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);
>
