Re: [Xen-devel] [patch] netback: Xennet half die---netback driver didn't detect the jiffies wrapping correctly.

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 30.11.12 at 10:44, Ian Campbell <Ian.Campbell@xxxxxxxxxx> wrote:
> On Fri, 2012-11-30 at 09:25 +0000, Jan Beulich wrote:
>> >>> On 30.11.12 at 09:35, Ian Campbell <Ian.Campbell@xxxxxxxxxx> wrote:
>> > On Fri, 2012-11-30 at 08:09 +0000, Yi, Shunli wrote:
>> >> Netback driver use " time_after_eq()" to check the jiffies wrapping,
>> >> while this function was only called when the credit is  running out.
>> >> So, if the jiffies wrapped and the credit isn't run out in first half
>> >> jiffies circle, the time_after_eq() cannot check the wrapping any
>> >> more.
>> > 
>> > Which tree is this against? It doesn't appear to be mainline Linux,
>> > which is all I am really interested in these days.
>> > 
>> > Also your patch is missing a Signed-off-by and is whitespace damaged.
>> > Please read Documentation/SubmittingPatches and
>> > Documentation/SubmitChecklist.
>> > 
>> >> This will cause the credit_timerout.expires is set to dozens of days
>> >> in future.
>> >>
>> >>  The netback will stop receiving data from netfront. 
>> >> 
>> >> For example: 
>> >> Jiffies initialized to 0xffffff-(300*HZ), and the
>> >> credit_timeout.expires was initialized to 0xffffff00, 
>> >> After dozens of days,  when the jiffies grow to upper than 0x80000000,
>> >> and the time_after_eq() will cannot check for the wrapping.
>> >> 
>> >> 
>> >> 
>> >> --- drivers/xen/netback/netback.c.org     2012-11-30 15:48:13.109039998 
>> >> -0500
>> >> +++ drivers/xen/netback/netback.c 2012-11-30 15:48:55.212072898 -0500
>> >> @@ -1272,6 +1272,10 @@ static void net_tx_action(unsigned long
>> >>           rmb(); /* Ensure that we see the request before we copy it. */
>> >>           memcpy(&txreq, RING_GET_REQUEST(&netif->tx, i), sizeof(txreq));
>> >>  
>> >> +        /* Check for the jiffies wrapping */
>> >> +        if (time_after_eq(jiffies, netif->credit_timeout.expires))
>> >> +            netif->credit_timeout.expires = jiffies;
>> > 
>> > Do you not need to remove the similar check from the following block?
>> 
>> I don't think so, but I also can't see how that adjustment would
>> help in the first place: If it gets executed after a very long period
>> of no traffic, it would itself not be able to reliably tell whether the
>> clock wrapped.
> 
> Hrm, yes, This change would help in the case of a dribble of traffic
> which never hits the limit, but not in the case of no timer at all.
> 
>> That said, I agree that the code as is appears to have a problem
>> (with 32-bit jiffies at least), but I can't see how to easily deal with
>> it.
> 
> Would it help to always have the pending timer armed, for either the
> next tick if credit needs replenishing or for, say MAX_JIFFIES/4 as a
> backstop to avoid wrapping issues?

If that can be made work cleanly, that would probably be the
easiest solution. But I don't see MAX_JIFFIES being defined
anywhere, and I'm unsure ULONG_MAX/4 would be well received
as a timeout on 64-bit systems.

Jan


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