netdev - bnx2x statistics occasionally wrong by near multiples of 4GB.

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Date:	Fri, 15 Mar 2013 14:03:05 -0700
From:	Maciej Żenczykowski <maze@...gle.com>
To:	Mintz Yuval <yuvalmin@...adcom.com>
Cc:	Linux NetDev <netdev@...r.kernel.org>
Subject: bnx2x statistics occasionally wrong by near multiples of 4GB.

bnx2x_stats.h:

#define UPDATE_QSTAT(s, t) \
     do { \
              qstats->t##_hi = qstats_old->t##_hi + le32_to_cpu(s.hi); \
              qstats->t##_lo = qstats_old->t##_lo + le32_to_cpu(s.lo); \
      } while (0)

Looks wrong because it doesn't deal with overflow of _lo.

I'm not 100% sure if I tracked it down correctly, but I think this is
the root cause of us occasionally seeing stats (/proc/net/dev tx_bytes
in this case) jump back by roughly N*4GB and then go forward again.

In particular we saw successive reads of /proc/net/dev eth0 Transmit bytes show:

0x8F6 45356E60
0x8F4 4C8D72B9
0x8F6 5420D096

(which since this is a sum of 3 values (ucase/mcast/bcast) from all
queues, means we actually wrongly accounted for overflow twice...)

- Maciej
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