netdev - Re: [PATCH] net: Add etun driver

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Message-ID: <m1tzvt3pih.fsf@ebiederm.dsl.xmission.com>
Date:	Fri, 06 Apr 2007 20:08:22 -0600
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Roland Dreier <rdreier@...co.com>
Cc:	Jeff Garzik <jgarzik@...ox.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	David Miller <davem@...emloft.net>, <netdev@...r.kernel.org>,
	Alexey Kuznetsov <kuznet@....inr.ac.ru>,
	Ben Greear <greearb@...delatech.com>,
	Daniel Lezcano <dlezcano@...ibm.com>,
	Dmitry Mishin <dim@...nvz.org>,
	Linux Containers <containers@...ts.osdl.org>
Subject: Re: [PATCH] net: Add etun driver

Roland Dreier <rdreier@...co.com> writes:

>  > +/*
>  > + * The higher levels take care of making this non-reentrant (it's
>  > + * called with bh's disabled).
>  > + */
>  > +static int etun_xmit(struct sk_buff *skb, struct net_device *tx_dev)
>
> You have this comment, but then...
>
>  > +	dev->features		= NETIF_F_FRAGLIST
>  > +				  | NETIF_F_HIGHDMA
>  > +				  | NETIF_F_LLTX;
>
> you set LLTX, which means that the upper layers _don't_ make sure that
> your xmit routine is not reentrant.
>
> It looks like the impact of multiple simultaneous xmit calls is just
> the possibility of screwing up the statistics, but still I think you
> want to drop the LLTX feature (since you have no lock of your own to
> try and take).

Yup.  That is an inconsistency and probably a bug.  I have to think
through what makes most sense in this case.  Though the cheap answer
is clearly to remove NETIF_F_LLTX.


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