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Message-ID: <21d7e9970812091746u5b6e8594s8567007ec1e26bac@mail.gmail.com>
Date: Wed, 10 Dec 2008 11:46:13 +1000
From: "Dave Airlie" <airlied@gmail.com>
To: "Linus Torvalds" <torvalds@linux-foundation.org>
Subject: Re: [patch 021/104] lib/idr.c: fix rcu related race with idr_find
Cc: "Greg KH" <gregkh@suse.de>, linux-kernel@vger.kernel.org,
       "Randy Dunlap" <rdunlap@xenotime.net>,
       "Chuck Ebbert" <cebbert@redhat.com>,
       "Domenico Andreoli" <cavokz@gmail.com>, akpm@linux-foundation.org,
       alan@lxorguk.ukuu.org.uk, "Manfred Spraul" <manfred@colorfullife.com>,
       "Clement Calmels" <cboulte@gmail.com>,
       "Nadia Derbey" <Nadia.Derbey@bull.net>,
       "Pierre Peiffer" <peifferp@gmail.com>
In-Reply-To: <21d7e9970812091643n7e3e4fe5g8e44030adfbababa@mail.gmail.com>
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>>
>> 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.
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