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SubjectRe: [PATCH v3] mm/slab_common: Deleting kobject in kmem_cache_destroy() without holding slab_mutex/cpu_hotplug_lock
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!

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