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

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Message-ID: <21d7e9970812091746u5b6e8594s8567007ec1e26bac@mail.gmail.com>
Date:	Wed, 10 Dec 2008 11:46:13 +1000
From:	"Dave Airlie" <airlied@...il.com>
To:	"Linus Torvalds" <torvalds@...ux-foundation.org>
Cc:	"Greg KH" <gregkh@...e.de>, linux-kernel@...r.kernel.org,
	"Randy Dunlap" <rdunlap@...otime.net>,
	"Chuck Ebbert" <cebbert@...hat.com>,
	"Domenico Andreoli" <cavokz@...il.com>, akpm@...ux-foundation.org,
	alan@...rguk.ukuu.org.uk,
	"Manfred Spraul" <manfred@...orfullife.com>,
	"Clement Calmels" <cboulte@...il.com>,
	"Nadia Derbey" <Nadia.Derbey@...l.net>,
	"Pierre Peiffer" <peifferp@...il.com>
Subject: Re: [patch 021/104] lib/idr.c: fix rcu related race with idr_find

>>
>> On Wed, 10 Dec 2008, Dave Airlie wrote:
>>>
>>> On Thu, Dec 4, 2008 at 5:49 AM, Greg KH <gregkh@...e.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.
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