From: Kent Overstreet <kmo@daterainc.com>
To: Tejun Heo <tj@kernel.org>
Cc: linux-kernel@vger.kernel.org
Subject: Re: [PATCH percpu/for-3.16 2/2] percpu-refcount: implement percpu_ref_tryget()
Date: Sun, 11 May 2014 17:06:19 -0700 [thread overview]
Message-ID: <20140512000619.GC10617@kmo-pixel> (raw)
In-Reply-To: <20140509195522.GF4486@htj.dyndns.org>
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.
next prev parent reply other threads:[~2014-05-12 0:06 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-05-07 15:55 [PATCH percpu/for-3.16 1/2] percpu-refcount: rename percpu_ref_tryget() to percpu_ref_tryget_live() Tejun Heo
2014-05-07 15:56 ` [PATCH REPOST " Tejun Heo
2014-05-07 15:58 ` [PATCH percpu/for-3.16 2/2] percpu-refcount: implement percpu_ref_tryget() Tejun Heo
2014-05-09 19:12 ` Tejun Heo
2014-05-09 19:40 ` Kent Overstreet
2014-05-09 19:41 ` Tejun Heo
2014-05-09 19:51 ` Kent Overstreet
2014-05-09 19:55 ` Tejun Heo
2014-05-12 0:06 ` Kent Overstreet [this message]
2014-05-12 20:07 ` Tejun Heo
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