net/sched: netem: fix probability gaps in 4-state loss model

The 4-state Markov chain in loss_4state() has gaps at the boundaries
between transition probability ranges. The comparisons use:

  if (rnd < a4)
  else if (a4 < rnd && rnd < a1 + a4)

When rnd equals a boundary value exactly, neither branch matches and
no state transition occurs. The redundant lower-bound check (a4 < rnd)
is already implied by being in the else branch.

Remove the unnecessary lower-bound comparisons so the ranges are
contiguous and every random value produces a transition, matching
the GI (General and Intuitive) loss model specification.

This bug goes back to original implementation of this model.

Fixes: 661b79725fea ("netem: revised correlated loss generator")
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20260418032027.900913-2-stephen@networkplumber.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

diff --git a/net/sched/sch_netem.c b/net/sched/sch_netem.c
index 20df1c08b1e9d04e9495f1a69eff0dd96049f914..8ee72cac1faf09e577b45f357b9df9a8a551a9a5 100644 (file)
--- a/net/sched/sch_netem.c
+++ b/net/sched/sch_netem.c
@@ -227,10 +227,10 @@ static bool loss_4state(struct netem_sched_data *q)
                if (rnd < clg->a4) {
                        clg->state = LOST_IN_GAP_PERIOD;
                        return true;
-               } else if (clg->a4 < rnd && rnd < clg->a1 + clg->a4) {
+               } else if (rnd < clg->a1 + clg->a4) {
                        clg->state = LOST_IN_BURST_PERIOD;
                        return true;
-               } else if (clg->a1 + clg->a4 < rnd) {
+               } else {
                        clg->state = TX_IN_GAP_PERIOD;
                }
 
@@ -247,9 +247,9 @@ static bool loss_4state(struct netem_sched_data *q)
        case LOST_IN_BURST_PERIOD:
                if (rnd < clg->a3)
                        clg->state = TX_IN_BURST_PERIOD;
-               else if (clg->a3 < rnd && rnd < clg->a2 + clg->a3) {
+               else if (rnd < clg->a2 + clg->a3) {
                        clg->state = TX_IN_GAP_PERIOD;
-               } else if (clg->a2 + clg->a3 < rnd) {
+               } else {
                        clg->state = LOST_IN_BURST_PERIOD;
                        return true;
                }