[DCCP]: Make `before' relation unambiguous
Problem:
--------
before48(a, b) returns the same as before48(b, a) for all a, b which
are 2^47 apart (modulo-2^48). This is because the difference a-b is used
and `true' is returned whenever 2^47 <= a-b <= 2^48-1. This has the
disadvantage that a positive difference a-b = 2^47 can not be distinguished
from a negative difference b-a = 2^47. As a result, an ambiguity arises in
2^47 cases.
Fix:
----
The fix is the same as the one suggested for TCP: define a `before' b
whenever 1 <= b-a <= 2^47-1 (positive signed 48-bit numbers). As a result,
the ambiguity disappears. (Note: we could have used dccp_delta_seqno, but
Arnaldo's concept of using shift and subtraction requires fewer operations
and is thus better).
The patch further defines after48 as a macro, since it is in actual fact just
before48 with the parameters swapped.
Many thanks to Eddie for help and clarifying discussion with this.
Signed-off-by: Gerrit Renker <gerrit@xxxxxxxxxxxxxx>
---
net/dccp/dccp.h | 9 +++------
1 file changed, 3 insertions(+), 6 deletions(-)
--- a/net/dccp/dccp.h
+++ b/net/dccp/dccp.h
@@ -126,14 +126,11 @@ static inline s64 dccp_delta_seqno(const
/* is seq1 < seq2 ? */
static inline int before48(const u64 seq1, const u64 seq2)
{
- return (s64)((seq1 << 16) - (seq2 << 16)) < 0;
+ return (s64)((seq2 << 16) - (seq1 << 16)) > 0;
}
/* is seq1 > seq2 ? */
-static inline int after48(const u64 seq1, const u64 seq2)
-{
- return (s64)((seq2 << 16) - (seq1 << 16)) < 0;
-}
+#define after48(seq1, seq2) before48((seq2), (seq1))
/* is seq2 <= seq1 <= seq3 ? */
static inline int between48(const u64 seq1, const u64 seq2, const u64 seq3)
@@ -143,7 +140,7 @@ static inline int between48(const u64 se
static inline u64 max48(const u64 seq1, const u64 seq2)
{
- return after48(seq1, seq2) ? seq1 : seq2;
+ return before48(seq2, seq1) ? seq1 : seq2;
}
/* is seq1 next seqno after seq2 */
-
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