| Re: [PATCH] fix bad behavior in use_hierarchy file [message #46908 is a reply to message #46905] |
Mon, 25 June 2012 12:08   |
Michal Hocko
Messages: 109
Registered: December 2011
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Senior Member |
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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.
> /*
> * If parent's use_hierarchy is set, we can't make any modifications
> * in the child subtrees. If it is unset, then the change can
> @@ -3795,6 +3799,8 @@ static int mem_cgroup_hierarchy_write(struct cgroup *cont, struct cftype *cft,
> retval = -EBUSY;
> } else
> retval = -EINVAL;
> +
> +out:
> cgroup_unlock();
>
> return retval;
> --
> 1.7.10.2
>
--
Michal Hocko
SUSE Labs
SUSE LINUX s.r.o.
Lihovarska 1060/12
190 00 Praha 9
Czech Republic
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