linux-kernel - Re: [PATCH] netpoll: use non-BH variant of RCU

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Date:	Fri, 13 Aug 2010 13:51:57 -0400
From:	Herbert Xu <herbert@...dor.apana.org.au>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	David Miller <davem@...emloft.net>, linville@...driver.com,
	netdev@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] netpoll: use non-BH variant of RCU

On Fri, Aug 13, 2010 at 09:29:12AM -0700, Paul E. McKenney wrote:
>
> > But this doesn't really solve the problem for netif_rx.  The reason
> > is that netif_rx can either be called with IRQs on OR off.  So we
> > need to take the right precautions in the case where IRQs are
> > enabled along with BH.
> 
> Interesting...
> 
> Is it possible that IRQs are off at rcu_read_lock_bh_irqsoff() time, but
> enabled by the time we get to rcu_read_unlock_bh_irqsoff()?  I hope not,
> but have to ask.  If I am guaranteed of the same state in both cases,
> I can do something like the following:

Yes in our case it's certainly guaranteed that IRQs will remain
off.
 
> But all in all, mightn't it be easier to remove the checks from
> _local_bh_enable(), and then just use rcu_read_lock_bh()?  Have those
> checks really been that helpful in finding bugs?  ;-)

You are right.  It would be much simpler to simply have it not
warn.

Cheers,
-- 
Email: Herbert Xu <herbert@...dor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt
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