um: vector: Replace locks guarding queue depth with atomics

UML vector drivers use ring buffer structures which map
preallocated skbs onto mmsg vectors for use with sendmmsg
and recvmmsg. They are designed around a single consumer,
single producer pattern allowing simultaneous enqueue and
dequeue.

Lock debugging with preemption showed possible races when
locking the queue depth. This patch addresses this by
removing extra locks, adding barriers and making queue
depth inc/dec and access atomic.
Signed-off-by: Anton Ivanov <anton.ivanov@cambridgegreys.com>
Signed-off-by: Richard Weinberger <richard@nod.at>

arch/um/drivers/vector_kern.c

@@ -22,6 +22,7 @@
 #include <linux/interrupt.h>
 #include <linux/firmware.h>
 #include <linux/fs.h>
+#include <asm/atomic.h>
 #include <uapi/linux/filter.h>
 #include <init.h>
 #include <irq_kern.h>
@@ -102,18 +103,33 @@ static const struct {
 
 static void vector_reset_stats(struct vector_private *vp)
 {
+	/* We reuse the existing queue locks for stats */
+
+	/* RX stats are modified with RX head_lock held
+	 * in vector_poll.
+	 */
+
+	spin_lock(&vp->rx_queue->head_lock);
 	vp->estats.rx_queue_max = 0;
 	vp->estats.rx_queue_running_average = 0;
-	vp->estats.tx_queue_max = 0;
-	vp->estats.tx_queue_running_average = 0;
 	vp->estats.rx_encaps_errors = 0;
+	vp->estats.sg_ok = 0;
+	vp->estats.sg_linearized = 0;
+	spin_unlock(&vp->rx_queue->head_lock);
+
+	/* TX stats are modified with TX head_lock held
+	 * in vector_send.
+	 */
+
+	spin_lock(&vp->tx_queue->head_lock);
 	vp->estats.tx_timeout_count = 0;
 	vp->estats.tx_restart_queue = 0;
 	vp->estats.tx_kicks = 0;
 	vp->estats.tx_flow_control_xon = 0;
 	vp->estats.tx_flow_control_xoff = 0;
-	vp->estats.sg_ok = 0;
-	vp->estats.sg_linearized = 0;
+	vp->estats.tx_queue_max = 0;
+	vp->estats.tx_queue_running_average = 0;
+	spin_unlock(&vp->tx_queue->head_lock);
 }
 
 static int get_mtu(struct arglist *def)
@@ -232,12 +248,6 @@ static int get_transport_options(struct arglist *def)
 
 static char *drop_buffer;
 
-/* Array backed queues optimized for bulk enqueue/dequeue and
- * 1:N (small values of N) or 1:1 enqueuer/dequeuer ratios.
- * For more details and full design rationale see
- * http://foswiki.cambridgegreys.com/Main/EatYourTailAndEnjoyIt
- */
-
 
 /*
  * Advance the mmsg queue head by n = advance. Resets the queue to
@@ -247,27 +257,13 @@ static char *drop_buffer;
 
 static int vector_advancehead(struct vector_queue *qi, int advance)
 {
-	int queue_depth;
-
 	qi->head =
 		(qi->head + advance)
 			% qi->max_depth;
 
 
-	spin_lock(&qi->tail_lock);
-	qi->queue_depth -= advance;
-
-	/* we are at 0, use this to
-	 * reset head and tail so we can use max size vectors
-	 */
-
-	if (qi->queue_depth == 0) {
-		qi->head = 0;
-		qi->tail = 0;
-	}
-	queue_depth = qi->queue_depth;
-	spin_unlock(&qi->tail_lock);
-	return queue_depth;
+	atomic_sub(advance, &qi->queue_depth);
+	return atomic_read(&qi->queue_depth);
 }
 
