Subject: DomU's network interface will hung when Dom0 running 32bit

From: jianhai luan <[email-masked]>
Date: Sat, 12 Oct 2013 16:53:18 +0800

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 some error exist in above explain, please help me point it out.

Thanks,
Jason