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From: KAMEZAWA Hiroyuki on 3 Aug 2010 01:00
On Tue, 3 Aug 2010 13:51:29 +0900
Daisuke Nishimura <nishimura(a)mxp.nes.nec.co.jp> wrote:

> On Tue, 3 Aug 2010 13:37:23 +0900
> KAMEZAWA Hiroyuki <kamezawa.hiroyu(a)jp.fujitsu.com> wrote:
>
> > On Tue, 3 Aug 2010 13:31:09 +0900
> > Daisuke Nishimura <nishimura(a)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.

ok, will rewrite in that style.

-Kame

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From: Daisuke Nishimura on 3 Aug 2010 01:00
On Tue, 3 Aug 2010 13:37:23 +0900
KAMEZAWA Hiroyuki <kamezawa.hiroyu(a)jp.fujitsu.com> wrote:

> On Tue, 3 Aug 2010 13:31:09 +0900
> Daisuke Nishimura <nishimura(a)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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From: Daisuke Nishimura on 3 Aug 2010 01:20
On Tue, 3 Aug 2010 13:54:13 +0900
KAMEZAWA Hiroyuki <kamezawa.hiroyu(a)jp.fujitsu.com> wrote:

> On Tue, 3 Aug 2010 13:51:29 +0900
> Daisuke Nishimura <nishimura(a)mxp.nes.nec.co.jp> wrote:
>
> > On Tue, 3 Aug 2010 13:37:23 +0900
> > KAMEZAWA Hiroyuki <kamezawa.hiroyu(a)jp.fujitsu.com> wrote:
> >
> > > On Tue, 3 Aug 2010 13:31:09 +0900
> > > Daisuke Nishimura <nishimura(a)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.
> > > >
Ah, looking into [2/5], this would not be true necessarily..
So, it might be better to leave as it is.

Sorry for noise.

> > >
> > > 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.
>
> ok, will rewrite in that style.
>
> -Kame
>
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