[SRU][M][L][J][PATCH 1/1] net/smc: Fix pos miscalculation in statistics

[frank.heimes at canonical.com]

From: Nils Hoppmann <niho at linux.ibm.com>

BugLink: https://bugs.launchpad.net/bugs/2039575

SMC_STAT_PAYLOAD_SUB(_smc_stats, _tech, key, _len, _rc) will calculate
wrong bucket positions for payloads of exactly 4096 bytes and
(1 << (m + 12)) bytes, with m == SMC_BUF_MAX - 1.

Intended bucket distribution:
Assume l == size of payload, m == SMC_BUF_MAX - 1.

Bucket 0                : 0 < l <= 2^13
Bucket n, 1 <= n <= m-1 : 2^(n+12) < l <= 2^(n+13)
Bucket m                : l > 2^(m+12)

Current solution:
_pos = fls64((l) >> 13)
[...]
_pos = (_pos < m) ? ((l == 1 << (_pos + 12)) ? _pos - 1 : _pos) : m

For l == 4096, _pos == -1, but should be _pos == 0.
For l == (1 << (m + 12)), _pos == m, but should be _pos == m - 1.

In order to avoid special treatment of these corner cases, the
calculation is adjusted. The new solution first subtracts the length by
one, and then calculates the correct bucket by shifting accordingly,
i.e. _pos = fls64((l - 1) >> 13), l > 0.
This not only fixes the issues named above, but also makes the whole
bucket assignment easier to follow.

Same is done for SMC_STAT_RMB_SIZE_SUB(_smc_stats, _tech, k, _len),
where the calculation of the bucket position is similar to the one
named above.

Fixes: e0e4b8fa5338 ("net/smc: Add SMC statistics support")
Suggested-by: Halil Pasic <pasic at linux.ibm.com>
Signed-off-by: Nils Hoppmann <niho at linux.ibm.com>
Reviewed-by: Halil Pasic <pasic at linux.ibm.com>
Reviewed-by: Wenjia Zhang <wenjia at linux.ibm.com>
Reviewed-by: Dust Li <dust.li at linux.alibaba.com>
Signed-off-by: David S. Miller <davem at davemloft.net>
(cherry picked from commit a950a5921db450c74212327f69950ff03419483a)
Signed-off-by: Frank Heimes <frank.heimes at canonical.com>
---
 net/smc/smc_stats.h | 14 +++++++++-----
 1 file changed, 9 insertions(+), 5 deletions(-)

diff --git a/net/smc/smc_stats.h b/net/smc/smc_stats.h
index b60fe1eb37ab..a7bbc095894c 100644
--- a/net/smc/smc_stats.h
+++ b/net/smc/smc_stats.h
@@ -92,13 +92,14 @@ do { \
 	typeof(_smc_stats) stats = (_smc_stats); \
 	typeof(_tech) t = (_tech); \
 	typeof(_len) l = (_len); \
-	int _pos = fls64((l) >> 13); \
+	int _pos; \
 	typeof(_rc) r = (_rc); \
 	int m = SMC_BUF_MAX - 1; \
 	this_cpu_inc((*stats).smc[t].key ## _cnt); \
-	if (r <= 0) \
+	if (r <= 0 || l <= 0) \
 		break; \
-	_pos = (_pos < m) ? ((l == 1 << (_pos + 12)) ? _pos - 1 : _pos) : m; \
+	_pos = fls64((l - 1) >> 13); \
+	_pos = (_pos <= m) ? _pos : m; \
 	this_cpu_inc((*stats).smc[t].key ## _pd.buf[_pos]); \
 	this_cpu_add((*stats).smc[t].key ## _bytes, r); \
 } \
@@ -138,9 +139,12 @@ while (0)
 do { \
 	typeof(_len) _l = (_len); \
 	typeof(_tech) t = (_tech); \
-	int _pos = fls((_l) >> 13); \
+	int _pos; \
 	int m = SMC_BUF_MAX - 1; \
-	_pos = (_pos < m) ? ((_l == 1 << (_pos + 12)) ? _pos - 1 : _pos) : m; \
+	if (_l <= 0) \
+		break; \
+	_pos = fls((_l - 1) >> 13); \
+	_pos = (_pos <= m) ? _pos : m; \
 	this_cpu_inc((*(_smc_stats)).smc[t].k ## _rmbsize.buf[_pos]); \
 } \
 while (0)
-- 
2.25.1