netdev - Re: changing dev->needed_headroom/needed

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Message-ID: <1375711240.8120.11.camel@jlt4.sipsolutions.net>
Date:	Mon, 05 Aug 2013 16:00:40 +0200
From:	Johannes Berg <johannes@...solutions.net>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Ben Hutchings <bhutchings@...arflare.com>, netdev@...r.kernel.org,
	linux-wireless@...r.kernel.org
Subject: Re: changing dev->needed_headroom/needed_tailroom?

On Fri, 2013-08-02 at 06:11 -0700, Eric Dumazet wrote:
> On Fri, 2013-08-02 at 10:55 +0200, Ben Hutchings wrote:
> 
> > I don't think this is safe when the interface is running (even if
> > carrier is off).  Some functions may read dev->needed_headroom twice and
> > rely on getting the same value each time.
> 
> It should be no problem. Remaining unsafe places should be fixed.

Most interesting would be stack devs, which I hadn't even considered. In
any case, since I can't completely _rely_ on it, it's an optimisation,
the only bugs would be around the double-access and then running
over/under the SKB or so?

> We already had this discussion in the past, and some patches were
> issued. Check commit ae641949df01b85117845bec45328eab6d6fada1
> ("net: Remove all uses of LL_ALLOCATED_SPACE")

That would have addressed some of that, I guess.

I'm asking because some of the crypto stuff we do has fairly large
head/tailroom requirements and it seems I may need to add more. But if
you don't have crypto, it would be much smaller, so I figured we could
switch it.

johannes

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