linux-kernel - Re: [PATCH -mm] memrlimit: fix task

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Message-ID: <48D2A21E.7050806@linux.vnet.ibm.com>
Date:	Thu, 18 Sep 2008 11:46:54 -0700
From:	Balbir Singh <balbir@...ux.vnet.ibm.com>
To:	righi.andrea@...il.com
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Paul Menage <menage@...gle.com>,
	containers@...ts.linux-foundation.org, linux-mm@...ck.org,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH -mm] memrlimit: fix task_lock() recursive locking

Andrea Righi wrote:
> cgroup_mm_owner_callbacks() can be called with task_lock() held in
> mm_update_next_owner(), and all the .mm_owner_changed callbacks seem to
> be *always* called with task_lock() held.
> 
> Actually, memrlimit is using task_lock() via get_task_mm() in
> memrlimit_cgroup_mm_owner_changed(), raising the following recursive locking
> trace:

[snip]

Thanks for the BUG report()

[snip]

>  static void memrlimit_cgroup_mm_owner_changed(struct cgroup_subsys *ss,
>  						struct cgroup *old_cgrp,
> @@ -246,7 +246,7 @@ static void memrlimit_cgroup_mm_owner_changed(struct cgroup_subsys *ss,
>  						struct task_struct *p)
>  {
>  	struct memrlimit_cgroup *memrcg, *old_memrcg;
> -	struct mm_struct *mm = get_task_mm(p);
> +	struct mm_struct *mm = get_task_mm_task_locked(p);
> 

Since we hold task_lock(), we know that p->mm cannot change and we don't have to
worry about incrementing mm_users. I think using just p->mm will work, we do
have checks to make sure we don't pick a kernel thread. I vote for going down
that road.


-- 
	Balbir
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