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SubjectRe: [btrfs] BUG: unable to handle kernel NULL pointer dereference at 0000000000000038
Hello, David, Fengguang, Chris.

On Fri, Feb 07, 2014 at 01:13:06PM -0800, David Rientjes wrote:
> On Fri, 7 Feb 2014, Fengguang Wu wrote:
>
> > On Fri, Feb 07, 2014 at 02:13:59AM -0800, David Rientjes wrote:
> > > On Fri, 7 Feb 2014, Fengguang Wu wrote:
> > >
> > > > [ 1.625020] BTRFS: selftest: Running btrfs_split_item tests
> > > > [ 1.627004] BTRFS: selftest: Running find delalloc tests
> > > > [ 2.289182] tsc: Refined TSC clocksource calibration: 2299.967 MHz
> > > > [ 292.084537] kthreadd invoked oom-killer: gfp_mask=0x3000d0, order=1, oom_score_adj=0
> > > > [ 292.086439] kthreadd cpuset=
> > > > [ 292.087072] BUG: unable to handle kernel NULL pointer dereference at 0000000000000038
> > > > [ 292.087372] IP: [<ffffffff812119de>] pr_cont_kernfs_name+0x1b/0x6c
> > >
> > > This looks like a problem with the cpuset cgroup name, are you sure this
> > > isn't related to the removal of cgroup->name?
> >
> > It looks not related to patch "cgroup: remove cgroup->name", because
> > that patch lies in the cgroup tree and not contained in output of "git log BAD_COMMIT".
> >
>
> It's dying on pr_cont_kernfs_name which is some tree that has "kernfs:
> implement kernfs_get_parent(), kernfs_name/path() and friends", which is
> not in linux-next, and is obviously printing the cpuset cgroup name.
>
> It doesn't look like it has anything at all to do with btrfs or why they
> would care about this failure.

Yeah, this is from a patch in cgroup/review-post-kernfs-conversion
branch which updates cgroup to use pr_cont_kernfs_name(). I forget
that cgrp->kn is NULL for the dummy_root's top cgroup and thus it ends
up calling the kernfs functions with NULL kn and thus the oops. I
posted an updated patch and the git branch has been updated.

http://lkml.kernel.org/g/20140208200640.GB10975@htj.dyndns.org

So, nothing to do with btrfs and it looks like somehow the test
appratus is mixing up branches?

Thanks!

--
tejun


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