netdev - Re: [net-next 01/16] ixgbe: add check for netif_carrier_ok in ixgbe_xmit

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Message-ID: <1394842754.15764.13.camel@deadeye.wl.decadent.org.uk>
Date:	Sat, 15 Mar 2014 00:19:14 +0000
From:	Ben Hutchings <ben@...adent.org.uk>
To:	David Miller <davem@...emloft.net>
Cc:	emil.s.tantilov@...el.com, jeffrey.t.kirsher@...el.com,
	netdev@...r.kernel.org, gospo@...hat.com, sassmann@...hat.com,
	asharma@...com, stable@...r.kernel.org
Subject: Re: [net-next 01/16] ixgbe: add check for netif_carrier_ok in
 ixgbe_xmit_frame

On Fri, 2014-03-14 at 17:51 -0400, David Miller wrote:
> From: "Tantilov, Emil S" <emil.s.tantilov@...el.com>
> Date: Fri, 14 Mar 2014 20:19:38 +0000
> 
> > That would work, but what if there are other callers of
> > ndo_start_xmit that don't have this check? Handling this in the
> > driver takes care of all instances.
> 
> netif_carrier_off() is supposed to stop traffic from flowing to
> the device.
> 
> The qdisc layer should be honoring this property, and my
> suspicion is that it is just netpoll operates in the unique
> environment that doesn't.

There is no check for netif_carrier_ok() in the qdisc transmit path.
Instead netif_carrier_off() eventually causes link_watch to call
dev_deactivate() which installs the noop qdisc.

It does seem like transmitters that bypass the qdisc should be checking
netif_carrier_ok(), but drivers must also accept that the effect of
netif_carrier_off() is not immediate.

Maybe ixgbe is not calling netif_stop_all_queues() at quite the right
time?

Ben.

-- 
Ben Hutchings
When you say `I wrote a program that crashed Windows', people just stare ...
and say `Hey, I got those with the system, *for free*'. - Linus Torvalds

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