Subject: Re: [Xen-devel] [PATCH net] xen-netback: add the scenario which now beyond the range time_after_eq().

From: David Vrabel <[email-masked]>
Date: Thu, 17 Oct 2013 10:15:42 +0100

On 17/10/13 10:02, jianhai luan wrote:
> 
> On 2013-10-17 16:26, Jan Beulich wrote:
>>>>> On 16.10.13 at 19:22, Jason Luan <[email-masked]> wrote:
>>> time_after_eq() only works if the delta is < MAX_ULONG/2.
>>>
>>> If netfront sends 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 (e.g., if prior to the long gap, all
>>> credit was exhausted).
>>>
>>> We should add the scenario which now beyond next_credit+MAX_UNLONG/2.
>>> Because
>>> the fact now must be not before than expire, time_before(now, expire)
>>> == true
>>> will verify the scenario.
>>>      time_after_eq(now, next_credit) || time_before (now, expire)
>>>      ==
>>>      !time_in_range_open(now, expire, next_credit)
>> So first of all this must be with a 32-bit netback. And the not
>> coverable gap between activity is well over 240 days long. _If_
>> this really needs dealing with, then why is extending this from
>> 240+ to 480+ days sufficient? I.e. why don't you simply
>> change to 64-bit jiffy values, and use time_after_eq64()?
> 
> Yes, the issue only can be  reproduced in 32-bit Dom0 (Beyond
> MAX_ULONG/2 in 64-bit will need long long time)
> 
> I think the gap should be think all environment even now extending 480+.
> if now fall in the gap,  one timer will be pending and replenish will be
> in time.  Please run the attachment test program.
> 
> If use time_after_eq64(), expire ,next_credit and other member will must
> be u64.

Yes, you'll need to store next_credit as a u64 in vif instead of
calculating it in tx_credit_exceeded from expires (which is only an
unsigned long).

David