linux-kernel - Re: [Experimental][PATCH 19/21] memcg-fix-pre-destroy.patch

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Date:	Thu, 4 Dec 2008 18:34:28 +0900
From:	Daisuke Nishimura <nishimura@....nes.nec.co.jp>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"balbir@...ux.vnet.ibm.com" <balbir@...ux.vnet.ibm.com>,
	"kosaki.motohiro@...fujitsu.com" <kosaki.motohiro@...fujitsu.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"lizf@...fujitsu.com" <lizf@...fujitsu.com>,
	Paul Menage <menage@...gle.com>, nishimura@....nes.nec.co.jp
Subject: Re: [Experimental][PATCH 19/21] memcg-fix-pre-destroy.patch

Added CC: Paul Menage <menage@...gle.com>

> @@ -2096,7 +2112,7 @@ static void mem_cgroup_get(struct mem_cg
>  static void mem_cgroup_put(struct mem_cgroup *mem)
>  {
>  	if (atomic_dec_and_test(&mem->refcnt)) {
> -		if (!mem->obsolete)
> +		if (!css_under_removal(&mem->css))
>  			return;
>  		mem_cgroup_free(mem);
>  	}
I don't think it's safe to check css_under_removal here w/o cgroup_lock.
(It's safe *NOW* just because memcg is the only user of css->refcnt.)

As Li said before, css_under_removal doesn't necessarily mean
this this group has been destroyed, but mem_cgroup will be freed.

But adding cgroup_lock/unlock here causes another dead lock,
because mem_cgroup_get_next_node calls mem_cgroup_put.

hmm.. hierarchical reclaim code will be re-written completely by [21/21],
so would it be better to change patch order or to take another approach ?


Thanks,
Daisuke Nishimura.
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