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

Li Zefan
2009-08-04T01:10:57Z

Benjamin Blum wrote:
> On Sun, Aug 2, 2009 at 8:00 PM, Li Zefan<lizf@cn.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.
>

Maybe my English sucks..

By "gained a reference", doesn't it mean get_css_set()? But this
put_css_set() is not against the get() just called.

And in fact the ref can be 0 before this put(), because task_exit
can drop the last ref, but put_css_set() will check this case,
so it's Ok.

>>> +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.
>

Quoted from Documentation/CodingStyle:

...Such a value can be represented as an error-code integer
(-Exxx = failure, 0 = success) or a "succeeded" boolean (0 = failure,
non-zero = success).