linux-kernel - Re: RCU idle CPU detection is broken in linux-next

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Date:	Wed, 26 Sep 2012 16:56:55 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	paulmck@...ux.vnet.ibm.com
Cc:	Sasha Levin <levinsasha928@...il.com>,
	Michael Wang <wangyun@...ux.vnet.ibm.com>,
	Dave Jones <davej@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: RCU idle CPU detection is broken in linux-next

2012/9/25 Paul E. McKenney <paulmck@...ux.vnet.ibm.com>:
> On Tue, Sep 25, 2012 at 01:59:26PM +0200, Frederic Weisbecker wrote:
>> Given that we have:
>>
>> rcu_irq_enter()
>>       rcu_user_exit()
>>       rcu_user_enter()
>> rcu_irq_exit()
>
> Indeed, the code to deal with irq misnestings won't like that at all.
> And we are in the kernel between rcu_user_exit() and rcu_user_enter()
> (right?), so we could in fact see irq misnestings.

Exactly.

>
>> And we already have rcu_user_exit_after_irq(), this starts to be confusing
>> if we allow that nesting. Although if we find a solution that, in the end,
>> merge rcu_user_exit() with rcu_user_exit_after_irq() and same for the enter version,
>> this would probably be a good thing. Provided this doesn't involve some more
>> complicated rdtp->dyntick_nesting trickies nor more overhead.
>>
>> Otherwise we could avoid to call rcu_user_* when we are in an irq. When we'll have
>> the user_hooks layer, we can perhaps manage that from that place. For
>> now may be we can return after in_interrupt() in the rcu user apis.
>
> This last sounds best.

Ok.

>
> My main concern is irq misnesting.  We might need to do something ugly
> like record the interrupt nesting level at rcu_user_exit() and restore
> it at rcu_user_enter().  Sigh!!!

Right, but that doesn't seem to apply in x86? I suggest we start
simple and think
about some wider solution when more architecture implement this.
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