net/smc: Fix pos miscalculation in statistics

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: e0e4b8f ("net/smc: Add SMC statistics support")
Suggested-by: Halil Pasic <[email protected]>
Signed-off-by: Nils Hoppmann <[email protected]>
Reviewed-by: Halil Pasic <[email protected]>
Reviewed-by: Wenjia Zhang <[email protected]>
Reviewed-by: Dust Li <[email protected]>
Signed-off-by: David S. Miller <[email protected]>
(cherry picked from commit a950a59)
Signed-off-by: Frank Heimes <[email protected]>
Acked-by: Stefan Bader <[email protected]>
Acked-by: Roxana Nicolescu <[email protected]>
Signed-off-by: Roxana Nicolescu <[email protected]>

Author: Nils Hoppmann

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)