Patch "af_packet: don't pass empty blocks for PACKET_V3" has been added to the 3.18-stable tree

This is a note to let you know that I've just added the patch titled

af_packet: don't pass empty blocks for PACKET_V3

to the 3.18-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
af_packet-don-t-pass-empty-blocks-for-packet_v3.patch
and it can be found in the queue-3.18 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Alexander Drozdov <[email protected]>
Date: Tue, 24 Feb 2015 08:18:28 +0300
Subject: af_packet: don't pass empty blocks for PACKET_V3

From: Alexander Drozdov <[email protected]>

commit 41a50d621a321b4c15273cc1b5ed41437f4acdfb upstream.

Before da413eec729d ("packet: Fixed TPACKET V3 to signal poll when block is closed rather than every packet") poll listening for an af_packet socket was not signaled if there was no packets to process. After the patch poll is signaled evety time when block retire timer expires. That happens because af_packet closes the current block on timeout even if the block is empty.

Passing empty blocks to the user not only wastes CPU but also wastes ring buffer space increasing probability of packets dropping on small timeouts.

---
 net/packet/af_packet.c |   10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

@@ -697,6 +697,10 @@ static void prb_retire_rx_blk_timer_expi
 
 	if (pkc->last_kactive_blk_num == pkc->kactive_blk_num) {
 		if (!frozen) {
+			if (!BLOCK_NUM_PKTS(pbd)) {
+				/* An empty block. Just refresh the timer. */
+				goto refresh_timer;
+			}
 			prb_retire_current_block(pkc, po, TP_STATUS_BLK_TMO);
 			if (!prb_dispatch_next_block(pkc, po))
 				goto refresh_timer;

@@ -796,7 +800,11 @@ static void prb_close_block(struct tpack
 		h1->ts_last_pkt.ts_sec = last_pkt->tp_sec;
 		h1->ts_last_pkt.ts_nsec	= last_pkt->tp_nsec;
 	} else {
-		/* Ok, we tmo'd - so get the current time */
+		/* Ok, we tmo'd - so get the current time.
+		 *
+		 * It shouldn't really happen as we don't close empty
+		 * blocks. See prb_retire_rx_blk_timer_expired().
+		 */
 		struct timespec ts;
 		getnstimeofday(&ts);
 		h1->ts_last_pkt.ts_sec = ts.tv_sec;

Patches currently in stable-queue which might be from [email protected] are

queue-3.18/af_packet-don-t-pass-empty-blocks-for-packet_v3.patch