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Date: Mon, 9 Dec 2019 20:33:13 +0000 (UTC)
From: Christopher Lameter <cl@linux.com>
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To: Daniel Wagner <dwagner@suse.de>
cc: Pekka Enberg <penberg@kernel.org>, David Rientjes <rientjes@google.com>, 
    Joonsoo Kim <iamjoonsoo.kim@lge.com>, 
    Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org, 
    linux-kernel@vger.kernel.org
Subject: Re: lockdep warns: cpu_hotplug_lock.rw_sem --> slab_mutex -->
 kn->count#39
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References: <20191209182418.7vxer6vmre67ewvt@beryllium.lan>
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On Mon, 9 Dec 2019, Daniel Wagner wrote:

> [    5.038862]
> [    5.038862] -> #2 (kn->count#39){++++}:
> [    5.039329]        __kernfs_remove+0x240/0x2e0
> [    5.039717]        kernfs_remove_by_name_ns+0x3c/0x80
> [    5.040159]        sysfs_slab_add+0x184/0x250

sysfs_slab_add should not be called under any lock. But it happens during
an initcall (sysfs_slab_init) when the kmalloc slab array is being set up.

And the problems results from a hotplug event? During system bringup when
the slab caches have not been setup yet?

Is this really something that can happen?