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 15:25:07 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Gleb Natapov <gleb@...hat.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

2012/11/28 Gleb Natapov <gleb@...hat.com>:
> On Wed, Nov 28, 2012 at 01:55:42PM +0100, Frederic Weisbecker wrote:
>> 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?

Because we are not supposed to fault in idle, at least I hope there
are no case around. Except on cases like here with KVM I guess but we
have that special async handler for that.

Note exception_enter() only takes cares of RCU user mode, not idle. If
the need arise in the future we can extend it to handle idle.

>> >
>> > 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.

 Thanks!
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