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From: Américo Wang on 12 Mar 2010 04:10
On Fri, Mar 12, 2010 at 4:07 PM, David Miller <davem(a)davemloft.net> wrote:
> From: Américo Wang <xiyou.wangcong(a)gmail.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.

Thanks much, David!
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From: Eric Dumazet on 12 Mar 2010 06:20
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(a)davemloft.net> wrote:
> > From: Américo Wang <xiyou.wangcong(a)gmail.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)



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From: Américo Wang on 12 Mar 2010 08:20
On Fri, Mar 12, 2010 at 7:11 PM, Eric Dumazet <eric.dumazet(a)gmail.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(a)davemloft.net> wrote:
>> > From: Américo Wang <xiyou.wangcong(a)gmail.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?
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From: Eric Dumazet on 12 Mar 2010 08:40
Le vendredi 12 mars 2010 à 21:11 +0800, Américo Wang a écrit :

> Oh, but lockdep complains about rcu_read_lock(), it said
> rcu_read_lock() can't be used in softirq context.
>
> Am I missing something?

Well, lockdep might be dumb, I dont know...

I suggest you read rcu_read_lock_bh kernel doc :

/**
* rcu_read_lock_bh - mark the beginning of a softirq-only RCU critical
section
*
* This is equivalent of rcu_read_lock(), but to be used when updates
* are being done using call_rcu_bh(). Since call_rcu_bh() callbacks
* consider completion of a softirq handler to be a quiescent state,
* a process in RCU read-side critical section must be protected by
* disabling softirqs. Read-side critical sections in interrupt context
* can use just rcu_read_lock().
*
*/


Last sentence being perfect :

Read-side critical sections in interrupt context
can use just rcu_read_lock().



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From: Paul E. McKenney on 12 Mar 2010 17:10
On Fri, Mar 12, 2010 at 09:11:02PM +0800, Am�rico Wang wrote:
> On Fri, Mar 12, 2010 at 7:11 PM, Eric Dumazet <eric.dumazet(a)gmail.com> wrote:
> > Le vendredi 12 mars 2010 � 16:59 +0800, Am�rico Wang a �crit :
> >> On Fri, Mar 12, 2010 at 4:07 PM, David Miller <davem(a)davemloft.net> wrote:
> >> > From: Am�rico Wang <xiyou.wangcong(a)gmail.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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