linux-kernel - Re: [PATCH] hotplug: Optimize {get,put}_online

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Message-ID: <20131001144820.GP3657@laptop.programming.kicks-ass.net>
Date:	Tue, 1 Oct 2013 16:48:20 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Oleg Nesterov <oleg@...hat.com>, Mel Gorman <mgorman@...e.de>,
	Rik van Riel <riel@...hat.com>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...nel.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Linux-MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH] hotplug: Optimize {get,put}_online_cpus()

On Tue, Oct 01, 2013 at 07:45:37AM -0700, Paul E. McKenney wrote:
> If you don't have cpuhp_seq, you need some other way to avoid
> counter overflow.  Which might be provided by limited number of
> tasks, or, on 64-bit systems, 64-bit counters.

How so? PID space is basically limited to 30 bits, so how could we
overflow a 32bit reference counter?
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