linux-kernel - Re: [PATCH v3] mm/slab_common: Deleting kobject in kmem_cache_destroy() without holding slab_mutex/cpu_hotplug

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Date:   Mon, 15 Aug 2022 10:01:50 -0400
From:   Waiman Long <longman@...hat.com>
To:     Roman Gushchin <roman.gushchin@...ux.dev>
Cc:     Christoph Lameter <cl@...ux.com>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Hyeonggon Yoo <42.hyeyoo@...il.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] mm/slab_common: Deleting kobject in
 kmem_cache_destroy() without holding slab_mutex/cpu_hotplug_lock

On 8/13/22 14:25, Roman Gushchin wrote:
> 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@...hat.com>
> Reviewed-by: Roman Gushchin <roman.gushchin@...ux.dev>
>
> Thank you, Waiman!
>
Thanks for your suggestions that make this patch better.

Cheers,
Longman