Subject: Re: [PATCH] natsemi: netpoll fixes

From: Mark Huth <[email-masked]>
Date: Mon, 05 Mar 2007 21:10:20 -0700

Mark Brown wrote:
> [Once more with CCs]
>
> On Tue, Mar 06, 2007 at 12:10:08AM +0400, Sergei Shtylyov wrote:
>
>   
>>  #ifdef CONFIG_NET_POLL_CONTROLLER
>>  static void natsemi_poll_controller(struct net_device *dev)
>>  {
>> +     struct netdev_private *np = netdev_priv(dev);
>> +
>>       disable_irq(dev->irq);
>> -     intr_handler(dev->irq, dev);
>> +
>> +     /*
>> +      * A real interrupt might have already reached us at this point
>> +      * but NAPI might still haven't called us back.  As the
>> interrupt
>> +      * status register is cleared by reading, we should prevent an
>> +      * interrupt loss in this case...
>> +      */
>> +     if (!np->intr_status)
>> +             intr_handler(dev->irq, dev);
>> +
>>       enable_irq(dev->irq);
>>     
>
> Is it possible for this to run at the same time as the NAPI poll?  If so
> then it is possible for the netpoll poll to run between np->intr_status
> being cleared and netif_rx_complete() being called.  If the hardware
> asserts an interrupt at the wrong moment then this could cause the
>   
Well, there is a whole task of analyzing the netpoll conditions under 
smp.  There appears to me to be a race with netpoll and NAPI on another 
processor, given that netpoll can be called with virtually any system 
condition on a debug breakpoint or crash dump initiation.  I'm spending 
some time looking into it, but don't have a smoking gun immediately.  
Regardless, if such a condition does exist, it is shared across many or 
all of the potential netpolled devices.  Since that is exactly the 
condition  the suggested patch purports to solve, it is pointless if the 
whole NAPI/netpoll race exists.  Such a race would lead to various and 
imaginative failures in the system.  So don't fix that problem in a 
particular driver.  If it exists, fix it generally in the netpoll/NAPI 
infrastructure.
> In any case, this is a problem independently of netpoll if the chip
> shares an interrupt with anything so the interrupt handler should be
> fixed to cope with this situation instead.
>   
Yes, that would appear so.  If an interrupt line is shared with this 
device, then the interrupt handler can be called again, even though the 
device's interrupts are disabled on the interface.  So, in the actual 
interrupt handler, check the dev->state __LINK_STATE_SCHED flag - if 
it's set, leave immediately, it can't be our interrupt. If it's clear, 
read the irq enable hardware register.  If enabled, do the rest of the 
interrupt handler. Since the isr is disabled only by the interrupt 
handler, and enabled only by the poll routine, the race on the interrupt 
cause register is prevented.  And, as a byproduct, the netpoll race is 
also prevented.  You could just always read the isr enable hardware 
register, but that means you always do an operation to the chip, which 
can be painfully slow.  I guess the tradeoff depends on the probability 
of getting the isr called when NAPI is active for the device.

If this results in netpoll not getting a packet right away, that's okay, 
since the netpoll users should try again.

Mark Huth