[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 */ - To unsubscribe from this list: send the line "unsubscribe dccp" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html