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

[Vlastimil Babka]

On 8/12/22 20:30, 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@xxxxxxxxxx>

Thanks, added to slab.git for-6.0/fixes

> ---
> 
>  [v3] Move sysfs_slab_unlink() out to kmem_cache_release() and move
>       schedule_work() back right after list_add_tail().
> 
>  mm/slab_common.c | 45 +++++++++++++++++++++++++++++----------------
>  1 file changed, 29 insertions(+), 16 deletions(-)
> 
> diff --git a/mm/slab_common.c b/mm/slab_common.c
> index 17996649cfe3..07b948288f84 100644
> --- a/mm/slab_common.c
> +++ b/mm/slab_common.c
> @@ -392,6 +392,28 @@ kmem_cache_create(const char *name, unsigned int size, unsigned int align,
>  }
>  EXPORT_SYMBOL(kmem_cache_create);
>  
> +#ifdef SLAB_SUPPORTS_SYSFS
> +/*
> + * For a given kmem_cache, kmem_cache_destroy() should only be called
> + * once or there will be a use-after-free problem. The actual deletion
> + * and release of the kobject does not need slab_mutex or cpu_hotplug_lock
> + * protection. So they are now done without holding those locks.
> + *
> + * Note that there will be a slight delay in the deletion of sysfs files
> + * if kmem_cache_release() is called indrectly from a work function.
> + */
> +static void kmem_cache_release(struct kmem_cache *s)
> +{
> +	sysfs_slab_unlink(s);
> +	sysfs_slab_release(s);
> +}
> +#else
> +static void kmem_cache_release(struct kmem_cache *s)
> +{
> +	slab_kmem_cache_release(s);
> +}
> +#endif
> +
>  static void slab_caches_to_rcu_destroy_workfn(struct work_struct *work)
>  {
>  	LIST_HEAD(to_destroy);
> @@ -418,11 +440,7 @@ static void slab_caches_to_rcu_destroy_workfn(struct work_struct *work)
>  	list_for_each_entry_safe(s, s2, &to_destroy, list) {
>  		debugfs_slab_release(s);
>  		kfence_shutdown_cache(s);
> -#ifdef SLAB_SUPPORTS_SYSFS
> -		sysfs_slab_release(s);
> -#else
> -		slab_kmem_cache_release(s);
> -#endif
> +		kmem_cache_release(s);
>  	}
>  }
>  
> @@ -437,20 +455,11 @@ static int shutdown_cache(struct kmem_cache *s)
>  	list_del(&s->list);
>  
>  	if (s->flags & SLAB_TYPESAFE_BY_RCU) {
> -#ifdef SLAB_SUPPORTS_SYSFS
> -		sysfs_slab_unlink(s);
> -#endif
>  		list_add_tail(&s->list, &slab_caches_to_rcu_destroy);
>  		schedule_work(&slab_caches_to_rcu_destroy_work);
>  	} else {
>  		kfence_shutdown_cache(s);
>  		debugfs_slab_release(s);
> -#ifdef SLAB_SUPPORTS_SYSFS
> -		sysfs_slab_unlink(s);
> -		sysfs_slab_release(s);
> -#else
> -		slab_kmem_cache_release(s);
> -#endif
>  	}
>  
>  	return 0;
> @@ -465,14 +474,16 @@ void slab_kmem_cache_release(struct kmem_cache *s)
>  
>  void kmem_cache_destroy(struct kmem_cache *s)
>  {
> +	int refcnt;
> +
>  	if (unlikely(!s) || !kasan_check_byte(s))
>  		return;
>  
>  	cpus_read_lock();
>  	mutex_lock(&slab_mutex);
>  
> -	s->refcount--;
> -	if (s->refcount)
> +	refcnt = --s->refcount;
> +	if (refcnt)
>  		goto out_unlock;
>  
>  	WARN(shutdown_cache(s),
> @@ -481,6 +492,8 @@ void kmem_cache_destroy(struct kmem_cache *s)
>  out_unlock:
>  	mutex_unlock(&slab_mutex);
>  	cpus_read_unlock();
> +	if (!refcnt && !(s->flags & SLAB_TYPESAFE_BY_RCU))
> +		kmem_cache_release(s);
>  }
>  EXPORT_SYMBOL(kmem_cache_destroy);
>