linux-kernel - Re: [RFC PATCH] Fix abnormal rcu dynticks_nesting values related to async page fault

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Date:	Tue, 27 Nov 2012 17:31:14 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	paulmck@...ux.vnet.ibm.com
Cc:	Li Zhong <zhong@...ux.vnet.ibm.com>,
	linux-next list <linux-next@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>, sasha.levin@...cle.com,
	gleb@...hat.com, avi@...hat.com
Subject: Re: [RFC PATCH] Fix abnormal rcu dynticks_nesting values related to
 async page fault

2012/11/27 Paul E. McKenney <paulmck@...ux.vnet.ibm.com>:
> It is OK to call rcu_irq_exit() without a matching rcu_irq_enter() -only-
> if you have also called rcu_idle_exit() since the last rcu_idle_enter().
> There will be a similar rule for rcu_user_exit().
>
> More generally, it is OK to call rcu_irq_exit() without a matching
> rcu_irq_enter() only if RCU believes that the CPU you are running on is
> non-idle.

Perfect!

>  On 32-bit systems, you are only allowed a few tens of million
> such unmatched rcu_irq_enter() calls in a given RCU-non-idle region.
>
> All courtesy of RCU's need to tolerate architectures that enter
> interrupt handlers without ever leaving them and vice versa.  ;-)

RCU idle mode in a hostile environment ;-)
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