netdev - Re: 2.6.34-rc1: rcu lockdep bug?

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Date:	Fri, 12 Mar 2010 14:03:19 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Américo Wang <xiyou.wangcong@...il.com>
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	David Miller <davem@...emloft.net>, peterz@...radead.org,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: 2.6.34-rc1: rcu lockdep bug?

On Fri, Mar 12, 2010 at 09:11:02PM +0800, Américo Wang wrote:
> On Fri, Mar 12, 2010 at 7:11 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> > Le vendredi 12 mars 2010 à 16:59 +0800, Américo Wang a écrit :
> >> On Fri, Mar 12, 2010 at 4:07 PM, David Miller <davem@...emloft.net> wrote:
> >> > From: Américo Wang <xiyou.wangcong@...il.com>
> >> > Date: Fri, 12 Mar 2010 15:56:03 +0800
> >> >
> >> >> Ok, after decoding the lockdep output, it looks like that
> >> >> netif_receive_skb() should call rcu_read_lock_bh() instead of rcu_read_lock()?
> >> >> But I don't know if all callers of netif_receive_skb() are in softirq context.
> >> >
> >> > Normally, netif_receive_skb() is invoked from softirq context.
> >> >
> >> > However, via netpoll it can be invoked essentially from any context.
> >> >
> >> > But, when this happens, the networking receive path makes amends such
> >> > that this works fine.  That's what the netpoll_receive_skb() check in
> >> > netif_receive_skb() is for.  That check makes it bail out early if the
> >> > call to netif_receive_skb() is via a netpoll invocation.
> >> >
> >>
> >> Oh, I see. This means we should call rcu_read_lock_bh() instead.
> >> If Paul has no objections, I will send a patch for this.
> >>
> >
> > Nope, its calling rcu_read_lock() from interrupt context and it should
> > stay as is (we dont need to disable bh, this has a cpu cost)
> >
> 
> Oh, but lockdep complains about rcu_read_lock(), it said
> rcu_read_lock() can't be used in softirq context.
> 
> Am I missing something?

Hmmm...  It is supposed to be OK to use rcu_read_lock() in pretty much
any context, even NMI.  I will take a look.

							Thanx, Paul
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