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

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Message-ID: <1488287047.9415.231.camel@edumazet-glaptop3.roam.corp.google.com>
Date:   Tue, 28 Feb 2017 05:04:07 -0800
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     David Laight <David.Laight@...LAB.COM>
Cc:     Stephen Hemminger <stephen@...workplumber.org>,
        David Miller <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH v3 net] net: solve a NAPI race

On Tue, 2017-02-28 at 10:14 +0000, David Laight wrote:
> 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.

Have you seen the mlx4 patch I provided ?

Then, I did not want to review 100+ NAPI drivers and provide patches for
them.

This generic solution is basically free. Same number of atomic
operations.

Given it took more than 2 years to even spot the bug, I have no idea how
people on netdev expect me to review all drivers. This is crazy.