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

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Date:   Tue, 28 Feb 2017 05:20:52 -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.
> 

You understand that a 'reschedule' is only firing another invocation of
napi->poll() right away ?

hot cache lines, basically 0 cost.

In my stress tests, this happens 0.01 % of the times. Bug is tiny.

(Otherwise we would have spotted it earlier)

> 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.

Well, no :

Interrupt has been masked before the napi poll was scheduled.

As David and Stephen says, this condition must not happen, unless your
are really unlucky while doing busy polling, which is opportunistic call
of napi->poll() while you have idle cycles on your cpu.

> 
> 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.

Seems irrelevant to the bug we are discussing.