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 <redacted>
CC: Dhaval Giani <redacted>
CC: Michal Hocko <redacted>
CC: Kamezawa Hiroyuki <redacted>
CC: Johannes Weiner <redacted>

Acked-by: Johannes Weiner <redacted>