 /*	Advance the queue tail by n = advance.
@@ -277,16 +273,11 @@ static int vector_advancehead(struct vector_queue *qi, int advance)
 
 static int vector_advancetail(struct vector_queue *qi, int advance)
 {
-	int queue_depth;
-
 	qi->tail =
 		(qi->tail + advance)
 			% qi->max_depth;
-	spin_lock(&qi->head_lock);
-	qi->queue_depth += advance;
-	queue_depth = qi->queue_depth;
-	spin_unlock(&qi->head_lock);
-	return queue_depth;
+	atomic_add(advance, &qi->queue_depth);
+	return atomic_read(&qi->queue_depth);
 }
 
 static int prep_msg(struct vector_private *vp,
@@ -339,9 +330,7 @@ static int vector_enqueue(struct vector_queue *qi, struct sk_buff *skb)
 	int iov_count;
 
 	spin_lock(&qi->tail_lock);
-	spin_lock(&qi->head_lock);
-	queue_depth = qi->queue_depth;
-	spin_unlock(&qi->head_lock);
+	queue_depth = atomic_read(&qi->queue_depth);
 
 	if (skb)
 		packet_len = skb->len;
@@ -360,6 +349,7 @@ static int vector_enqueue(struct vector_queue *qi, struct sk_buff *skb)
 		mmsg_vector->msg_hdr.msg_iovlen = iov_count;
 		mmsg_vector->msg_hdr.msg_name = vp->fds->remote_addr;
 		mmsg_vector->msg_hdr.msg_namelen = vp->fds->remote_addr_size;
+		wmb(); /* Make the packet visible to the NAPI poll thread */
 		queue_depth = vector_advancetail(qi, 1);
 	} else
 		goto drop;
@@ -398,7 +388,7 @@ static int consume_vector_skbs(struct vector_queue *qi, int count)
 }
 
 /*
- * Generic vector deque via sendmmsg with support for forming headers
+ * Generic vector dequeue via sendmmsg with support for forming headers
  * using transport specific callback. Allows GRE, L2TPv3, RAW and
  * other transports to use a common dequeue procedure in vector mode
  */
@@ -408,16 +398,13 @@ static int vector_send(struct vector_queue *qi)
 {
 	struct vector_private *vp = netdev_priv(qi->dev);
 	struct mmsghdr *send_from;
-	int result = 0, send_len, queue_depth = qi->max_depth;
+	int result = 0, send_len;
 
 	if (spin_trylock(&qi->head_lock)) {
-		if (spin_trylock(&qi->tail_lock)) {
 		/* update queue_depth to current value */
-			queue_depth = qi->queue_depth;
-			spin_unlock(&qi->tail_lock);
-			while (queue_depth > 0) {
+		while (atomic_read(&qi->queue_depth) > 0) {
 			/* Calculate the start of the vector */
-				send_len = queue_depth;
+			send_len = atomic_read(&qi->queue_depth);
 			send_from = qi->mmsg_vector;
 			send_from += qi->head;
 			/* Adjust vector size if wraparound */
@@ -447,7 +434,6 @@ static int vector_send(struct vector_queue *qi)
 				result = send_len;
 			}
 			if (result > 0) {
-					queue_depth =
 				consume_vector_skbs(qi, result);
 				/* This is equivalent to an TX IRQ.
 				 * Restart the upper layers to feed us
@@ -460,17 +446,16 @@ static int vector_send(struct vector_queue *qi)
 			}
 			netif_wake_queue(qi->dev);
 			/* if TX is busy, break out of the send loop,
-				 *  poll write IRQ will reschedule xmit for us
+			 *  poll write IRQ will reschedule xmit for us.
 			 */
 			if (result != send_len) {
 				vp->estats.tx_restart_queue++;
 				break;
 			}
 		}
-		}
 		spin_unlock(&qi->head_lock);
 	}
-	return queue_depth;
+	return atomic_read(&qi->queue_depth);
 }
 
 /* Queue destructor. Deliberately stateless so we can use
@@ -589,7 +574,7 @@ static struct vector_queue *create_queue(
 	}
 	spin_lock_init(&result->head_lock);
 	spin_lock_init(&result->tail_lock);
-	result->queue_depth = 0;
+	atomic_set(&result->queue_depth, 0);
 	result->head = 0;
 	result->tail = 0;
 	return result;
@@ -668,18 +653,27 @@ static struct sk_buff *prep_skb(
 }
 
