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Date: Wed, 2 Apr 2008 12:27:38 -0700
From: "Paul Menage" <menage@google.com>
To: balbir@linux.vnet.ibm.com
Subject: Re: [RFC][-mm] Add an owner to the mm_struct (v4)
Cc: "KAMEZAWA Hiroyuki" <kamezawa.hiroyu@jp.fujitsu.com>,
       "Pavel Emelianov" <xemul@openvz.org>, "Hugh Dickins" <hugh@veritas.com>,
       "Sudhir Kumar" <skumar@linux.vnet.ibm.com>,
       "YAMAMOTO Takashi" <yamamoto@valinux.co.jp>, lizf@cn.fujitsu.com,
       linux-kernel@vger.kernel.org, taka@valinux.co.jp, linux-mm@kvack.org,
       "David Rientjes" <rientjes@google.com>,
       "Andrew Morton" <akpm@linux-foundation.org>
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On Tue, Apr 1, 2008 at 8:25 PM, Balbir Singh <balbir@linux.vnet.ibm.com> wrote:
>
>  > +assign_new_owner:
>  > +     rcu_read_unlock();
>  > +     BUG_ON(c == p);
>  > +     task_lock(c);
>  > +     if (c->mm != mm) {
>  > +             task_unlock(c);
>  > +             goto retry;
>  > +     }
>  > +     cgroup_mm_owner_callbacks(mm->owner, c);
>  > +     mm->owner = c;
>  > +     task_unlock(c);
>  > +}
>  > Why rcu_read_unlock() before changing owner ? Is it safe ?
>  >
>
>  It should be safe, since we take task_lock(), but to be doubly sure, we can drop
>  rcu read lock after taking the task_lock().
>

I agree with Kamezawa - the task can technically disappear as soon as
we leave the RCU critical section. (In practice, it'll only happen
with CONFIG_PREEMPT).

Paul