linux-kernel - Re: [PATCH 14/14] cgroup: implement css

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Message-ID: <20140511123837.GC22945@htj.dyndns.org>
Date:	Sun, 11 May 2014 08:38:37 -0400
From:	Tejun Heo <tj@...nel.org>
To:	Johannes Weiner <hannes@...xchg.org>
Cc:	lizefan@...wei.com, cgroups@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 14/14] cgroup: implement css_tryget()

On Sun, May 11, 2014 at 12:54:59AM -0400, Johannes Weiner wrote:
> >  /**
> > + * css_tryget - try to obtain a reference on the specified css
> > + * @css: target css
> > + *
> > + * Obtain a reference on @css unless it already has reached zero and is
> > + * being released.  This function doesn't care whether @css is on or
> > + * offline.  The caller naturally needs to ensure that @css is accessible
> > + * but doesn't have to be holding a reference on it - IOW, RCU protected
> > + * access is good enough for this function.  Returns %true if a reference
> > + * count was successfully obtained; %false otherwise.
> > + */
> > +static inline bool css_tryget(struct cgroup_subsys_state *css)
> > +{
> > +	return percpu_ref_tryget(&css->refcnt);
> > +}
> 
> percpu_ref_tryget() fails once killed (transitioned from per-cpu to
> atomic mode), but exactly this happens during offlining and so this
> would actually be equivalent to css_tryget_online(), no?

Not any more.  percpu/for-3.16 already contains the updates to
percpu_ref_tryget[_live]().  percpu_ref_tryget() succeeds as long as
the refcnt is above zero.

Thanks.

-- 
tejun
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