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

[David Rientjes <rientjes@google.com>]

On Fri, 12 Aug 2022, 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>

Acked-by: David Rientjes <rientjes@google.com>