Route refresh in tables uses a stale counter.

Until now, we were marking routes as REF_STALE and REF_DISCARD to
cleanup old routes after route refresh. This needed a synchronous route
table walk at both beginning and the end of route refresh routine,
marking the routes by the flags.

We avoid these walks by using a stale counter. Every route contains:
  u8 stale_cycle;
Every import hook contains:
  u8 stale_set;
  u8 stale_valid;
  u8 stale_pruned;
  u8 stale_pruning;

In base_state, stale_set == stale_valid == stale_pruned == stale_pruning
and all routes' stale_cycle also have the same value.

The route refresh looks like follows:
+ ----------- + --------- + ----------- + ------------- + ------------ +
|             | stale_set | stale_valid | stale_pruning | stale_pruned |
| Base        |     x     |      x      |        x      |       x      |
| Begin       |    x+1    |      x      |        x      |       x      |
  ... now routes are being inserted with stale_cycle == (x+1)
| End         |    x+1    |     x+1     |        x      |       x      |
  ... now table pruning routine is scheduled
| Prune begin |    x+1    |     x+1     |       x+1     |       x      |
  ... now routes with stale_cycle not between stale_set and stale_valid
      are deleted
| Prune end   |    x+1    |     x+1     |       x+1     |      x+1     |
+ ----------- + --------- + ----------- + ------------- + ------------ +

The pruning routine is asynchronous and may have high latency in
high-load environments. Therefore, multiple route refresh requests may
happen before the pruning routine starts, leading to this situation:

| Prune begin |    x+k    |     x+k     |    x -> x+k   |       x      |
  ... or even
| Prune begin |   x+k+1   |     x+k     |    x -> x+k   |       x      |
  ... if the prune event starts while another route refresh is running.

In such a case, the pruning routine still deletes routes not fitting
between stale_set and and stale_valid, effectively pruning the remnants
of all unpruned route refreshes from before:

| Prune end   |    x+k    |     x+k     |       x+k     |      x+k     |

In extremely rare cases, there may happen too many route refreshes
before any route prune routine finishes. If the difference between
stale_valid and stale_pruned becomes more than 128 when requesting for
another route refresh, the routine walks the table synchronously and
resets all the stale values to a base state, while logging a warning.

1 files changed, 9 insertions, 9 deletions

diff --git a/proto/bgp/bgp.c b/proto/bgp/bgp.c
index 6ffe8824..d9008b9a 100644
--- a/proto/bgp/bgp.c
+++ b/proto/bgp/bgp.c
@@ -760,16 +760,16 @@ bgp_handle_graceful_restart(struct bgp_proto *p)
       {
       case BGP_GRS_NONE:
 	c->gr_active = BGP_GRS_ACTIVE;
-	rt_refresh_begin(c->c.table, &c->c.in_req);
+	rt_refresh_begin(&c->c.in_req);
 	break;
 
       case BGP_GRS_ACTIVE:
-	rt_refresh_end(c->c.table, &c->c.in_req);
-	rt_refresh_begin(c->c.table, &c->c.in_req);
+	rt_refresh_end(&c->c.in_req);
+	rt_refresh_begin(&c->c.in_req);
 	break;
 
       case BGP_GRS_LLGR:
-	rt_refresh_begin(c->c.table, &c->c.in_req);
+	rt_refresh_begin(&c->c.in_req);
 	rt_modify_stale(c->c.table, &c->c.in_req);
 	break;
       }
@@ -777,8 +777,8 @@ bgp_handle_graceful_restart(struct bgp_proto *p)
     else
     {
       /* Just flush the routes */
-      rt_refresh_begin(c->c.table, &c->c.in_req);
-      rt_refresh_end(c->c.table, &c->c.in_req);
+      rt_refresh_begin(&c->c.in_req);
+      rt_refresh_end(&c->c.in_req);
     }
 
     /* Reset bucket and prefix tables */
@@ -819,7 +819,7 @@ bgp_graceful_restart_done(struct bgp_channel *c)
     BGP_TRACE(D_EVENTS, "Neighbor graceful restart done");
 
   tm_stop(c->stale_timer);
-  rt_refresh_end(c->c.table, &c->c.in_req);
+  rt_refresh_end(&c->c.in_req);
 }
 
 /**
@@ -899,7 +899,7 @@ bgp_refresh_begin(struct bgp_channel *c)
   { log(L_WARN "%s: BEGIN-OF-RR received before END-OF-RIB, ignoring", p->p.name); return; }
 
   c->load_state = BFS_REFRESHING;
-  rt_refresh_begin(c->c.table, &c->c.in_req);
+  rt_refresh_begin(&c->c.in_req);
 }
 
 /**
@@ -920,7 +920,7 @@ bgp_refresh_end(struct bgp_channel *c)
   { log(L_WARN "%s: END-OF-RR received without prior BEGIN-OF-RR, ignoring", p->p.name); return; }
 
   c->load_state = BFS_NONE;
-  rt_refresh_end(c->c.table, &c->c.in_req);
+  rt_refresh_end(&c->c.in_req);
 }