linux-kernel - Re: [PATCH] generic dynamic per cpu refcounting

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Message-ID: <20130128215042.GN22465@mtj.dyndns.org>
Date:	Mon, 28 Jan 2013 13:50:42 -0800
From:	Tejun Heo <tj@...nel.org>
To:	Kent Overstreet <koverstreet@...gle.com>
Cc:	Oleg Nesterov <oleg@...hat.com>, srivatsa.bhat@...ux.vnet.ibm.com,
	rusty@...tcorp.com.au, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] generic dynamic per cpu refcounting

Hello, Kent.

On Mon, Jan 28, 2013 at 01:45:06PM -0800, Kent Overstreet wrote:
> Ahh. Bias value sounds... hacky (i.e. harder to convince myself it's
> correct) but I see what you're getting at.

I don't think it's that hacky.  Just push the base point to the
opposite of zero - LLONG_MAX.  The global counter can start biased so
that it gets unbiased only once dying is confirmed and everything can
blindly do atomic64_dec_and_test() and trust the result.

> Something to consider is wrapping; after we set state to dying but
> before we've collected the percpu counters, the atomic counter may be
> negative.

There's a reason why we use 64bit vars for this type of global
counting.  They virtually don't wrap.  Here, you'll need 1<<63 for the
counter to unintentionally reach 0, which we consider practically
impossible.

Thanks.

-- 
tejun
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