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SubjectRe: [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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