Both races may happen when a device (e.g. YOTA 4G LTE Modem) is
unplugged while the system is downloading a large file from the Net.
Hardware breakpoints and Kprobes with delays were used to confirm that
the races do actually happen.
1. The first race is on skb_queue ('next' pointer) between usbnet_stop()
and rx_complete(), which, in turn, calls usbnet_bh().
Here is a part of the call stack with the code where the changes to the
queue happen. The line numbers are for the kernel 4.1.0:
*0 __skb_unlink (skbuff.h:1517)
prev->next = next;
*1 defer_bh (usbnet.c:430)
spin_lock_irqsave(&list->lock, flags);
old_state = entry->state;
entry->state = state;
__skb_unlink(skb, list);
spin_unlock(&list->lock);
spin_lock(&dev->done.lock);
__skb_queue_tail(&dev->done, skb);
if (dev->done.qlen == 1)
tasklet_schedule(&dev->bh);
spin_unlock_irqrestore(&dev->done.lock, flags);
*2 rx_complete (usbnet.c:640)
state = defer_bh(dev, skb, &dev->rxq, state);
At the same time, the following code repeatedly checks if the queue is
empty and reads these values concurrently with the above changes:
*0 usbnet_terminate_urbs (usbnet.c:765)
/* maybe wait for deletions to finish. */
while (!skb_queue_empty(&dev->rxq)
&& !skb_queue_empty(&dev->txq)
&& !skb_queue_empty(&dev->done)) {
schedule_timeout(msecs_to_jiffies(UNLINK_TIMEOUT_MS));
set_current_state(TASK_UNINTERRUPTIBLE);
netif_dbg(dev, ifdown, dev->net,
"waited for %d urb completions\n", temp);
}
*1 usbnet_stop (usbnet.c:806)
if (!(info->flags & FLAG_AVOID_UNLINK_URBS))
usbnet_terminate_urbs(dev);
As a result, it is possible, for example, that the skb is removed from
dev->rxq by __skb_unlink() before the check
"!skb_queue_empty(&dev->rxq)" in usbnet_terminate_urbs() is made. It is
also possible in this case that the skb is added to dev->done queue
after "!skb_queue_empty(&dev->done)" is checked. So
usbnet_terminate_urbs() may stop waiting and return while dev->done
queue still has an item.
Locking in defer_bh() and usbnet_terminate_urbs() was revisited to avoid
this race.
2. The second race is on dev->flags.
dev->flags is set to 0 here:
*0 usbnet_stop (usbnet.c:816)
/* deferred work (task, timer, softirq) must also stop.
* can't flush_scheduled_work() until we drop rtnl (later),
* else workers could deadlock; so make workers a NOP.
*/
dev->flags = 0;
del_timer_sync (&dev->delay);
tasklet_kill (&dev->bh);
And here, the code clears EVENT_RX_KILL bit in dev->flags, which may
execute concurrently with the above operation:
*0 clear_bit (bitops.h:113, inlined)
*1 usbnet_bh (usbnet.c:1475)
/* restart RX again after disabling due to high error rate */
clear_bit(EVENT_RX_KILL, &dev->flags);
It seems, setting dev->flags to 0 is not necessarily atomic w.r.t.
clear_bit() and other bit operations with dev->flags. It is safer to
make it atomic and this way, make the race harmless.
While at it, the checking of EVENT_NO_RUNTIME_PM bit of dev->flags in
usbnet_stop() was fixed too: the bit should be checked before dev->flags
is cleared.
Signed-off-by: Eugene Shatokhin <eugene.shatokhin@xxxxxxxxxx>
---
drivers/net/usb/usbnet.c | 49 ++++++++++++++++++++++++++++++++--------------
include/linux/usb/usbnet.h | 33 +++++++++++++++++++------------
2 files changed, 54 insertions(+), 28 deletions(-)
diff --git a/drivers/net/usb/usbnet.c b/drivers/net/usb/usbnet.c
index 3c86b10..a53124c 100644
--- a/drivers/net/usb/usbnet.c
+++ b/drivers/net/usb/usbnet.c
@@ -428,12 +428,18 @@ static enum skb_state defer_bh(struct usbnet *dev, struct sk_buff *skb,
old_state = entry->state;
entry->state = state;
__skb_unlink(skb, list);
- spin_unlock(&list->lock);
- spin_lock(&dev->done.lock);
+
+ /* defer_bh() is never called with list == &dev->done.
