Date: Tue, 15 Mar 2011 17:13:53 -0400
From: Ben Blum <bblum@andrew.cmu.edu>
To: Paul Menage <menage@google.com>
Cc: Ben Blum <bblum@andrew.cmu.edu>, linux-kernel@vger.kernel.org,
        containers@lists.linux-foundation.org, akpm@linux-foundation.org,
        ebiederm@xmission.com, lizf@cn.fujitsu.com, matthltc@us.ibm.com,
        oleg@redhat.com, David Rientjes <rientjes@google.com>,
        Miao Xie <miaox@cn.fujitsu.com>
Subject: Re: [PATCH v8 3/3] cgroups: make procs file writable
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On Thu, Mar 10, 2011 at 12:01:29PM -0800, Paul Menage wrote:
> On Wed, Mar 9, 2011 at 10:18 PM, Ben Blum <bblum@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...

OK; I guess it can go.

> > 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.

Sounds good.

> >> > + ? ? ? ? ? ? ? ? ? ? ? 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()?

Hmm, you may be right; my understanding of RCU is not complete. But
actually I think the BUG_ON should just be removed, since we're about to
drop locks before handing off to cgroup_attach_proc anyway (so at no
important part is the assertion guaranteed), which will detect and
EAGAIN if such a race happened.

> > (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
> 

Hmm, well, should I make this assumption, then? The code would not be
more complicated either way, really. I kind of prefer it as it is...

-- Ben
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