buf_ring/drbr: Add buf_ring_peek_clear_sc and use it in drbr_peek

Unlike buf_ring_peek, it only supports single consumer mode, and it
clears the cons_head if DEBUG_BUFRING/INVARIANTS is defined.

The normal use case of drbr_peek for network drivers is:

m = drbr_peek(br);
err = hw_spec_encap(&m); /* could m_defrag/m_collapse */
(*)
if (err) {
    if (m == NULL)
        drbr_advance(br);
    else
        drbr_putback(br, m);
    /* break the loop */
}
drbr_advance(br);

The race is:
If hw_spec_encap() m_defrag or m_collapse the mbuf, i.e. the old mbuf
was freed, or like the Hyper-V's network driver, that transmission-
done does not even require the TX lock; then on the other CPU at the
(*) time, the freed mbuf could be recycled and being drbr_enqueue even
before the current CPU had the chance to call drbr_{advance,putback}.
This triggers a panic in drbr_enqueue duplicated element check, if
DEBUG_BUFRING/INVARIANTS is defined.

Use buf_ring_peek_clear_sc() in drbr_peek() to fix the above race.

This change is a NO-OP, if neither DEBUG_BUFRING nor INVARIANTS are
defined.

MFC after:	1 week
Sponsored by:	Microsoft OSTC
Differential Revision:	https://reviews.freebsd.org/D5416

2 files changed, 32 insertions, 1 deletions

diff --git a/sys/net/ifq.h b/sys/net/ifq.h
index 769d5a5..98a84aa 100644
--- a/sys/net/ifq.h
+++ b/sys/net/ifq.h
@@ -369,7 +369,7 @@ drbr_peek(struct ifnet *ifp, struct buf_ring *br)
 		return (m);
 	}
 #endif
-	return(buf_ring_peek(br));
+	return(buf_ring_peek_clear_sc(br));
 }
 
 static __inline void
diff --git a/sys/sys/buf_ring.h b/sys/sys/buf_ring.h
index 012aabf..6a6bfc4 100644
--- a/sys/sys/buf_ring.h
+++ b/sys/sys/buf_ring.h
@@ -268,6 +268,37 @@ buf_ring_peek(struct buf_ring *br)
 	return (br->br_ring[br->br_cons_head]);
 }
 
+static __inline void *
+buf_ring_peek_clear_sc(struct buf_ring *br)
+{
+#ifdef DEBUG_BUFRING
+	void *ret;
+
+	if (!mtx_owned(br->br_lock))
+		panic("lock not held on single consumer dequeue");
+#endif	
+	/*
+	 * I believe it is safe to not have a memory barrier
+	 * here because we control cons and tail is worst case
+	 * a lagging indicator so we worst case we might
+	 * return NULL immediately after a buffer has been enqueued
+	 */
+	if (br->br_cons_head == br->br_prod_tail)
+		return (NULL);
+
+#ifdef DEBUG_BUFRING
+	/*
+	 * Single consumer, i.e. cons_head will not move while we are
+	 * running, so atomic_swap_ptr() is not necessary here.
+	 */
+	ret = br->br_ring[br->br_cons_head];
+	br->br_ring[br->br_cons_head] = NULL;
+	return (ret);
+#else
+	return (br->br_ring[br->br_cons_head]);
+#endif
+}
+
 static __inline int
 buf_ring_full(struct buf_ring *br)
 {