Re: [PATCH 1/2] fix bad behavior in use_hierarchy file

[Johannes Weiner <hannes@cmpxchg.org>]

On Tue, Jun 26, 2012 at 07:47:13PM +0400, 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.
> 
> 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>

Acked-by: Johannes Weiner <hannes@cmpxchg.org>

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