On Thursday 19 July 2007 12:58, Ingo Molnar wrote:
> i.e. it's the classic 'eth0 got stuck somehow' tx/rx state machine
> hickup symptoms, with no other bad symptoms such as lockups or crashes.
Duh, I found it.
The e1000 poll routine does this to leave polling mode.
netif_rx_complete(poll_dev);
e1000_irq_enable(adapter);
return 0;
Which looks innocent enough, except that e1000_irq_enable has
this little irq_sem counter:
if (likely(atomic_dec_and_test(&adapter->irq_sem))) {
E1000_WRITE_REG(&adapter->hw, IMS, IMS_ENABLE_MASK);
E1000_WRITE_FLUSH(&adapter->hw);
}
So as poll_napi calls the poll() routine repeatedly, the irq_sem
counter is decremented by one each time. During the first call,
it re-enables the interrupt. During the next calls, irq_sem goes
negative.
Then an interrupt comes in, e1000_intr disables the interrupt,
increments irq_sem by one, and schedules the device for rx_action.
rx_action calls dev->poll(), which finishes cleaning rx/rx rings,
and when it finds there's no more work, it calls rx_complete and
irq_enable. Except irq_enable doesn't enable anything now, since
irq_sem is <= 0, and dec_and_test returns false.
The whole irq_sem accounting in the e1000 does not rhyme well with
netpoll's way of exercising dev->poll(). The reason my patch triggers
the problem reliably for you is that now, we always get at least
two invocations of dev->poll: once from poll_napi - where we do not
remove the device from the poll list any longer - and another one
from net_rx_action.
I don't have a fix ready yet - I hope I'll have something later
this afternoon.
Olaf
--
Olaf Kirch | --- o --- Nous sommes du soleil we love when we play
[EMAIL PROTECTED] | / | \ sol.dhoop.naytheet.ah kin.ir.samse.qurax
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