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

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Message-ID: <20140512000619.GC10617@kmo-pixel>
Date:	Sun, 11 May 2014 17:06:19 -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 03:55:22PM -0400, Tejun Heo wrote:
> 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.

No, it's not needed with RCU... look at the aio code for an example (or don't,
save your eyes instead).

Conceptually the RCU data structure should own a refcount on the things that are
accessible via it; that ref shouldn't be dropped until after it's removed and an
RCU barrier has happened.
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