linux-kernel - Re: [PATCH 1/4] lockd: convert nlm_host.h_count from atomic_t to refcount

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Date:   Fri, 22 Dec 2017 09:25:53 -0500
From:   "J. Bruce Fields" <bfields@...ldses.org>
To:     "Reshetova, Elena" <elena.reshetova@...el.com>
Cc:     "linux-nfs@...r.kernel.org" <linux-nfs@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "jlayton@...nel.org" <jlayton@...nel.org>,
        "trond.myklebust@...marydata.com" <trond.myklebust@...marydata.com>,
        "anna.schumaker@...app.com" <anna.schumaker@...app.com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "keescook@...omium.org" <keescook@...omium.org>
Subject: Re: [PATCH 1/4] lockd: convert nlm_host.h_count from atomic_t to
 refcount_t

On Fri, Dec 22, 2017 at 09:29:15AM +0000, Reshetova, Elena wrote:
> 
> On Wed, Nov 29, 2017 at 01:15:43PM +0200, Elena Reshetova wrote:
> > atomic_t variables are currently used to implement reference
> > counters with the following properties:
> >  - counter is initialized to 1 using atomic_set()
> >  - a resource is freed upon counter reaching zero
> >  - once counter reaches zero, its further
> >    increments aren't allowed
> >  - counter schema uses basic atomic operations
> >    (set, inc, inc_not_zero, dec_and_test, etc.)
> 
> >Whoops, I forgot that this doesn't apply to h_count.
> 
> >Well, it's confusing, because h_count is actually used in two different
> >ways: depending on whether a nlm_host represents a client or server, it
> >may have the above properties or not.
> 
> 
> So, what happens when it is not having the above properties? Is the object 
> being reused or? 

The object isn't destroyed when the counter hits zero--zero is just
taken as a hint to some garbage collection algorithm that it would be OK
to destroy it.  So decrementing to or incrementing from zero is OK.

--b.