linux-kernel - Re: [PATCH 6/6] Makes procs file writable to move all threads by tgid at once

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Date:	Mon, 3 Aug 2009 17:56:51 -0700
From:	Benjamin Blum <bblum@...gle.com>
To:	Li Zefan <lizf@...fujitsu.com>
Cc:	linux-kernel@...r.kernel.org,
	containers@...ts.linux-foundation.org, akpm@...ux-foundation.org,
	serue@...ibm.com, menage@...gle.com
Subject: Re: [PATCH 6/6] Makes procs file writable to move all threads by tgid 
	at once

On Sun, Aug 2, 2009 at 8:00 PM, Li Zefan<lizf@...fujitsu.com> wrote:
> Ben Blum wrote:
>> +     }
>> +     write_unlock(&css_set_lock);
>> +
>> +     /*
>> +      * We just gained a reference on oldcg by taking it from the task. As
>
> This comment is incorrect, the ref we just got has been dropped by
> the above put_css_set(oldcg).

No, the idea is that even though we had a reference that we already
dropped, we in effect "traded" the newcg to the task for its oldcg,
giving it our reference on newcg and gaining its reference on oldcg. I
believe the cgroup_mutex guarantees that it'll still be there when we
do the trade - perhaps a BUG_ON(tsk->cgroups != oldcg) is wanted
inside the second task_lock section there? At the very least, a
clearer comment.

>> +static int css_set_check_fetched(struct cgroup *cgrp, struct task_struct *tsk,
>> +                              struct css_set *cg,
>> +                              struct list_head *newcg_list)
>> +{
>> +     struct css_set *newcg;
>> +     struct cg_list_entry *cg_entry;
>> +     struct cgroup_subsys_state *template[CGROUP_SUBSYS_COUNT];
>> +     read_lock(&css_set_lock);
>> +     newcg = find_existing_css_set(cg, cgrp, template);
>> +     if (newcg)
>> +             get_css_set(newcg);
>> +     read_unlock(&css_set_lock);
>> +     /* doesn't exist at all? */
>> +     if (!newcg)
>> +             return 1;
>
> I think it's more intuitive to return 1 if found and 0 if not found.

I was sticking with the convention of nonzero return values indicating
failure, as is used everywhere else in this context.
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