>>> + 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
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