This fixes a out-of-order problem with several

of the newer drivers. The basic problem was
that the driver was pulling the mbuf off the
drbr ring and then when sending with xmit(), encounting
a full transmit ring. Thus the lower layer
xmit() function would return an error, and the
drivers would then append the data back on to the ring.
For TCP this is a horrible scenario sure to bring
on a fast-retransmit.

The fix is to use drbr_peek() to pull the data pointer
but not remove it from the ring. If it fails then
we either call the new drbr_putback or drbr_advance
method. Advance moves it forward (we do this sometimes
when the xmit() function frees the mbuf). When
we succeed we always call advance. The
putback will always copy the mbuf back to the top
of the ring. Note that the putback *cannot* be used
with a drbr_dequeue() only with drbr_peek(). We most
of the time, in putback, would not need to copy it
back since most likey the mbuf is still the same, but
sometimes xmit() functions will change the mbuf via
a pullup or other call. So the optimial case for
the single consumer is to always copy it back. If
we ever do a multiple_consumer (for lagg?) we
will  need a test and atomic in the put back possibly
a seperate putback_mc() in the ring buf.

Reviewed by:	jhb@freebsd.org, jlv@freebsd.org

1 files changed, 52 insertions, 1 deletions

diff --git a/sys/net/if_var.h b/sys/net/if_var.h
index 2ec73e4..7fcbd38 100644
--- a/sys/net/if_var.h
+++ b/sys/net/if_var.h
@@ -622,6 +622,45 @@ drbr_enqueue(struct ifnet *ifp, struct buf_ring *br, struct mbuf *m)
 }
 
 static __inline void
+drbr_putback(struct ifnet *ifp, struct buf_ring *br, struct mbuf *new)
+{
+	/*
+	 * The top of the list needs to be swapped 
+	 * for this one.
+	 */
+#ifdef ALTQ
+	if (ifp != NULL && ALTQ_IS_ENABLED(&ifp->if_snd)) {
+		/* 
+		 * Peek in altq case dequeued it
+		 * so put it back.
+		 */
+		IFQ_DRV_PREPEND(&ifp->if_snd, new);
+		return;
+	}
+#endif
+	buf_ring_putback_sc(br, new);
+}
+
+static __inline struct mbuf *
+drbr_peek(struct ifnet *ifp, struct buf_ring *br)
+{
+#ifdef ALTQ
+	struct mbuf *m;
+	if (ifp != NULL && ALTQ_IS_ENABLED(&ifp->if_snd)) {
+		/* 
+		 * Pull it off like a dequeue
+		 * since drbr_advance() does nothing
+		 * for altq and drbr_putback() will
+		 * use the old prepend function.
+		 */
+		IFQ_DEQUEUE(&ifp->if_snd, m);
+		return (m);
+	}
+#endif
+	return(buf_ring_peek(br));
+}
+
+static __inline void
 drbr_flush(struct ifnet *ifp, struct buf_ring *br)
 {
 	struct mbuf *m;
@@ -648,7 +687,7 @@ drbr_dequeue(struct ifnet *ifp, struct buf_ring *br)
 #ifdef ALTQ
 	struct mbuf *m;
 
-	if (ALTQ_IS_ENABLED(&ifp->if_snd)) {	
+	if (ifp != NULL && ALTQ_IS_ENABLED(&ifp->if_snd)) {	
 		IFQ_DEQUEUE(&ifp->if_snd, m);
 		return (m);
 	}
@@ -656,6 +695,18 @@ drbr_dequeue(struct ifnet *ifp, struct buf_ring *br)
 	return (buf_ring_dequeue_sc(br));
 }
 
+static __inline void
+drbr_advance(struct ifnet *ifp, struct buf_ring *br)
+{
+#ifdef ALTQ
+	/* Nothing to do here since peek dequeues in altq case */
+	if (ifp != NULL && ALTQ_IS_ENABLED(&ifp->if_snd))
+		return;
+#endif
+	return (buf_ring_advance_sc(br));
+}
+
+
 static __inline struct mbuf *
 drbr_dequeue_cond(struct ifnet *ifp, struct buf_ring *br,
     int (*func) (struct mbuf *, void *), void *arg)