linux-kernel - Re: [RFC PATCH v2] Add rcu user eqs exception hooks for async page fault

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Date:	Wed, 28 Nov 2012 15:53:45 +0200
From:	Gleb Natapov <gleb@...hat.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Li Zhong <zhong@...ux.vnet.ibm.com>,
	linux-next list <linux-next@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>, paulmck@...ux.vnet.ibm.com,
	sasha.levin@...cle.com, avi@...hat.com
Subject: Re: [RFC PATCH v2] Add rcu user eqs exception hooks for async page
 fault

On Wed, Nov 28, 2012 at 01:55:42PM +0100, Frederic Weisbecker wrote:
> > So I still want to remove it. And later if it shows that we really needs
> > rcu somewhere in this code path, maybe we could use RCU_NONIDLE() to
> > protect it. ( The suspicious RCU usage reported in commit
> > c5e015d4949aa665 seems related to schedule(), which is not in the code
> > path if we are in cpu idle eqs )
> 
> Yes but if rcu_irq_*() calls are fine to be called there, and I
> believe they are because exception_enter() exits the user mode, we
> should start to protect there right now instead of waiting for a
> potential future warning of illegal RCU use.
> 
Async page not present is not much different from regular page fault
exception when it happens not on idle task (regular #PF cannot happen
on idle task), but code have a special handling for idle task. So why
do you think rcu_irq_*() is required here, but not in page fault
handler?

> >
> > I think we still need Gleb's patch about the idle check in
> > kvm_async_pf_task_wait(), and maybe another patch for the
> > exit_idle()/enter_idle() issue.
> 
> Right.
> 
Done.

--
			Gleb.
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