[PATCH 5/5] can: kvaser_usb: Fix tx queue start/stop race conditions

Ahmed S. Darwish
2015-02-26T15:29:22Z

From: Ahmed S. Darwish <ahmed.darwish@valeo.com>

A number of tx queue wake-up events went missing due to the
outlined scenario below. Start state is a pool of 16 tx URBs,
active tx_urbs count = 15, with the netdev tx queue open.

start_xmit() tx_acknowledge()
............ ................
atomic_inc(&tx_urbs);
if (atomic_read(&tx_urbs) >= 16) {
URB completion IRQ!
-->
atomic_dec(&tx_urbs);
netif_wake_queue();
return;
<--
end of IRQ!
netif_stop_queue();
}

At the end, the correct state expected is a 15 tx_urbs count
value with the tx queue state _open_. Due to the race, we get
the same tx_urbs value but with the tx queue state _stopped_.
The wake-up event is completely lost.

Thus avoid hand-rolled concurrency mechanisms and use a proper
lock for contexts protection.

Signed-off-by: Ahmed S. Darwish <ahmed.darwish@valeo.com>
---
drivers/net/can/usb/kvaser_usb.c | 82 +++++++++++++++++++++++++---------------
1 file changed, 51 insertions(+), 31 deletions(-)

diff --git a/drivers/net/can/usb/kvaser_usb.c b/drivers/net/can/usb/kvaser_usb.c
index 13bae86..807ab0c 100644
--- a/drivers/net/can/usb/kvaser_usb.c
+++ b/drivers/net/can/usb/kvaser_usb.c
@@ -14,6 +14,7 @@
* Copyright (C) 2015 Valeo S.A.
*/

+#include <linux/spinlock.h>
#include <linux/kernel.h>
#include <linux/completion.h>
#include <linux/module.h>
@@ -471,10 +472,12 @@ struct kvaser_usb {
struct kvaser_usb_net_priv {
struct can_priv can;

- atomic_t active_tx_urbs;
- struct usb_anchor tx_submitted;
+ spinlock_t tx_contexts_lock;
+ int active_tx_contexts;
struct kvaser_usb_tx_urb_context *tx_contexts;

+ struct usb_anchor tx_submitted;
+
struct completion start_comp, stop_comp;

struct kvaser_usb *dev;
@@ -702,6 +705,7 @@ static void kvaser_usb_tx_acknowledge(const struct kvaser_usb *dev,
struct kvaser_usb_net_priv *priv;
struct sk_buff *skb;
struct can_frame *cf;
+ unsigned long flags;
u8 channel, tid;

channel = msg->u.tx_acknowledge_header.channel;
@@ -745,12 +749,15 @@ static void kvaser_usb_tx_acknowledge(const struct kvaser_usb *dev,

stats->tx_packets++;
stats->tx_bytes += context->dlc;
- can_get_echo_skb(priv->netdev, context->echo_index);

- context->echo_index = dev->max_tx_urbs;
- atomic_dec(&priv->active_tx_urbs);
+ spin_lock_irqsave(&priv->tx_contexts_lock, flags);

+ can_get_echo_skb(priv->netdev, context->echo_index);
+ context->echo_index = dev->max_tx_urbs;
+ --priv->active_tx_contexts;
netif_wake_queue(priv->netdev);
+
+ spin_unlock_irqrestore(&priv->tx_contexts_lock, flags);
}

static void kvaser_usb_simple_msg_callback(struct urb *urb)
@@ -811,18 +818,6 @@ static int kvaser_usb_simple_msg_async(struct kvaser_usb_net_priv *priv,
return 0;
}

-static void kvaser_usb_unlink_tx_urbs(struct kvaser_usb_net_priv *priv)
-{
- struct kvaser_usb *dev = priv->dev;
- int i;
-
- usb_kill_anchored_urbs(&priv->tx_submitted);
- atomic_set(&priv->active_tx_urbs, 0);
-
- for (i = 0; i < dev->max_tx_urbs; i++)
- priv->tx_contexts[i].echo_index = dev->max_tx_urbs;
-}
-
static void kvaser_usb_rx_error_update_can_state(struct kvaser_usb_net_priv *priv,
const struct kvaser_usb_error_summary *es,
struct can_frame *cf)
@@ -1524,6 +1519,26 @@ error:
return err;
}

