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

From: annie li <[email-masked]>
Date: Mon, 28 Oct 2013 10:58:32 +0800

On 2013-10-28 10:36, annie li wrote:
>
> On 2013-10-27 19:11, Wei Liu wrote:
>> 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.
>>
>> Suggested-by: Jan Beulich <[email-masked]>
>> Suggested-by: David Vrabel <[email-masked]>
>> Signed-off-by: Wei Liu <[email-masked]>
>> Cc: Ian Campbell <[email-masked]>
>> Cc: Jason Luan <[email-masked]>
>> ---
>>   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 */
>> +    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);
>
> You simply replace "credit_timeout.expires" with 
> "vif->credit_window_start" here, and never update 
> "vif->credit_window_start" in following code.
>
>>         /* 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;
>
> updates credit_window_start as following,
> vif->credit_window_start = (unsigned long)now;

both credit_window_start and credit_timeout.expires need to be updated 
here,

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

>
>>           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);
>
> vif->credit_timeout.expires is unsigned long, and this still causes 
> original issue on 32bit system, which works well on 64bit. Or 
> rewriting code to avoid uses of vif->credit_timeout.expires, but it is 
> complex.
My understanding here is wrong, please ignore this.

Thanks
Annie