On Mon, 23 Aug 2010 12:35:33 +0900
Daisuke Nishimura <nishimura@xxxxxxxxxxxxxxxxx> wrote:
> Hi,
>
> > +/* 0 is unused */
> > +static atomic_t mem_cgroup_num;
> > +#define NR_MEMCG_GROUPS (CONFIG_MEM_CGROUP_MAX_GROUPS + 1)
> > +static struct mem_cgroup *mem_cgroups[NR_MEMCG_GROUPS] __read_mostly;
> > +
> > +/* Must be called under rcu_read_lock */
> > +static struct mem_cgroup *id_to_memcg(unsigned short id)
> > +{
> > + struct mem_cgroup *ret;
> > + /* see mem_cgroup_free() */
> > + ret = rcu_dereference_check(mem_cgroups[id], rch_read_lock_held());
> > + if (likely(ret && ret->valid))
> > + return ret;
> > + return NULL;
> > +}
> > +
> I prefer "mem" to "ret".
>
Hmm, ok.
> > @@ -2231,7 +2244,7 @@ __mem_cgroup_commit_charge_swapin(struct
> >
> > id = swap_cgroup_record(ent, 0);
> > rcu_read_lock();
> > - memcg = mem_cgroup_lookup(id);
> > + memcg = id_to_memcg(id);
> > if (memcg) {
> > /*
> > * This recorded memcg can be obsolete one. So, avoid
> > @@ -2240,9 +2253,10 @@ __mem_cgroup_commit_charge_swapin(struct
> > if (!mem_cgroup_is_root(memcg))
> > res_counter_uncharge(&memcg->memsw, PAGE_SIZE);
> > mem_cgroup_swap_statistics(memcg, false);
> > + rcu_read_unlock();
> > mem_cgroup_put(memcg);
> > - }
> > - rcu_read_unlock();
> > + } else
> > + rcu_read_unlock();
> > }
> > /*
> > * At swapin, we may charge account against cgroup which has no tasks.
> > @@ -2495,7 +2509,7 @@ void mem_cgroup_uncharge_swap(swp_entry_
> >
> > id = swap_cgroup_record(ent, 0);
> > rcu_read_lock();
> > - memcg = mem_cgroup_lookup(id);
> > + memcg = id_to_memcg(id);
> > if (memcg) {
> > /*
> > * We uncharge this because swap is freed.
> > @@ -2504,9 +2518,10 @@ void mem_cgroup_uncharge_swap(swp_entry_
> > if (!mem_cgroup_is_root(memcg))
> > res_counter_uncharge(&memcg->memsw, PAGE_SIZE);
> > mem_cgroup_swap_statistics(memcg, false);
> > + rcu_read_unlock();
> > mem_cgroup_put(memcg);
> > - }
> > - rcu_read_unlock();
> > + } else
> > + rcu_read_unlock();
> > }
> >
> > /**
> Could you explain why we need rcu_read_unlock() before mem_cgroup_put() ?
> I suspect that it's because mem_cgroup_put() can free the memcg, but do we
> need mem->valid then ?
>
mem_cgroup_put() may call synchronize_rcu(). So, we have to unlock before it.
Thanks,
-Kame
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