Re: [RFC PATCH 06/10] memcg: do obj_cgroup_put inside drain_obj_stock

[Sebastian Andrzej Siewior]

On 2025-03-14 11:17:28 [+0100], Vlastimil Babka wrote:
> On 3/14/25 07:15, Shakeel Butt wrote:
> > Previously we could not call obj_cgroup_put() inside the local lock
> > because on the put on the last reference, the release function
> > obj_cgroup_release() may try to re-acquire the local lock. However that
> > chain has been broken. Now simply do obj_cgroup_put() inside
> > drain_obj_stock() instead of returning the old objcg.
> > 
> > Signed-off-by: Shakeel Butt <shakeel.butt@xxxxxxxxx>
> 
> Hm is this really safe? I can see obj_cgroup_release() doing
> percpu_ref_exit() -> kfree(), do we have guaranteed that allocation won't be
> also in a kmemcg and recurse?

This was like this until commit
	5675114623872 ("mm/memcg: protect memcg_stock with a local_lock_t")

at which point the put had to happen outside. This "percpu_ref_exit() ->
kfree()" was also prior this commit.

Sebastian