+static void kvaser_usb_reset_tx_urb_contexts(struct kvaser_usb_net_priv *priv)
+{
+ int i, max_tx_urbs;
+
+ max_tx_urbs = priv->dev->max_tx_urbs;
+
+ priv->active_tx_contexts = 0;
+ for (i = 0; i < max_tx_urbs; i++)
+ priv->tx_contexts[i].echo_index = max_tx_urbs;
+}
+
+/* This method might sleep. Do not call it in the atomic context
+ * of URB completions.
+ */
+static void kvaser_usb_unlink_tx_urbs(struct kvaser_usb_net_priv *priv)
+{
+ usb_kill_anchored_urbs(&priv->tx_submitted);
+ kvaser_usb_reset_tx_urb_contexts(priv);
+}
+
static void kvaser_usb_unlink_all_urbs(struct kvaser_usb *dev)
{
int i;
@@ -1643,6 +1658,7 @@ static netdev_tx_t kvaser_usb_start_xmit(struct sk_buff *skb,
struct kvaser_msg *msg;
int i, err, ret = NETDEV_TX_OK;
u8 *msg_tx_can_flags = NULL; /* GCC */
+ unsigned long flags;

if (can_dropped_invalid_skb(netdev, skb))
return NETDEV_TX_OK;
@@ -1696,12 +1712,21 @@ static netdev_tx_t kvaser_usb_start_xmit(struct sk_buff *skb,
if (cf->can_id & CAN_RTR_FLAG)
*msg_tx_can_flags |= MSG_FLAG_REMOTE_FRAME;

+ spin_lock_irqsave(&priv->tx_contexts_lock, flags);
for (i = 0; i < dev->max_tx_urbs; i++) {
if (priv->tx_contexts[i].echo_index == dev->max_tx_urbs) {
context = &priv->tx_contexts[i];
+
+ context->echo_index = i;
+ can_put_echo_skb(skb, netdev, context->echo_index);
+ ++priv->active_tx_contexts;
+ if (priv->active_tx_contexts >= dev->max_tx_urbs)
+ netif_stop_queue(netdev);
+
break;
}
}
+ spin_unlock_irqrestore(&priv->tx_contexts_lock, flags);

/* This should never happen; it implies a flow control bug */
if (!context) {
@@ -1713,7 +1738,6 @@ static netdev_tx_t kvaser_usb_start_xmit(struct sk_buff *skb,
}

context->priv = priv;
- context->echo_index = i;
context->dlc = cf->can_dlc;

msg->u.tx_can.tid = context->echo_index;
@@ -1725,18 +1749,17 @@ static netdev_tx_t kvaser_usb_start_xmit(struct sk_buff *skb,
kvaser_usb_write_bulk_callback, context);
usb_anchor_urb(urb, &priv->tx_submitted);

- can_put_echo_skb(skb, netdev, context->echo_index);
-
- atomic_inc(&priv->active_tx_urbs);
-
- if (atomic_read(&priv->active_tx_urbs) >= dev->max_tx_urbs)
- netif_stop_queue(netdev);
-
err = usb_submit_urb(urb, GFP_ATOMIC);
if (unlikely(err)) {
+ spin_lock_irqsave(&priv->tx_contexts_lock, flags);
+
can_free_echo_skb(netdev, context->echo_index);
+ context->echo_index = dev->max_tx_urbs;
+ --priv->active_tx_contexts;
+ netif_wake_queue(netdev);
+
+ spin_unlock_irqrestore(&priv->tx_contexts_lock, flags);

- atomic_dec(&priv->active_tx_urbs);
usb_unanchor_urb(urb);

stats->tx_dropped++;
@@ -1863,7 +1886,7 @@ static int kvaser_usb_init_one(struct usb_interface *intf,
struct kvaser_usb *dev = usb_get_intfdata(intf);
struct net_device *netdev;
struct kvaser_usb_net_priv *priv;
- int i, err;
+ int err;

err = kvaser_usb_send_simple_msg(dev, CMD_RESET_CHIP, channel);
if (err)
@@ -1892,10 +1915,7 @@ static int kvaser_usb_init_one(struct usb_interface *intf,
priv->channel = channel;

init_usb_anchor(&priv->tx_submitted);
- atomic_set(&priv->active_tx_urbs, 0);
-
- for (i = 0; i < dev->max_tx_urbs; i++)
- priv->tx_contexts[i].echo_index = dev->max_tx_urbs;
+ kvaser_usb_reset_tx_urb_contexts(priv);

priv->can.state = CAN_STATE_STOPPED;
priv->can.clock.freq = CAN_USB_CLOCK;
--
1.9.1