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Re: [Xen-devel] DomU's network interface will hung when Dom0 running 32bit

On Sat, Oct 12, 2013 at 04:53:18PM +0800, jianhai luan wrote:
> Hi Ian,
>   I meet the DomU's network interface hung issue recently, and have
> been working on the issue from that time. I find that DomU's network
> interface, which send lesser package, will hung if Dom0 running
> 32bit and DomU's up-time is very long.  I think that one jiffies
> overflow bug exist in the function tx_credit_exceeded().
>   I know the inline function time_after_eq(a,b) will process jiffies
> overflow, but the function have one limit a should little that (b +
> MAX_SIGNAL_LONG). If a large than the value, time_after_eq will
> return false. The MAX_SINGNAL_LONG should be 0x7fffffff at 32-bit
> machine.
>   If DomU's network interface send lesser package (<0.5k/s if
> jiffies=250 and credit_bytes=ULONG_MAX), jiffies will beyond out
> (credit_timeout.expires + MAX_SIGNAL_LONG) and time_after_eq(now,
> next_credit) will failure (should be true). So one timer which will
> not be trigger in short time, and later process will be aborted when
> timer_pending(&vif->credit_timeout) is true. The result will be
> DomU's network interface will be hung in long time (> 40days).
>   Please think about the below scenario:
>   Condition:
>     Dom0 running 32-bit and HZ = 1000
>     vif->credit_timeout->expire = 0xffffffff, vif->remaining_credit
> = 0xffffffff, vif->credit_usec=0 jiffies=0
>     vif receive lesser package (DomU send lesser package). If the
> value is litter than 2K/s, consume 4G(0xffffffff) will need 582.55
> hours. jiffies will large than 0x7ffffff. we guess jiffies =
> 0x800000ff, time_after_eq(0x800000ff, 0xffffffff) will failure, and
> one time which expire is 0xfffffff will be pended into system. So
> the interface will hung until jiffies recount 0xffffffff (that will
> need very long time).

If I'm not mistaken you meant time_after_eq(now, next_credit) in
netback. How does next_credit become 0xffffffff?


>   If some error exist in above explain, please help me point it out.
> Thanks,
> Jason

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