Re: [PATCH -mm 1/5] quick lookup memcg by ID

[Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>]

On Tue, 3 Aug 2010 13:37:23 +0900
KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> wrote:

> On Tue, 3 Aug 2010 13:31:09 +0900
> Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp> wrote:
> 
(snip)
> > > +/* 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;
> > > +
> > > +static struct mem_cgroup *id_to_memcg(unsigned short id)
> > > +{
> > > +	/*
> > > +	 * This array is set to NULL when mem_cgroup is freed.
> > > +	 * IOW, there are no more references && rcu_synchronized().
> > > +	 * This lookup-caching is safe.
> > > +	 */
> > > +	if (unlikely(!mem_cgroups[id])) {
> > > +		struct cgroup_subsys_state *css;
> > > +
> > > +		rcu_read_lock();
> > > +		css = css_lookup(&mem_cgroup_subsys, id);
> > > +		rcu_read_unlock();
> > > +		if (!css)
> > > +			return NULL;
> > > +		mem_cgroups[id] = container_of(css, struct mem_cgroup, css);
> > > +	}
> > > +	return mem_cgroups[id];
> > > +}
> > id_to_memcg() seems to be called under rcu_read_lock() already, so I think
> > rcu_read_lock()/unlock() would be unnecessary.
> > 
> 
> Maybe. I thought about which is better to add
> 
> 	VM_BUG_ON(!rcu_read_lock_held);
> or
> 	rcu_read_lock()
> 	..
> 	rcu_read_unlock()
> 
> Do you like former ? If so, it's ok to remove rcu-read-lock.
> 
Yes, I personally like the former.

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