MIME-Version: 1.0
Date: Sat, 25 Apr 2015 00:07:34 +1000
Message-ID: <CAOviyagQTu_ARVURBmtHtXGMugMsgSyZ2Es3ix8mQASCNkuPyA@mail.gmail.com>
Subject: Re: [PATCH v10 4/4] cgroups: implement the PIDs subsystem
From: Aleksa Sarai <cyphar@cyphar.com>
To: Tejun Heo <tj@kernel.org>
Cc: lizefan@huawei.com, mingo@redhat.com, peterz@infradead.org, richard@nod.at,
        =?UTF-8?B?RnLDqWTDqXJpYyBXZWlzYmVja2Vy?= <fweisbec@gmail.com>,
        linux-kernel@vger.kernel.org, cgroups@vger.kernel.org
Content-Type: text/plain; charset=UTF-8
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 1637
Lines: 48

>>> +     rcu_read_lock();
>>> +     css = task_css(current, pids_cgrp_id);
>>> +     if (!css_tryget_online(css)) {
>>> +             retval = -EBUSY;
>>> +             goto err_rcu_unlock;
>>> +     }
>>> +     rcu_read_unlock();
>>
>> Hmmm... so, the above is guaranteed to succeed in finite amount of
>> time (the race window is actually very narrow) and it'd be silly to
>> fail fork because a task was being moved across cgroups.
>>
>> I think it'd be a good idea to implement task_get_css() which loops
>> and returns the current css for the requested subsystem with reference
>> count bumped and it can use css_tryget() too.  Holding a ref doesn't
>> prevent css from dying anyway, so it doesn't make any difference.
>
> Hmmm, okay. I'll work on this later.

Would something like this suffice?

struct cgroup_subsys_state *task_get_css(struct task_struct *task, int
subsys_id) {
        bool have_ref = false;
        struct cgroup_subsys_state *css;

        while(!have_ref) {
                rcu_read_lock();
                css = task_css(task, subsys_id);
                have_ref = !css_tryget(css);
                rcu_read_unlock();
        }

        return css;
}

Also, as a side note (in the same vein I guess), does a ref on a
css_set give you an implicit ref on a css inside that css_set, or are
those two orthogonal operations?

--
Aleksa Sarai (cyphar)
www.cyphar.com
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/