Re: [patch 021/104] lib/idr.c: fix rcu related race with idr_find

Dave Airlie
2008-12-10T01:46:25Z

>>
>> On Wed, 10 Dec 2008, Dave Airlie wrote:
>>>
>>> On Thu, Dec 4, 2008 at 5:49 AM, Greg KH <gregkh@suse.de> wrote:
>>> > 2.6.27-stable review patch. If anyone has any objections, please let us know.
>>> >
>>> Revert.
>>>
>>> This caused problems in the F10 kernel with idr, the drm device alloc
>>> went all wierd,
>>> it might be a drm bug but changing this code triggers it and so it
>>> isn't really "stable"
>>
>> Well, maybe it should be reverted in mainlne too, then?
>
> It appears idr_replace is broken at least in stable with this patch.
>
> I'm trying to track down where the problem is (idr_replace doesn't look like
> idr_find in a lot of places and I wonder if this has ever been tested.)
>
(cc-trimmed).

Okay I'm not idr expert and maybe what the drm is doing is illegal but
it never caused a problem up to now.

The drm grabs an idr minor number using a NULL pointer to reserve the
number, it then uses idr_replace later
to stick a pointer into the reserved number. However this seems to be
what is broken, I'm not sure if this is a legal
use of idrs but has worked like that for a long time now.

I can fix the drm to workaround this, and allocate my pointers before
I try to get a minor number, but I'd like to know
if my usage is illegal over just overlooked.