+ * spin_lock_nested() tells lockdep that it is OK to take
+ * dev->done.lock here with list->lock held. *
+ */
+ spin_lock_nested(&dev->done.lock, SINGLE_DEPTH_NESTING);
+
__skb_queue_tail(&dev->done, skb);
if (dev->done.qlen == 1)
tasklet_schedule(&dev->bh);
- spin_unlock_irqrestore(&dev->done.lock, flags);
+ spin_unlock(&dev->done.lock);
+ spin_unlock_irqrestore(&list->lock, flags);
return old_state;
}
@@ -749,6 +755,20 @@ EXPORT_SYMBOL_GPL(usbnet_unlink_rx_urbs);
/*-------------------------------------------------------------------------*/
+static void wait_skb_queue_empty(struct sk_buff_head *q)
+{
+ unsigned long flags;
+
+ spin_lock_irqsave(&q->lock, flags);
+ while (!skb_queue_empty(q)) {
+ spin_unlock_irqrestore(&q->lock, flags);
+ schedule_timeout(msecs_to_jiffies(UNLINK_TIMEOUT_MS));
+ set_current_state(TASK_UNINTERRUPTIBLE);
+ spin_lock_irqsave(&q->lock, flags);
+ }
+ spin_unlock_irqrestore(&q->lock, flags);
+}
+
// precondition: never called in_interrupt
static void usbnet_terminate_urbs(struct usbnet *dev)
{
@@ -762,14 +782,11 @@ static void usbnet_terminate_urbs(struct usbnet *dev)
unlink_urbs(dev, &dev->rxq);
/* maybe wait for deletions to finish. */
- while (!skb_queue_empty(&dev->rxq)
- && !skb_queue_empty(&dev->txq)
- && !skb_queue_empty(&dev->done)) {
- schedule_timeout(msecs_to_jiffies(UNLINK_TIMEOUT_MS));
- set_current_state(TASK_UNINTERRUPTIBLE);
- netif_dbg(dev, ifdown, dev->net,
- "waited for %d urb completions\n", temp);
- }
+ wait_skb_queue_empty(&dev->rxq);
+ wait_skb_queue_empty(&dev->txq);
+ wait_skb_queue_empty(&dev->done);
+ netif_dbg(dev, ifdown, dev->net,
+ "waited for %d urb completions\n", temp);
set_current_state(TASK_RUNNING);
remove_wait_queue(&dev->wait, &wait);
}
@@ -778,7 +795,8 @@ int usbnet_stop (struct net_device *net)
{
struct usbnet *dev = netdev_priv(net);
struct driver_info *info = dev->driver_info;
- int retval, pm;
+ int retval, pm, mpn;
+ int e;
clear_bit(EVENT_DEV_OPEN, &dev->flags);
netif_stop_queue (net);
@@ -813,14 +831,15 @@ int usbnet_stop (struct net_device *net)
* can't flush_scheduled_work() until we drop rtnl (later),
* else workers could deadlock; so make workers a NOP.
*/
- dev->flags = 0;
+ mpn = !test_and_clear_bit(EVENT_NO_RUNTIME_PM, &dev->flags);
+ for (e = 0; e < EVENT_NUM_EVENTS; ++e)
+ clear_bit(e, &dev->flags);
del_timer_sync (&dev->delay);
tasklet_kill (&dev->bh);
if (!pm)
usb_autopm_put_interface(dev->intf);
- if (info->manage_power &&
- !test_and_clear_bit(EVENT_NO_RUNTIME_PM, &dev->flags))
+ if (info->manage_power && mpn)
info->manage_power(dev, 0);
else
usb_autopm_put_interface(dev->intf);
diff --git a/include/linux/usb/usbnet.h b/include/linux/usb/usbnet.h
index 6e0ce8c..39a57f5 100644
--- a/include/linux/usb/usbnet.h
+++ b/include/linux/usb/usbnet.h
@@ -22,6 +22,23 @@
#ifndef __LINUX_USB_USBNET_H
#define __LINUX_USB_USBNET_H
+enum usbnet_event_bits {
+ EVENT_TX_HALT = 0,
+ EVENT_RX_HALT = 1,
+ EVENT_RX_MEMORY = 2,
+ EVENT_STS_SPLIT = 3,
+ EVENT_LINK_RESET = 4,
+ EVENT_RX_PAUSED = 5,
+ EVENT_DEV_ASLEEP = 6,
+ EVENT_DEV_OPEN = 7,
+ EVENT_DEVICE_REPORT_IDLE = 8,
+ EVENT_NO_RUNTIME_PM = 9,
+ EVENT_RX_KILL = 10,
+ EVENT_LINK_CHANGE = 11,
+ EVENT_SET_RX_MODE = 12,
+ EVENT_NUM_EVENTS /* keep it last */
+};
+
/* interface from usbnet core to each USB networking link we handle */
struct usbnet {
/* housekeeping */
@@ -65,20 +82,10 @@ struct usbnet {
struct tasklet_struct bh;
struct work_struct kevent;
+
+ /* See enum usbnet_event_bits for the meaning of the individial
+ * bits in 'flags'. */
unsigned long flags;
-# define EVENT_TX_HALT 0
-# define EVENT_RX_HALT 1
-# define EVENT_RX_MEMORY 2
-# define EVENT_STS_SPLIT 3
-# define EVENT_LINK_RESET 4
-# define EVENT_RX_PAUSED 5
-# define EVENT_DEV_ASLEEP 6
-# define EVENT_DEV_OPEN 7
-# define EVENT_DEVICE_REPORT_IDLE 8
-# define EVENT_NO_RUNTIME_PM 9
-# define EVENT_RX_KILL 10
-# define EVENT_LINK_CHANGE 11
-# define EVENT_SET_RX_MODE 12
};
static inline struct usb_driver *driver_of(struct usb_interface *intf)
--
2.3.2
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