[PATCH v8 3/3] cgroups: make procs file writable

[Paul Menage]

On Wed, Mar 9, 2011 at 10:18 PM, Ben Blum <bblum at andrew.cmu.edu> wrote:
>> This BUG_ON() seems unnecessary, given the i++ directly above it.
>
> It's meant to communicate that the loop must go through at least once,
> so that 'struct cgroup *oldcgrp' will be initialised within a loop later
> (setting it to NULL in the beginning is just to shut up the compiler.)

Right, but it's a do {} while() loop with no break in it - it's
impossible to not go through at least once...

>> Should we be setting failed_ss here? Doesn't that mean that if all
>> subsystems pass the can_attach() check but the first one fails a
>> can_attach_task() check, we don't call any cancel_attach() methods?
>>
>> What are the rollback semantics for failing a can_attach_task() check?
>
> They are not called in that order - it's for_each_subsys { can_attach();
> can_attach_task(); }.

Oh, fair point - I misread that.

> Although if the deal is that cancel_attach reverts
> the things that can_attach does (and can_attach_task is separate) (is
> this the case? it should probably go in the documentation), then passing
> a can_attach and failing a can_attach_task should cause cancel_attach to
> get called for that subsystem, which in this code it doesn't. Something
> like:
>
>    retval = ss->can_attach();
>    if (retval) {
>        failed_ss = ss;
>        goto out_cancel_attach;
>    }
>    retval = ss->can_attach_task();
>    if (retval) {
>        failed_ss = ss;
>        cancel_extra_ss = true;
>        goto out_cancel_attach;
>    }

Yes, but maybe call the flag cancel_failed_ss? Slightly more obvious,
to me at least.

>> > + ? ? ? ? ? ? ? ? ? ? ? BUG_ON(!thread_group_leader(tsk));
>>
>> Can this race with an exiting/execing group leader?
>
> No, rcu_read_lock() is held.
>

But rcu_read_lock() doesn't stop any actions - it just stops the data
structures from going away. Can't leadership change during an
execve()?

> (However, I did try to test it, and it looks like if a leader calls
> sys_exit() then the whole group goes away; is this actually guaranteed?)

I think so, but maybe not instantaneously.

Paul