linux-kernel - Re: [PATCH 1/8] cgroup: kill cgroup_subsys->__DEPRECATED_clear_css

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Date:	Wed, 31 Oct 2012 09:38:48 -0700
From:	Tejun Heo <tj@...nel.org>
To:	Glauber Costa <glommer@...allels.com>
Cc:	lizefan@...wei.com, hannes@...xchg.org, mhocko@...e.cz,
	bsingharora@...il.com, kamezawa.hiroyu@...fujitsu.com,
	cgroups@...r.kernel.org, containers@...ts.linux-foundation.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/8] cgroup: kill
 cgroup_subsys->__DEPRECATED_clear_css_refs

Hello, Glauber.

On Wed, Oct 31, 2012 at 05:21:29PM +0400, Glauber Costa wrote:
> > +
> > +	local_irq_disable();
> > +
> > +	/* block new css_tryget() by deactivating refcnt */
> > +	for_each_subsys(cgrp->root, ss) {
> > +		struct cgroup_subsys_state *css = cgrp->subsys[ss->subsys_id];
> > +
> > +		WARN_ON(atomic_read(&css->refcnt) < 0);
> > +		atomic_add(CSS_DEACT_BIAS, &css->refcnt);
> >  	}
> > -	/* NO css_tryget() can success after here. */
> > +
> > +	/*
> > +	 * Set REMOVED.  All in-progress css_tryget() will be released.
> > +	 * Put all the base refs.  Each css holds an extra reference to the
> > +	 * cgroup's dentry and cgroup removal proceeds regardless of css
> > +	 * refs.  On the last put of each css, whenever that may be, the
> > +	 * extra dentry ref is put so that dentry destruction happens only
> > +	 * after all css's are released.
> > +	 */
> > +	for_each_subsys(cgrp->root, ss) {
> > +		struct cgroup_subsys_state *css = cgrp->subsys[ss->subsys_id];
> > +
> > +		set_bit(CSS_REMOVED, &css->flags);
> > +		css_put(css);
> > +	}
> > +
> > +	local_irq_enable();
> > +
> >  	finish_wait(&cgroup_rmdir_waitq, &wait);
> 
> It seems to me that the main reason for doing this in two passes was to
> derive a global "failed" state on the first pass. Now that we can't
> fail, why can't you just loop through the subsystems just once ?

Later in the series, pre_destroy() happens inbetween.  The first loop
marks the CSSes and cgrp dead, ->pre_destroy()s are called, and the
later loop will put the final ref.  I'll mention it in the commit
message.

Thanks.

-- 
tejun
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