netdev - Re: [PATCH RFX]: napi

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Message-ID: <ada6444y2nd.fsf@cisco.com>
Date:	Sat, 28 Jul 2007 08:21:58 -0700
From:	Roland Dreier <rdreier@...co.com>
To:	David Miller <davem@...emloft.net>
Cc:	netdev@...r.kernel.org, shemminger@...ux-foundation.org,
	rusty@...tcorp.com.au, jgarzik@...ox.com
Subject: Re: [PATCH RFX]: napi_struct V3

I've been on vacation so I missed most of this thread.  I'm just
catching up now...

 > Ok I converted everything with Rusty's suggestion to move napi_struct
 > out of net_device, this was mostly mechanical but some devices took
 > some unanticipated amount of work.

Actually you missed drivers/infiniband/ulp/ipoib, which has used NAPI
since 2.6.22.  I can take a stab at the conversion soon if you want...

 > Another area of consternation are drivers that were using
 > netif_rx_reschedule(), as that interface was removed because it
 > doesn't fit well with the caller managing the dev->quota et al.  I
 > left race conditions in the drivers that were using that interface,
 > but they should still basically work nonetheless.

...but this is a problem for IPoIB.  The underlying IB stuff only
allows us to register what is essentially a one-shot edge-triggered
interrupt.  So there is a race between calling netif_rx_complete() and
calling ib_req_notify_cq() (which enables the interrupt), since the
interrupt might never happen if a packet arrives between the two calls.

In the current driver we are OK because ib_req_notify_cq() can return
a hint if an event was missed, but to use this hint we need a way to
restart the NAPI poll without getting into trouble if the interrupt
handler calls netif_rx_schedule() too.

Thanks,
  Roland
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