linux-kernel - Re: [RFC][PATCH] oom: Be less verbose if the oom

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Message-ID: <20140605160029.GA28812@redhat.com>
Date:	Thu, 5 Jun 2014 18:00:29 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Richard Weinberger <richard@....at>
Cc:	hannes@...xchg.org, mhocko@...e.cz, bsingharora@...il.com,
	kamezawa.hiroyu@...fujitsu.com, akpm@...ux-foundation.org,
	vdavydov@...allels.com, tj@...nel.org, handai.szj@...bao.com,
	rientjes@...gle.com, rusty@...tcorp.com.au,
	kirill.shutemov@...ux.intel.com, linux-kernel@...r.kernel.org,
	cgroups@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [RFC][PATCH] oom: Be less verbose if the oom_control event fd
	has listeners

On 06/05, Richard Weinberger wrote:
>
> Am 05.06.2014 16:18, schrieb Oleg Nesterov:
> > On 06/05, Richard Weinberger wrote:
> >>
> >> +int mem_cgroup_has_listeners(struct mem_cgroup *memcg)
> >> +{
> >> +	int ret = 0;
> >> +
> >> +	if (!memcg)
> >> +		goto out;
> >> +
> >> +	spin_lock(&memcg_oom_lock);
> >> +	ret = !list_empty(&memcg->oom_notify);
> >> +	spin_unlock(&memcg_oom_lock);
> >> +
> >> +out:
> >> +	return ret;
> >> +}
> >
> > Do we really need memcg_oom_lock to check list_empty() ? With or without
> > this lock we can race with list_add/del anyway, and I guess we do not care.
>
> Hmm, in mm/memcontrol.c all list_dev/add are under memcg_oom_lock.

And? How this lock can help to check list_empty() ?

list_add/del can come right after mem_cgroup_has_listeners() and change
the value of list_empty() anyway.

> What do I miss?

Or me...

Oleg.

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