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

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Date:	Fri, 01 May 2015 11:42:23 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	alexander.h.duyck@...hat.com
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

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().

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.
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