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

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Message-ID: <20140509195522.GF4486@htj.dyndns.org>
Date:	Fri, 9 May 2014 15:55:22 -0400
From:	Tejun Heo <tj@...nel.org>
To:	Kent Overstreet <kmo@...erainc.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: [PATCH percpu/for-3.16 2/2] percpu-refcount: implement
 percpu_ref_tryget()

Hello,

On Fri, May 09, 2014 at 12:51:09PM -0700, Kent Overstreet wrote:
> 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...)

Oh sure, tryget can definitely be misunderstood but RCU protected
iteration is one valid use case.

	rcu_read_lock();
	locate the object of interest;
	tryget[_live]() depending on the use case;
	rcu_read_unlock();

	access the object.

It's not different from why we use atomic_inc_not_zero() in some
places.  The only difference is that percpu_ref distinguishes live
vs. dying states.  It's true that this can be used in pretty stupid
ways but I think the comments are pretty clear on that.  Do you think
we need more warning there?

Thanks.

-- 
tejun
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