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

Andrew Morton
2008-12-10T02:03:31Z

On Wed, 10 Dec 2008 11:46:13 +1000 "Dave Airlie" <airlied@gmail.com> wrote:

> >>
> >> 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.

<greps for a while>

I assume we're talking about drivers/gpu/drm/drm_stub.c:drm_minor_get_id()?

I don't immediately see anything in the idr code which special-cases a
NULL caller pointer?