Re: [PATCH] mm: kmem: add lockdep assertion to obj_cgroup_memcg

[Muchun Song]

> On Jul 24, 2024, at 02:39, Shakeel Butt <shakeel.butt@xxxxxxxxx> wrote:
> 
> On Mon, Jul 22, 2024 at 03:08:10PM GMT, Muchun Song wrote:
>> The obj_cgroup_memcg() is supposed to safe to prevent the returned
>> memory cgroup from being freed only when the caller is holding the
>> rcu read lock or objcg_lock or cgroup_mutex. It is very easy to
>> ignore thoes conditions when users call some upper APIs which call
>> obj_cgroup_memcg() internally like mem_cgroup_from_slab_obj() (See
>> the link below). So it is better to add lockdep assertion to
>> obj_cgroup_memcg() to find those issues ASAP.
>> 
>> Because there is no user of obj_cgroup_memcg() holding objcg_lock
>> to make the returned memory cgroup safe, do not add objcg_lock
>> assertion (We should export objcg_lock if we really want to do)
>> and leave a comment to indicate it is intentional.
>> 
> 
> Do we expect non-memcg code to access objcg_lock? To me this is some
> internal implementation detail of memcg and should not be accessible
> outside memcg code. So, I would recommend to not mention objcg_lock at
> all.

Also make sense. Will update next version.