linux-kernel - Re: [RFC PATCH 02/11] refcount: Implement inc/decrement-and-return functions

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Date:   Fri, 1 Sep 2017 18:42:33 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     David Howells <dhowells@...hat.com>
Cc:     linux-afs@...ts.infradead.org, linux-fsdevel@...r.kernel.org,
        Kees Cook <keescook@...omium.org>, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 02/11] refcount: Implement inc/decrement-and-return
 functions

On Fri, Sep 01, 2017 at 04:41:01PM +0100, David Howells wrote:
> Implement functions that increment or decrement a refcount_t object and
> return the value.  The dec-and-ret function can be used to maintain a
> counter in a cache where 1 means the object is unused, but available and
> the garbage collector can use refcount_dec_if_one() to make the object
> unavailable.  Further, both functions can be used to accurately trace the
> refcount (refcount_inc() followed by refcount_read() can't be considered
> accurate).
> 
> The interface is as follows:
> 
> 	unsigned int refcount_dec_return(refcount_t *r);
> 	unsigned int refcount_inc_return(refcount_t *r);
> 

I'm not immediately seeing how wanting 1 to mean unused leads to
requiring these two functions.

If you'll remember, I did that for inode_count and only needed
dec_unless().