Re: [PATCH v4 08/14] res_counter: return amount of charges after res_counter_uncharge

[Michal Hocko <mhocko@suse.cz>]

On Wed 10-10-12 13:03:39, Glauber Costa wrote:
> On 10/09/2012 07:35 PM, Michal Hocko wrote:
> > On Tue 09-10-12 19:14:57, Glauber Costa wrote:
> >> On 10/09/2012 07:08 PM, Michal Hocko wrote:
> >>> As I have already mentioned in my previous feedback this is cetainly not
> >>> atomic as you the lock protects only one group in the hierarchy. How is
> >>> the return value from this function supposed to be used?
> >>
> >> So, I tried to make that clearer in the updated changelog.
> >>
> >> Only the value of the base memcg (the one passed to the function) is
> >> returned, and it is atomic, in the sense that it has the same semantics
> >> as the atomic variables: If 2 threads uncharge 4k each from a 8 k
> >> counter, a subsequent read can return 0 for both. The return value here
> >> will guarantee that only one sees the drop to 0.
> >>
> >> This is used in the patch "kmem_accounting lifecycle management" to be
> >> sure that only one process will call mem_cgroup_put() in the memcg
> >> structure.
> > 
> > Yes, you are using res_counter_uncharge and its semantic makes sense.
> > I was refering to res_counter_uncharge_until (you removed that context
> > from my reply) because that one can race resulting that nobody sees 0
> > even though that parents get down to 0 as a result:
> > 	 A
> > 	 |
> > 	 B
> > 	/ \
> >       C(x)  D(y)
> > 
> > D and C uncharge everything.
> > 
> > CPU0				CPU1
> > ret += uncharge(D) [0]		ret += uncharge(C) [0]
> > ret += uncharge(B) [x-from C]
> > 				ret += uncharge(B) [0]
> > 				ret += uncharge(A) [y-from D]
> > ret += uncharge(A) [0]
> > 
> > ret == x			ret == y
> > 
> 
> Sorry Michal, I didn't realize you were talking about
> res_counter_uncharge_until.

I could have been more specific.

> I don't really need res_counter_uncharge_until to return anything, so I
> can just remove that if you prefer, keeping just the main
> res_counter_uncharge.
>
> However, I still can't make sense of your concern.
> 
> The return value will return the value of the counter passed as a
> parameter to the function:
> 
>                 r = res_counter_uncharge_locked(c, val);
>                 if (c == counter)
>                         ret = r;

Dohh. I have no idea where I took ret += r from. Sorry about the noise.
 
> So when you call res_counter_uncharge_until(D, whatever, x), you will
> see zero here as a result, and when you call
> res_counter_uncharge_until(D, whatever, y) you will see 0 here as well.
> 
> A doesn't get involved with that.

You are right.

-- 
Michal Hocko
SUSE Labs