Re: [PATCH v2] mm/slab_common: Deleting kobject in kmem_cache_destroy() without holding slab_mutex/cpu_hotplug_lock

[Waiman Long <longman@redhat.com>]

On 8/10/22 14:27, Waiman Long wrote:
> On 8/10/22 14:10, Roman Gushchin wrote:
>> On Wed, Aug 10, 2022 at 12:49:46PM -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.
>>>
>>> Signed-off-by: Waiman Long <longman@redhat.com>
>>> ---
>>>   [v2] Break kmem_cache_release() helper into 2 separate ones.
>>>
>>>   mm/slab_common.c | 54 
>>> +++++++++++++++++++++++++++++++++---------------
>>>   1 file changed, 37 insertions(+), 17 deletions(-)
>>>
>>> diff --git a/mm/slab_common.c b/mm/slab_common.c
>>> index 17996649cfe3..7742d0446d8b 100644
>>> --- a/mm/slab_common.c
>>> +++ b/mm/slab_common.c
>>> @@ -392,6 +392,36 @@ kmem_cache_create(const char *name, unsigned 
>>> int size, unsigned int align,
>>>   }
>>>   EXPORT_SYMBOL(kmem_cache_create);
>>>   +#ifdef SLAB_SUPPORTS_SYSFS
>>> +static void kmem_cache_workfn_release(struct kmem_cache *s)
>>> +{
>>> +    sysfs_slab_release(s);
>>> +}
>>> +#else
>>> +static void kmem_cache_workfn_release(struct kmem_cache *s)
>>> +{
>>> +    slab_kmem_cache_release(s);
>>> +}
>>> +#endif
>>> +
>>> +/*
>>> + * 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.
>>> + */
>>> +static void kmem_cache_release(struct kmem_cache *s)
>>> +{
>>> +#ifdef SLAB_SUPPORTS_SYSFS
>>> +    sysfs_slab_unlink(s);
>>> +#endif
>>> +
>>> +    if (s->flags & SLAB_TYPESAFE_BY_RCU)
>>> +        schedule_work(&slab_caches_to_rcu_destroy_work);
>>> +    else
>>> +        kmem_cache_workfn_release(s);
>>> +}
>>> +
>>>   static void slab_caches_to_rcu_destroy_workfn(struct work_struct 
>>> *work)
>>>   {
>>>       LIST_HEAD(to_destroy);
>>> @@ -418,11 +448,7 @@ 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_workfn_release(s);
>>>       }
>>>   }
>>>   @@ -437,20 +463,10 @@ 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);
>> Hi Waiman!
>>
>> This version is much more readable, thank you!
>>
>> But can we, please, leave this 
>> schedule_work(&slab_caches_to_rcu_destroy_work)
>> call here? I don't see a good reason to move it, do I miss something?
>> It's nice to have list_add_tail() and schedule_work() calls nearby, so
>> it's obvious we can't miss the latter.
>
> The reason that I need to move out schedule_work() as well is to make 
> sure that sysfs_slab_unlink() is called before sysfs_slab_release(). I 
> can't guarantee that if I do schedule_work() first. On the other hand, 
> moving sysfs_slab_unlink() into kmem_cache_workfn_release() introduces 
> unknown delay of when the sysfs file will be removed. I can add some 
> comment to make it more clear.

OK, I just realize that the current patch doesn't have the ordering 
guarantee either if another kmem_cache_destroy() is happening in 
parallel. I will have to push sysfs_slab_unlink() into 
kmem_cache_workfn_release() and tolerate some delay in the disappearance 
of the sysfs files. Now I can move schedule_work() back to after 
list_add_tail().

Cheers,
Longman