netdev - Re: [Xen-devel] [PATCH net] xen-netback: add the scenario which now beyond the range time_after

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Date:	Thu, 17 Oct 2013 23:41:54 +0800
From:	jianhai luan <jianhai.luan@...cle.com>
To:	David Vrabel <david.vrabel@...rix.com>
CC:	Wei Liu <wei.liu2@...rix.com>, Jan Beulich <JBeulich@...e.com>,
	ian.campbell@...rix.com, xen-devel@...ts.xenproject.org,
	annie.li@...cle.com, netdev@...r.kernel.org
Subject: Re: [Xen-devel] [PATCH net] xen-netback: add the scenario which now
 beyond the range time_after_eq().


On 2013-10-17 23:25, David Vrabel wrote:
> On 17/10/13 16:23, jianhai luan wrote:
>> On 2013-10-17 22:06, Wei Liu wrote:
>>> On Thu, Oct 17, 2013 at 09:59:30PM +0800, jianhai luan wrote:
>>> [...]
>>>>>>>> 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).
>>>>>> I know that.  Even we use u64, time_after_eq()  will also do wrong
>>>>>> judge
>>>>>> in theory (not in reality because need long long time).
>>>>> If jiffies_64 has millisecond resolution that would be more than
>>>>> 500,000,000 years.
>>>> Yes, I agree the fact.
>>>>>> I think the two better fixed way is below:
>>>>>>     - By time_before() to judge if now beyond MAX_ULONG/2
>>>>> This is broken, so no.
>>>> Where is broken?  would you like to help me point it out.
>>> I think David means you didn't actually fix the problem. Your solution is
>>> merely a workaround.
>> I have think  about using u64, but more code need to be modified and
>> that is not all.  Key point is how to change the element of struct
>> time_list (expires)  and don't affect other thing?
> I already suggested a way that didn't require changing the timer
> structure -- calculate and store next_credit in advanced.
I think that  modify next_credit only  will not fix the issue. please 
think about:
   - If jiffies have beyond 32 bit. i assume expire  is 0, jiffies_64 
is  0x1000000ff.
     next_credit = 0 +  <always 32-bit value >

     time_after_eq64(jiffies_64, next_credit ) will always true. 
replenish will always do, rate control will lost their function.

Jason
>
> David

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