netdev - RE: [PATCH v3 net] net: solve a NAPI race

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Date:   Tue, 28 Feb 2017 10:14:43 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Eric Dumazet' <eric.dumazet@...il.com>,
        Stephen Hemminger <stephen@...workplumber.org>
CC:     David Miller <davem@...emloft.net>, netdev <netdev@...r.kernel.org>
Subject: RE: [PATCH v3 net] net: solve a NAPI race

From: Eric Dumazet
> Sent: 27 February 2017 22:35
> On Mon, 2017-02-27 at 14:14 -0800, Stephen Hemminger wrote:
> 
> > The original design (as Davem mentioned) was that IRQ's must be disabled
> > during device polling. If that was true, then the race above
> > would be impossible.
> 
> I would love to see an alternative patch.

Can you test for 'receive data available' after telling the NAPI
logic that you've finished?
You'd then need to force a reschedule.

I think your proposed patch will do a reschedule if any packet arrives
during the receive processing, not just when one arrives right at the end.
You might want to 'unset' the reschedule flag before each check of the
receive ring.

I also wonder about the cost of processing the MSI-X (I guess) interrupts
compared to the cost of posted PCIe writes to disable and/or mask the
interrupt generation.
Clearly you don't want to do PCIe reads.

	David