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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
Date: Thu, 18 Nov 2021 11:49:39 +0100	[thread overview]
Message-ID: <YZYvw/uOvjKnY/jM@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <aecc489431c38f170097ae1eb37a7d22@linux.dev>

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.

      reply	other threads:[~2021-11-18 10:49 UTC|newest]

Thread overview: 6+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-11-18  3:53 [PATCH] refcount: introduce refcount_is_one() helper function Yajun Deng
2021-11-18  7:42 ` Peter Zijlstra
2021-11-18  8:12 ` yajun.deng
2021-11-18  8:44   ` Peter Zijlstra
2021-11-18 10:34   ` yajun.deng
2021-11-18 10:49     ` Peter Zijlstra [this message]

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