netdev - Re: DomU's network interface will hung when Dom0 running 32bit

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Message-ID: <20131015144910.GS11739@zion.uk.xensource.com>
Date:	Tue, 15 Oct 2013 15:49:10 +0100
From:	Wei Liu <wei.liu2@...rix.com>
To:	jianhai luan <jianhai.luan@...cle.com>
CC:	Wei Liu <wei.liu2@...rix.com>,
	Ian Campbell <Ian.Campbell@...rix.com>,
	<xen-devel@...ts.xenproject.org>, <netdev@...r.kernel.org>,
	ANNIE LI <annie.li@...cle.com>
Subject: Re: DomU's network interface will hung when Dom0 running 32bit

On Tue, Oct 15, 2013 at 10:29:15PM +0800, jianhai luan wrote:
> 
> On 2013-10-15 20:58, Wei Liu wrote:
> >On Tue, Oct 15, 2013 at 07:26:31PM +0800, jianhai luan wrote:
> >[...]
> >>>>>Can you propose a patch?
> >>>>Because credit_timeout.expire always after jiffies, i judge the
> >>>>value over the range of time_after_eq() by time_before(now,
> >>>>vif->credit_timeout.expires). please check the patch.
> >>>I don't think this really fix the issue for you. You still have chance
> >>>that now wraps around and falls between expires and next_credit. In that
> >>>case it's stalled again.
> >>if time_before(now, vif->credit_timeout.expires) is true, time wrap
> >>and do operation. Otherwise time_before(now,
> >>vif->credit_timeout.expires) isn't true, now -
> >>vif->credit_timeout.expires should be letter than ULONG_MAX/2.
> >>Because next_credit large than vif->credit_timeout.expires
> >>(next_crdit = vif->credit_timeout.expires +
> >>msecs_to_jiffies(vif->credit_usec/1000)), the delta between now and
> >>next_credit should be in range of time_after_eq().  So
> >>time_after_eq() do correctly judge.
> >>
> >Not sure I understand you. Consider "now" is placed like this:
> >
> >    expires   now   next_credit
> >    ----time increases this direction--->
> >
> >* time_after_eq(now, next_credit) -> false
> >* time_before(now, expires) -> false
> 
> If now is placed in above environment, the result will be correct
> (Sending package will be not allowed until next_credit).

No, it is not necessarily correct. Keep in mind that "now" wraps around,
which is the issue you try to fix. You still have a window to stall your
frontend.

> * time_after_eq(now, next_credit)  --> false will include two environment:
>   expires     now   next_credit
>   -----------time increases this direction ---->
> 
> Or
>   expires      next_credit             next_credit + MAX_LONG/2 now
>   -----------time increases this direction ---->
> 
> 
> the first environment should be correct to control transmit. the
> second environment is our included environment.
> 
> Jason
> >
> >Then it's stuck again. You're merely narrowing the window, not fixing
> >the real problem.
> >
> >Wei.
> >
> >>Jason
> >>>Wei.
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