From owner-freebsd-stable@freebsd.org  Fri Nov 13 21:30:48 2015
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Date: Sat, 14 Nov 2015 00:30:39 +0300
From: Slawa Olhovchenkov <slw@zxy.spb.ru>
To: freebsd-stable@freebsd.org
Subject: NETMAP and off-by-one?
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I am see strange things: like NETMAP stop transmit after `head` and `cur`
touch `tail`.

But:

/*
 * check if space is available in the ring.
 */
static inline int
nm_ring_empty(struct netmap_ring *ring)
{
        return (ring->cur == ring->tail);
}

i.e. if cur == (tail-1) mod ring_size -- space is available in the
ring and I can put packet in output buffer.
After put this packet ring is full, but no transmiting.

This is bug?