Re: [PATCH v10 4/4] cgroups: implement the PIDs subsystem

Aleksa Sarai
2015-04-23T00:43:12Z

Hi Tejun,

>> + 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.

>> + rcu_read_lock();
>> + css = task_css(task, pids_cgrp_id);
>> + css_get(css);
>
> Why is this safe? What guarantees that css's ref isn't already zero
> at this point?

Because it's already been exposed by pids_fork, so the current css_set
(which contains the current css)'s ref has been bumped. There isn't a
guarantee that there is a ref to css, but there is a guarantee the
css_set it is in has a ref. The problem with using tryget is that we
can't fail here.

>> +static ssize_t pids_max_write(struct kernfs_open_file *of, char *buf,
>> + size_t nbytes, loff_t off)
>> +{
>> + struct cgroup_subsys_state *css = of_css(of);
>> + struct pids_cgroup *pids = css_pids(css);
>> + int64_t limit;
>> + int err;
>> +
>> + buf = strstrip(buf);
>> + if (!strcmp(buf, PIDS_MAX_STR)) {
>> + limit = PIDS_MAX;
>> + goto set_limit;
>> + }
>> +
>> + err = kstrtoll(buf, 0, &limit);
>> + if (err)
>> + return err;
>> +
>> + /* We use INT_MAX as the maximum value of pid_t. */
>> + if (limit < 0 || limit > INT_MAX)
>
> This is kinda weird if we're using PIDS_MAX for max as it may end up
> showing "max" after some larger number is written to the file.

The reason for this is because I believe you said "PIDS_MAX isn't
meant to be exposed to userspace" (one of the previous patchsets used
PIDS_MAX as the maximum valid value).

--
Aleksa Sarai (cyphar)