linux-kernel - Re: [PATCH 5/6] Makes procs file writable to move all threads by tgid at once

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Message-ID: <20090724100807.GG11101@hawkmoon.kerlabs.com>
Date:	Fri, 24 Jul 2009 12:08:07 +0200
From:	Louis Rilling <Louis.Rilling@...labs.com>
To:	Ben Blum <bblum@...gle.com>, linux-kernel@...r.kernel.org,
	containers@...ts.linux-foundation.org, akpm@...ux-foundation.org,
	serue@...ibm.com, lizf@...fujitsu.com, menage@...gle.com
Subject: Re: [PATCH 5/6] Makes procs file writable to move all threads by
	tgid at once

On 24/07/09 12:02 +0200, Louis Rilling wrote:
> Hi Ben,
> 
> On 23/07/09 20:22 -0700, Ben Blum wrote:
> > Makes procs file writable to move all threads by tgid at once
> > 
> > This patch adds functionality that enables users to move all threads in a
> > threadgroup at once to a cgroup by writing the tgid to the 'cgroup.procs'
> > file. This current implementation makes use of a rwsem that's taken for
> > reading in the fork() path to prevent newly forking threads within the
> > threadgroup from "escaping" while moving is in progress.
> > 
> > Signed-off-by: Ben Blum <bblum@...gle.com>
> 

[...]

> > +int cgroup_attach_proc(struct cgroup *cgrp, struct task_struct *leader)
> > +{
> > +	int retval;
> > +	struct cgroup_subsys *ss;
> > +	struct cgroup *oldcgrp;
> > +	struct css_set *oldcg;
> > +	struct cgroupfs_root *root = cgrp->root;
> > +	int subsys_id;
> > +	/* threadgroup list cursor */
> > +	struct task_struct *tsk;
> > +	/*
> > +	 * we need to make sure we have css_sets for all the tasks we're
> > +	 * going to move -before- we actually start moving them, so that in
> > +	 * case we get an ENOMEM we can bail out before making any changes.
> > +	 */
> > +	struct list_head newcg_list;
> > +	struct cg_list_entry *cg_entry;
> > +
> > +	/* first, make sure this came from a valid tgid */
> > +	if (!thread_group_leader(leader))
> > +		return -EINVAL;
> > +	/*
> > +	 * check that we can legitimately attach to the cgroup.
> > +	 */
> > +	for_each_subsys(root, ss) {
> > +		if (ss->can_attach) {
> > +			retval = ss->can_attach(ss, cgrp, leader);
> > +			if (retval)
> > +				return retval;
> > +		}
> >  	}
> 
> So the semantics of ->can_attach() becomes: if called for a thread group leader,
> the result should be valid for the whole thread group, even if only the thread
> group leader is being attached. This looks a bit fuzzy and thus not desirable.
> Why not checking ->can_attach() for all threads (and lock cgroup_fork_mutex
> earlier)?

Ok I've read the next patch. Patch 6 should really go before this one, both for
better understanding and safety.

Thanks,

Louis

-- 
Dr Louis Rilling			Kerlabs
Skype: louis.rilling			Batiment Germanium
Phone: (+33|0) 6 80 89 08 23		80 avenue des Buttes de Coesmes
http://www.kerlabs.com/			35700 Rennes

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