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From: "Maciej Żenczykowski" <zenczykowski@gmail.com>
To: David Miller <davem@davemloft.net>
Cc: eilong@broadcom.com, eric.dumazet@gmail.com, dmitry@broadcom.com,
	netdev@vger.kernel.org, yuvalmin@broadcom.com
Subject: Re: [PATCH] bnx2x: fix occasional statistics off-by-4GB error
Date: Mon, 18 Mar 2013 13:36:21 -0700	[thread overview]
Message-ID: <CAHo-Ooz2W_jcbbOFLvihf+N-6fVSNoj0jGKTT8AXoOSC3_3DRg@mail.gmail.com> (raw)
In-Reply-To: <CAHo-OowoWD_BT2W8ok32pcSmcshXttpErna-X92Ojz_A8es8iw@mail.gmail.com>

Not quite the right call sequence above.
  UPDATE_FSTAT_QSTAT(total_bytes_received); --> SUB_64 --> DIFF_64
is probably more relevant.

Regardless:

/* difference = minuend - subtrahend */
#define DIFF_64(d_hi, m_hi, s_hi, d_lo, m_lo, s_lo) \
<------>do { \
<------><------>if (m_lo < s_lo) { \
<------><------><------>/* underflow */ \
<------><------><------>d_hi = m_hi - s_hi; \
<------><------><------>if (d_hi > 0) { \
<------><------><------><------>/* we can 'loan' 1 */ \
<------><------><------><------>d_hi--; \
<------><------><------><------>d_lo = m_lo + (UINT_MAX - s_lo) + 1; \
<------><------><------>} else { \
<------><------><------><------>/* m_hi <= s_hi */ \
<------><------><------><------>d_hi = 0; \
<------><------><------><------>d_lo = 0; \
<------><------><------>} \
<------><------>} else { \
...
I believe this fails.  All parameters are most likely going to be u32,
since that's used for stats pretty much everywhere.
As such after d_hi = m_hi - s_hi;  d_hi will be >= 0 since it's u32.

As such if "m_hi == s_hi" && "m_lo < s_lo" we will return (0,0)
however if "m_hi < s_hi" && "m_lo < s_lo" we will not return (0,0)

I'm not sure which behaviour is desired, but either way this is obviously wrong.

0 - 1 returns 0
0 - (4GB+1) returns -4GB-1

  parent reply	other threads:[~2013-03-18 20:36 UTC|newest]

Thread overview: 19+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2013-03-15 21:03 bnx2x statistics occasionally wrong by near multiples of 4GB Maciej Żenczykowski
2013-03-15 21:56 ` [PATCH] bnx2x: fix occasional statistics off-by-4GB error Maciej Żenczykowski
2013-03-17 13:10   ` Dmitry Kravkov
2013-03-17 18:24     ` David Miller
2013-03-17 19:37       ` Eilon Greenstein
2013-03-17 20:25         ` Eric Dumazet
2013-03-17 20:53           ` Eilon Greenstein
2013-03-17 23:17             ` Eric Dumazet
2013-03-18  7:27               ` Eilon Greenstein
2013-03-18  6:18             ` Maciej Żenczykowski
2013-03-18 10:06               ` Eilon Greenstein
2013-03-18 17:13                 ` David Miller
2013-03-18 19:54                   ` Maciej Żenczykowski
2013-03-18 20:05                     ` David Miller
2013-03-18 20:06                     ` Maciej Żenczykowski
2013-03-18 20:35                       ` Eric Dumazet
2013-03-18 20:36                       ` Maciej Żenczykowski [this message]
2013-03-21 21:23                         ` Maciej Żenczykowski
2013-03-21 21:25                           ` David Miller

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