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Date:   Sat, 13 Aug 2022 11:25:28 -0700
From:   Roman Gushchin <roman.gushchin@linux.dev>
To:     Waiman Long <longman@redhat.com>
Cc:     Christoph Lameter <cl@linux.com>,
        Pekka Enberg <penberg@kernel.org>,
        David Rientjes <rientjes@google.com>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Vlastimil Babka <vbabka@suse.cz>,
        Hyeonggon Yoo <42.hyeyoo@gmail.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v3] mm/slab_common: Deleting kobject in
 kmem_cache_destroy() without holding slab_mutex/cpu_hotplug_lock
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References: <20220812183033.346425-1-longman@redhat.com>
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On Fri, Aug 12, 2022 at 02:30:33PM -0400, Waiman Long wrote:
> A circular locking problem is reported by lockdep due to the following
> circular locking dependency.
> 
>   +--> cpu_hotplug_lock --> slab_mutex --> kn->active --+
>   |                                                     |
>   +-----------------------------------------------------+
> 
> The forward cpu_hotplug_lock ==> slab_mutex ==> kn->active dependency
> happens in
> 
>   kmem_cache_destroy():	cpus_read_lock(); mutex_lock(&slab_mutex);
>   ==> sysfs_slab_unlink()
>       ==> kobject_del()
>           ==> kernfs_remove()
> 	      ==> __kernfs_remove()
> 	          ==> kernfs_drain(): rwsem_acquire(&kn->dep_map, ...);
> 
> The backward kn->active ==> cpu_hotplug_lock dependency happens in
> 
>   kernfs_fop_write_iter(): kernfs_get_active();
>   ==> slab_attr_store()
>       ==> cpu_partial_store()
>           ==> flush_all(): cpus_read_lock()
> 
> One way to break this circular locking chain is to avoid holding
> cpu_hotplug_lock and slab_mutex while deleting the kobject in
> sysfs_slab_unlink() which should be equivalent to doing a write_lock
> and write_unlock pair of the kn->active virtual lock.
> 
> Since the kobject structures are not protected by slab_mutex or the
> cpu_hotplug_lock, we can certainly release those locks before doing
> the delete operation.
> 
> Move sysfs_slab_unlink() and sysfs_slab_release() to the newly
> created kmem_cache_release() and call it outside the slab_mutex &
> cpu_hotplug_lock critical sections. There will be a slight delay
> in the deletion of sysfs files if kmem_cache_release() is called
> indirectly from a work function.
> 
> Signed-off-by: Waiman Long <longman@redhat.com>

Reviewed-by: Roman Gushchin <roman.gushchin@linux.dev>

Thank you, Waiman!