[Devel] Re: [PATCH 4/5] freezer cgroup: Fix an RCU warning in cgroup_freezing_or_frozen()

[Peter Zijlstra]

On Fri, 2010-04-23 at 09:05 +0800, Li Zefan wrote:
> 
> 
> You are right in that taking task_lock() is sufficient (I forgot
> this lock rule), but it's not true that whatever locks are held
> in the ->attach method can pin a task's cgroup. 

Ah, can you be more specific about the ->attach() case?

The way I read it, cgroup_attach_task():

        for_each_subsys(root, ss) {
                if (ss->attach)
                        ss->attach(ss, cgrp, oldcgrp, tsk, false);
        }
        set_bit(CGRP_RELEASABLE, &oldcgrp->flags);
        synchronize_rcu();
        put_css_set(cg);

So if you hold a lock that any of those ->attach() methods will use, it
will in fact delay the put_css_set().

Ah, indeed I see your point, it doesn't indeed pin the task to the
cgroup, but does avoid the cgroup from being freed.

Hrmm,.. so anything wanting to really pin a task to its cgroup will have
to use task_lock()? I'll have to see if that works for
sched_setscheduler().

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