Small bug fix from DragonFly / review requested

[Joerg Sonnenberger]

On Sun, Feb 20, 2005 at 09:02:25PM +0000, Robert Watson wrote:
> 
> On Sun, 20 Feb 2005, Max Laier wrote:
> 
> > Joerg Sonnenberger has discovered a problem in the snc(4) driver that
> > might result in packets showing up on bpf multiple times.  See the
> > changelog for sys/dev/netif/snc/dp83932.c in their source tree[1]. 
> > 
> > Attached is a fix for HEAD.  I am almost sure that it is right, but I'd
> > welcome a review as there might be a chance that sonicput() and the TX
> > interrupt free the mbuf before it gets to bpf.  From my reading it seems
> > that the mbuf is "safe" until sc->mtd_free is altered (see comment). 
> 
> This change seems fine -- in the current world order, Giant will prevent
> the driver from preempting itself and freeing the mbuf, and if we get
> per-softc locking for snc, presumably that locking will provide the same
> protection.  It may be worth annotating that we rely on non-preemption by
> the interrupt handler to avoid the mbuf being freed with a comment. 

Quite a lot of the network drivers actually fully depend on that. So it
might be better to add it to a global list of stuff to keep in mind :)

Joerg