linux-kernel - Re: idr_get_new

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Date:	Mon, 02 Jul 2007 20:31:40 -0400
From:	Jim Houston <jim.houston@...r.com>
To:	Hoang-Nam Nguyen <hnguyen@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, linuxppc-dev@...abs.org,
	openib-general@...nib.org,
	Stefan Roscher <ossrosch@...ux.vnet.ibm.com>,
	raisch@...ibm.com, Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: idr_get_new_above() limitation?

On Mon, 2007-07-02 at 19:19 +0200, Hoang-Nam Nguyen wrote:

> i=3fffffff token=3fffffff t=000000003fffffff
> i=40000000 token=40000000 t=0000000000000000
> Invalid object 0000000000000000. Expected 40000000
> 
> That means token 0x40000000 seems to be the "upper boundary" of idr_find().
> However the behaviour is not consistent in that it was returned by
> idr_get_new_above().

Hi Nam,

Yes this is a bug.  Thanks for the great test module.

The problem is in idr_get_new_above_int() in the loop which
adds new layers to the top of the radix tree.  It is failing
the "layers < (MAX_LEVEL - 1)" test.  It doesn't allocate the
new layer but still calls sub_alloc() which relies on having
the new layer properly constructed.  I believe that it is
allocating the slot which corresponds to id = 0.

I believe this is an off by one error in calculating the
MAX_LEVEL value.  I will do a more careful review and post 
a fix in the next day or so.  I have been in Ottawa for OLS.
I'm flying home tomorrow.

Jim Houston - Concurrent Computer Corp.

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