 
-/* Prepare queue for recvmmsg one-shot rx - fill with fresh sk_buffs*/
+/* Prepare queue for recvmmsg one-shot rx - fill with fresh sk_buffs */
 
 static void prep_queue_for_rx(struct vector_queue *qi)
 {
 	struct vector_private *vp = netdev_priv(qi->dev);
 	struct mmsghdr *mmsg_vector = qi->mmsg_vector;
 	void **skbuff_vector = qi->skbuff_vector;
-	int i;
+	int i, queue_depth;
 
-	if (qi->queue_depth == 0)
+	queue_depth = atomic_read(&qi->queue_depth);
+
+	if (queue_depth == 0)
 		return;
-	for (i = 0; i < qi->queue_depth; i++) {
+
+	/* RX is always emptied 100% during each cycle, so we do not
+	 * have to do the tail wraparound math for it.
+	 */
+
+	qi->head = qi->tail = 0;
+
+	for (i = 0; i < queue_depth; i++) {
 		/* it is OK if allocation fails - recvmmsg with NULL data in
 		 * iov argument still performs an RX, just drops the packet
 		 * This allows us stop faffing around with a "drop buffer"
@@ -689,7 +683,7 @@ static void prep_queue_for_rx(struct vector_queue *qi)
 		skbuff_vector++;
 		mmsg_vector++;
 	}
-	qi->queue_depth = 0;
+	atomic_set(&qi->queue_depth, 0);
 }
 
 static struct vector_device *find_device(int n)
@@ -985,7 +979,7 @@ static int vector_mmsg_rx(struct vector_private *vp, int budget)
 	 * many do we need to prep the next time prep_queue_for_rx() is called.
 	 */
 
-	qi->queue_depth = packet_count;
+	atomic_add(packet_count, &qi->queue_depth);
 
 	for (i = 0; i < packet_count; i++) {
 		skb = (*skbuff_vector);
@@ -1172,6 +1166,7 @@ static int vector_poll(struct napi_struct *napi, int budget)
 
 	if ((vp->options & VECTOR_TX) != 0)
 		tx_enqueued = (vector_send(vp->tx_queue) > 0);
+	spin_lock(&vp->rx_queue->head_lock);
 	if ((vp->options & VECTOR_RX) > 0)
 		err = vector_mmsg_rx(vp, budget);
 	else {
@@ -1179,6 +1174,7 @@ static int vector_poll(struct napi_struct *napi, int budget)
 		if (err > 0)
 			err = 1;
 	}
+	spin_unlock(&vp->rx_queue->head_lock);
 	if (err > 0)
 		work_done += err;
 
@@ -1225,7 +1221,7 @@ static int vector_net_open(struct net_device *dev)
 			vp->rx_header_size,
 			MAX_IOV_SIZE
 		);
-		vp->rx_queue->queue_depth = get_depth(vp->parsed);
+		atomic_set(&vp->rx_queue->queue_depth, get_depth(vp->parsed));
 	} else {
 		vp->header_rxbuffer = kmalloc(
 			vp->rx_header_size,
@@ -1467,7 +1463,17 @@ static void vector_get_ethtool_stats(struct net_device *dev,
 {
 	struct vector_private *vp = netdev_priv(dev);
 
+	/* Stats are modified in the dequeue portions of
+	 * rx/tx which are protected by the head locks
+	 * grabbing these locks here ensures they are up
+	 * to date.
+	 */
+
+	spin_lock(&vp->tx_queue->head_lock);
+	spin_lock(&vp->rx_queue->head_lock);
 	memcpy(tmp_stats, &vp->estats, sizeof(struct vector_estats));
+	spin_unlock(&vp->rx_queue->head_lock);
+	spin_unlock(&vp->tx_queue->head_lock);
 }
 
 static int vector_get_coalesce(struct net_device *netdev,

arch/um/drivers/vector_kern.h

@@ -14,6 +14,7 @@
 #include <linux/ctype.h>
 #include <linux/workqueue.h>
 #include <linux/interrupt.h>
+#include <asm/atomic.h>
 
 #include "vector_user.h"
 
@@ -44,7 +45,8 @@ struct vector_queue {
 	struct net_device *dev;
 	spinlock_t head_lock;
 	spinlock_t tail_lock;
-	int queue_depth, head, tail, max_depth, max_iov_frags;
+	atomic_t queue_depth;
+	int head, tail, max_depth, max_iov_frags;
 	short options;
 };