linux-kernel - Re: [PATCH percpu/for-3.16 2/2] percpu-refcount: implement percpu_ref

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Message-ID: <20140509195109.GC2276@kmo>
Date:	Fri, 9 May 2014 12:51:09 -0700
From:	Kent Overstreet <kmo@...erainc.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: [PATCH percpu/for-3.16 2/2] percpu-refcount: implement
 percpu_ref_tryget()

On Fri, May 09, 2014 at 03:41:40PM -0400, Tejun Heo wrote:
> On Fri, May 09, 2014 at 12:40:21PM -0700, Kent Overstreet wrote:
> > On Wed, May 07, 2014 at 11:58:10AM -0400, Tejun Heo wrote:
> > > Implement percpu_ref_tryget() which fails if the refcnt already
> > > reached zero.  Note that this is different from the recently renamed
> > > percpu_ref_tryget_live() which fails if the refcnt has been killed and
> > > is draining the remaining references.  percpu_ref_tryget() succeeds on
> > > a killed refcnt as long as its current refcnt is above zero.
> > 
> > I'd still kind of prefer tryget() to be labelled "deprecated, don't use outside
> > the cgroup code" or somesuch, but it's not a huge deal :)
> 
> Hmmm... why would it be deprecated?  These are just two different
> operations.

Well not so much deprecated as "bad, avoid" - IMO using tryget() almost always
(I haven't seen a convincing counterexample) means you screwed up your
refcounting somewhere, if you need to take a ref on something whatever made that
object visible to you should have its own ref.

(I think we had this debate, but that was awhile ago...)
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