Do a lockless read of the BPF interface structure descriptor list head

before grabbing BPF locks to see if there are any entries in order to
avoid the cost of locking if there aren't any.  Avoids a mutex lock/
unlock for each packet received if there are no BPF listeners.

1 files changed, 21 insertions, 0 deletions

diff --git a/sys/net/bpf.c b/sys/net/bpf.c
index 7c00b9c..6f6c9fe 100644
--- a/sys/net/bpf.c
+++ b/sys/net/bpf.c
@@ -1147,6 +1147,13 @@ bpf_tap(bp, pkt, pktlen)
 	struct bpf_d *d;
 	u_int slen;
 
+	/*
+	 * Lockless read to avoid cost of locking the interface if there are
+	 * no descriptors attached.
+	 */
+	if (bp->bif_dlist == NULL)
+		return;
+
 	BPFIF_LOCK(bp);
 	for (d = bp->bif_dlist; d != NULL; d = d->bd_next) {
 		BPFD_LOCK(d);
@@ -1201,6 +1208,13 @@ bpf_mtap(bp, m)
 	struct bpf_d *d;
 	u_int pktlen, slen;
 
+	/*
+	 * Lockless read to avoid cost of locking the interface if there are
+	 * no descriptors attached.
+	 */
+	if (bp->bif_dlist == NULL)
+		return;
+
 	pktlen = m_length(m, NULL);
 	if (pktlen == m->m_len) {
 		bpf_tap(bp, mtod(m, u_char *), pktlen);
@@ -1240,6 +1254,13 @@ bpf_mtap2(bp, data, dlen, m)
 	struct bpf_d *d;
 	u_int pktlen, slen;
 
+	/*
+	 * Lockless read to avoid cost of locking the interface if there are
+	 * no descriptors attached.
+	 */
+	if (bp->bif_dlist == NULL)
+		return;
+
 	pktlen = m_length(m, NULL);
 	/*
 	 * Craft on-stack mbuf suitable for passing to bpf_filter.