linux-kernel - Re: [PATCH]rcu classic: update qlen when cpu offline

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Message-ID: <20080713211315.GA17442@elte.hu>
Date:	Sun, 13 Jul 2008 23:13:15 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Lai Jiangshan <laijs@...fujitsu.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH]rcu classic: update qlen when cpu offline


* Paul E. McKenney <paulmck@...ux.vnet.ibm.com> wrote:

> On Thu, Jun 26, 2008 at 10:06:43AM +0800, Lai Jiangshan wrote:
> > 
> > When callbacks are moved from offline cpu to this cpu,
> > the qlen field of this rdp should be updated.
> 
> Good catch!!!
> 
> The effect of this bug would be for force_quiescent_state() to be invoked
> when it should not and vice versa -- wasting cycles in the first case
> and letting RCU callbacks remain piled up in the second case.  The bug
> is thus "benign" in that it does not result in premature grace-period
> termination, but should of course be fixed nonetheless.
> 
> Preemption is disabled by the caller's get_cpu_var(), so we are guaranteed
> to remain on the same CPU, as required.  The local_irq_disable() is indeed
> needed, otherwise, an interrupt might invoke call_rcu() or call_rcu_bh(),
> which could cause that interrupt's increment of ->qlen to be lost.
> 
> So this patch looks correct to me.  Good job, Jiangshan!!!
> 
> Ingo, would you be willing to add this patch to tip/core/rcu?
> 
> Reviewed-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>

thanks, applied to tip/core/rcu. (sorry about the delay!)

	Ingo
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