netdev - Re: [PATCH 0/3] Stop using dev_kfree_skb

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Date:	Fri, 01 May 2015 09:45:55 -0700
From:	Alexander Duyck <alexander.h.duyck@...hat.com>
To:	David Miller <davem@...emloft.net>
CC:	netdev@...r.kernel.org, nic_swsd@...ltek.com,
	Dept-HSGLinuxNICDev@...gic.com, sony.chacko@...gic.com,
	intel-wired-lan@...ts.osuosl.org, jeffrey.t.kirsher@...el.com
Subject: Re: [PATCH 0/3] Stop using dev_kfree_skb_any in NAPI Tx clean-up

On 05/01/2015 08:42 AM, David Miller wrote:
> From: Alexander Duyck <alexander.h.duyck@...hat.com>
> Date: Thu, 30 Apr 2015 13:58:12 -0700
>
>> This change replaces the call to dev_kfree_skb_any in several NAPI based Tx
>> clean-up paths with dev_kfree_skb.  Where I made the replacement the
>> function appeared to always be in softirq context so we can avoid the
>> unnecessary time spent determining if we are in a hardirq context or not.
> It isn't always in softirq context.
>
> We can do a netconsole transmit from any context, and that code
> can and will invoke ->poll().

I knew ndo_start_xmit was affected by that, I didn't realize that the 
napi poll routine was as well.

> As Francois noted, netpoll_send_skb_on_dev() loops trying to transmit,
> and if the TX does nto give NETDEV_TX_OK it will try to execute TX
> reclaim by invoking ->poll().
>
> It's hard to trigger this, but if you do... the dev_kfree_skb_any()
> really is necessary.

Ugh, if anything it points out that there are probably a number of bugs 
in the various drivers out there since I think there were assumptions 
made on both sides about how all the NAPI and netpoll stuff works.  I'll 
probably be submitting fixes today for a number of drivers to address 
all the spots that have been missed and little things that need to be 
addressed.

- Alex
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