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SubjectRe: [LKP] d50d82faa0 [ 33.671845] WARNING: possible circular locking dependency detected
On Wed, 7 Nov 2018 15:43:36 -0800 Andrew Morton <akpm@linux-foundation.org> wrote:

> On Tue, 23 Oct 2018 08:30:04 +0800 kernel test robot <rong.a.chen@intel.com> wrote:
>
> > Greetings,
> >
> > 0day kernel testing robot got the below dmesg and the first bad commit is
> >
> > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master
> >
> > commit d50d82faa0c964e31f7a946ba8aba7c715ca7ab0
> > Author: Mikulas Patocka <mpatocka@redhat.com>
> > AuthorDate: Wed Jun 27 23:26:09 2018 -0700
> > Commit: Linus Torvalds <torvalds@linux-foundation.org>
> > CommitDate: Thu Jun 28 11:16:44 2018 -0700
> >
> > slub: fix failure when we delete and create a slab cache
>
> This is ugly. Is there an alternative way of fixing the race which
> Mikulas attempted to address? Possibly cancel the work and reuse the
> existing sysfs file, or is that too stupid to live?
>
> 3b7b314053d021 ("slub: make sysfs file removal asynchronous") was
> pretty lame, really. As mentioned,
>
> : It'd be the cleanest to deal with the issue by removing sysfs files
> : without holding slab_mutex before the rest of shutdown; however, given
> : the current code structure, it is pretty difficult to do so.
>
> Would be a preferable approach.
>
> >
> > This uncovered a bug in the slub subsystem - if we delete a cache and
> > immediatelly create another cache with the same attributes, it fails
> > because of duplicate filename in /sys/kernel/slab/. The slub subsystem
> > offloads freeing the cache to a workqueue - and if we create the new
> > cache before the workqueue runs, it complains because of duplicate
> > filename in sysfs.

Alternatively, could we flush the workqueue before attempting to
(re)create the sysfs file? Extra points for only doing this if the
first (re)creation attempt returned -EEXIST?

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