Date:   Thu, 18 Nov 2021 11:49:39 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     yajun.deng@linux.dev
Cc:     will@kernel.org, boqun.feng@gmail.com, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] refcount: introduce refcount_is_one() helper function
Message-ID: <YZYvw/uOvjKnY/jM@hirez.programming.kicks-ass.net>
References: <YZYSdJh7xpkHJjoa@hirez.programming.kicks-ass.net>
 <20211118074200.GF174703@worktop.programming.kicks-ass.net>
 <20211118035328.5730-1-yajun.deng@linux.dev>
 <a4b7401c06d3baa5d3d0a43d7aa6c62c@linux.dev>
 <aecc489431c38f170097ae1eb37a7d22@linux.dev>
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Disposition: inline
Content-Transfer-Encoding: 8bit
In-Reply-To: <aecc489431c38f170097ae1eb37a7d22@linux.dev>
Precedence: bulk

On Thu, Nov 18, 2021 at 10:34:44AM +0000, yajun.deng@linux.dev wrote:
> November 18, 2021 4:44 PM, "Peter Zijlstra" <peterz@infradead.org> wrote:
> 
> > On Thu, Nov 18, 2021 at 08:12:56AM +0000, yajun.deng@linux.dev wrote:
> > 
> >> November 18, 2021 3:42 PM, "Peter Zijlstra" <peterz@infradead.org> wrote:
> >> 
> >> On Thu, Nov 18, 2021 at 11:53:28AM +0800, Yajun Deng wrote:
> >> 
> >> There are many cases where it is necessary to determine if refcount is one,
> >> introduce refcount_is_one() helper function for these cases.
> >> 
> >> Give me one that is not racy?
> >> 
> >> The following determine refcount is one, 35 count.
> > 
> > Very good, now get me one that isn't broken :-)
> 
> Sorry, I didn't understand what is the 'isn't broken'。

What's the value of refcount_read() given that at any moment a
concurrent refcount_{inc,dec}() can happen?

If you can't know the current value (per the above) then what's the
value of knowing it was one some time ago?

Fundamentally using refcount_read() in control flow is broken, it's a
very bad anti-pattern.