Re: [Xen-devel] [PATCH net] xen-netback: use jiffies_64 value to calculate credit timeout

On 2013-10-28 16:21, David Vrabel wrote:

On 27/10/2013 11:11, Wei Liu wrote:

quoted

time_after_eq() only works if the delta is < MAX_ULONG/2.

For a 32bit Dom0, if netfront sends packets at a very low rate, the time
between subsequent calls to tx_credit_exceeded() may exceed MAX_ULONG/2
and the test for timer_after_eq() will be incorrect. Credit will not be
replenished and the guest may become unable to send packets (e.g., if
prior to the long gap, all credit was exhausted).

Use jiffies_64 variant to mitigate this problem for 32bit Dom0.

Thanks.

quoted

Suggested-by: Jan Beulich <redacted>
Suggested-by: David Vrabel <redacted>

Jan's suggestion; I just agreed with him.

quoted

Signed-off-by: Wei Liu <redacted>
Cc: Ian Campbell <redacted>
Cc: Jason Luan <redacted>
---
  drivers/net/xen-netback/common.h    |    1 +
  drivers/net/xen-netback/interface.c |    4 ++--
  drivers/net/xen-netback/netback.c   |   13 ++++++-------
  3 files changed, 9 insertions(+), 9 deletions(-)

diff --git a/drivers/net/xen-netback/common.h b/drivers/net/xen-netback/common.h
index 5715318..400fea1 100644
--- a/drivers/net/xen-netback/common.h
+++ b/drivers/net/xen-netback/common.h

@@ -163,6 +163,7 @@ struct xenvif {
  	unsigned long   credit_usec;
  	unsigned long   remaining_credit;
  	struct timer_list credit_timeout;
+	u64 credit_window_start;
  
  	/* Statistics */
  	unsigned long rx_gso_checksum_fixup;

diff --git a/drivers/net/xen-netback/interface.c b/drivers/net/xen-netback/interface.c
index 01bb854..8c45b63 100644
--- a/drivers/net/xen-netback/interface.c
+++ b/drivers/net/xen-netback/interface.c

@@ -312,8 +312,8 @@ struct xenvif *xenvif_alloc(struct device *parent, domid_t domid,
  	vif->credit_bytes = vif->remaining_credit = ~0UL;
  	vif->credit_usec  = 0UL;
  	init_timer(&vif->credit_timeout);
-	/* Initialize 'expires' now: it's used to track the credit window. */
-	vif->credit_timeout.expires = jiffies;
+	/* credit window is tracked in credit_window_start */

You could drop this comment as the field name makes this obvious.

quoted

+	vif->credit_window_start = get_jiffies_64();
  
  	dev->netdev_ops	= &xenvif_netdev_ops;
  	dev->hw_features = NETIF_F_SG | NETIF_F_IP_CSUM | NETIF_F_TSO;

diff --git a/drivers/net/xen-netback/netback.c b/drivers/net/xen-netback/netback.c
index f3e591c..1bc0688 100644
--- a/drivers/net/xen-netback/netback.c
+++ b/drivers/net/xen-netback/netback.c

@@ -1185,18 +1185,17 @@ out:
  
  static bool tx_credit_exceeded(struct xenvif *vif, unsigned size)
  {
-	unsigned long now = jiffies;
-	unsigned long next_credit =
-		vif->credit_timeout.expires +
-		msecs_to_jiffies(vif->credit_usec / 1000);
+	u64 now = get_jiffies_64();
+	u64 next_credit = vif->credit_window_start +
+		(u64)msecs_to_jiffies(vif->credit_usec / 1000);
  
  	/* Timer could already be pending in rare cases. */
  	if (timer_pending(&vif->credit_timeout))
  		return true;
  
  	/* Passed the point where we can replenish credit? */
-	if (time_after_eq(now, next_credit)) {
-		vif->credit_timeout.expires = now;
+	if (time_after_eq64(now, next_credit)) {
+		vif->credit_timeout.expires = (unsigned long)now;

Need to set vif->credit_window_start = now here instead of .expires.

I agree Annie's suggest. add the below line;

   vif->credit_window_start = now

   vif->credit_timeout.expires = (unsigned long)now;

quoted

  		tx_add_credit(vif);
  	}
  

@@ -1207,7 +1206,7 @@ static bool tx_credit_exceeded(struct xenvif *vif, unsigned size)
  		vif->credit_timeout.function =
  			tx_credit_callback;
  		mod_timer(&vif->credit_timeout,
-			  next_credit);
+			  (unsigned long)next_credit);

Need to set vif->credit_window_start = next_credit here as well.

I think that we shouldn't add the above line.

David

Jason