Messages in this thread | | | Date | Thu, 19 Jul 2007 08:42:52 -0700 | From | "Kok, Auke" <> | Subject | Re: [patch] revert: [NET]: Fix races in net_rx_action vs netpoll |
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Olaf Kirch wrote: > 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().
it's been accused of worse things ;)
> 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.
interesting, you seem to found the cause allright. I can't confirm the problem but I know that netpoll and NAPI has historically been an issue. I look forward to your suggestions...
Cheers,
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