
>> CC: Dhaval Giani <dhaval.giani@gmail.com>
>> CC: Michal Hocko <mhocko@suse.cz>
>> CC: Kamezawa Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
>> CC: Johannes Weiner <hannes@cmpxchg.org>
>
> One comment bellow...
> Acked-by: Michal Hocko <mhocko@suse.cz>
>
>> ---
>>   mm/memcontrol.c |    6 ++++++
>>   1 file changed, 6 insertions(+)
>>
>> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
>> index ac35bcc..cccebbc 100644
>> --- a/mm/memcontrol.c
>> +++ b/mm/memcontrol.c
>> @@ -3779,6 +3779,10 @@ static int mem_cgroup_hierarchy_write(struct cgroup *cont, struct cftype *cft,
>>   		parent_memcg = mem_cgroup_from_cont(parent);
>>
>>   	cgroup_lock();
>> +
>> +	if (memcg->use_hierarchy == val)
>> +		goto out;
>> +		
>
> Why do you need cgroup_lock to check the value? Even if we have 2
> CPUs racing (one trying to set to 0 other to 1 with use_hierarchy==0)
> then the "set to 0" operation might fail depending on who hits the
> cgroup_lock first anyway.
>
> So while this is correct I think there is not much point to take the global
> cgroup lock in this case.
>
Well, no.

All operations will succeed, unless the cgroup breeds new children.
That's the operation we're racing against.

So we need to guarantee a snapshot of what is the status of the file in 
the moment we said we'd create a new children.

Besides, I believe taking the lock is conceptually the right thing to 
do, even if by an ordering artifact we would happen to be safe.

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