linux-kernel - Re: [patch] revert: [NET]: Fix races in net_rx

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Date:	Thu, 19 Jul 2007 08:42:52 -0700
From:	"Kok, Auke" <auke-jan.h.kok@...el.com>
To:	Olaf Kirch <olaf.kirch@...cle.com>
CC:	Ingo Molnar <mingo@...e.hu>, Jarek Poplawski <jarkao2@...pl>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, davem@...emloft.net,
	Auke Kok <auke-jan.h.kok@...el.com>
Subject: Re: [patch] revert: [NET]: Fix races in net_rx_action vs netpoll

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,

Auke
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