On 06/26/2012 12:49 AM, Tejun Heo wrote:

On Mon, Jun 25, 2012 at 01:21:01PM +0400, Glauber Costa wrote:

quoted

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.

A bit of delta but is there any chance we can either deprecate
.use_hierarhcy or at least make it global toggle instead of subtree
thing?  This seems needlessly complicated. :(

I am for deprecating. If this is a long term goal, a two-phase process 
making it per-tree seems unnecessary and even more confusing.

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