[PATCH] neighbour.c: Avoid GC directly after state change

[Ulf Samuelsson <netdev@emagii.com>]

From: Ulf Samuelsson <ulf@emagii.com>

The neighbour state is changed in the ARP timer handler.
If the state is changed to NUD_STALE, then the neighbour
entry becomes a candidate for garbage collection.

The garbage collection is handled by a "periodic work" routine.

When :

	* noone refers to the entry
	* the state is no longer valid (I.E: NUD_STALE).
	* the timeout value has  been reached or state is FAILED

the "periodic work" routine will notify
the stack that the entry should be deleted.

A user application monitoring and controlling the neighbour table
using NETLINK may fail, if the "period work" routine is run
directly after the state has been changed to NUD_STALE,
but before the user application has had a chance to change
the state to something valid.

The "period work" routine will detect the NUD_STALE state
and if the timeout value has been reached, it will notify the stack
that the entry should be deleted.

The patch adds a check in the periodic work routine
which will skip test for garbage collection
unless a number of ticks has passed since the last time
the neighbour entry state was changed.

The feature is controlled through Kconfig

The featuree is enabled by setting ARP_GC_APPLY_GUARDBAND
The guardband time (in ticks) is set in ARP_GC_GUARDBAND
Default time is 100 ms if HZ_### is set.

Signed-off-by: Ulf Samuelsson <ulf@emagii.com>
---
 net/Kconfig          |   32 ++++++++++++++++++++++++++++++++
 net/core/neighbour.c |   15 ++++++++++++---
 2 files changed, 44 insertions(+), 3 deletions(-)

diff --git a/net/Kconfig b/net/Kconfig
index 44dd578..099a5dd 100644
--- a/net/Kconfig
+++ b/net/Kconfig
@@ -77,6 +77,38 @@ config INET
 	  Short answer: say Y.
 
 if INET
+
+#
+# Core Network configuration
+#
+
+config ARP_GC_APPLY_GUARDBAND
+	bool "IP: ARP: Avoid garbage collection directly after state change"
+	default n
+	---help---
+	  With this item selected, an entry in the neighbour table
+	  will not be garbage collected directly after the ARP state
+	  has changed to STALE of FAILED
+	  This allows an application program change the state to something valid
+	  before garbage colllection occurs.
+
+	  If unsure, say N.
+
+config ARP_GC_GUARDBAND
+	int "Guardband time on garbage collection"
+	depends on ARP_GC_APPLY_GUARDBAND
+	default 10 if HZ_100
+	default 25 if HZ_250
+	default 30 if HZ_300
+	default 100 if HZ_1000
+	default 100
+
+	---help---
+	  The number of ticks to delay garbage collection
+	  after the neighbour entry has been updated
+	  A delay of 100 ms is reasonable.
+	  With CONFIG_HZ = 250, this value should be 25
+
 source "net/ipv4/Kconfig"
 source "net/ipv6/Kconfig"
 source "net/netlabel/Kconfig"
diff --git a/net/core/neighbour.c b/net/core/neighbour.c
index 70fe9e1..194195d 100644
--- a/net/core/neighbour.c
+++ b/net/core/neighbour.c
@@ -786,13 +786,23 @@ static void neigh_periodic_work(struct work_struct *work)
 
 			state = n->nud_state;
 			if (state & (NUD_PERMANENT | NUD_IN_TIMER)) {
-				write_unlock(&n->lock);
 				goto next_elt;
 			}
 
 			if (time_before(n->used, n->confirmed))
 				n->used = n->confirmed;
 
+#if defined(CONFIG_ARP_GC_APPLY_GUARDBAND)
+			/* Do not garbage collect directly after we
+			 * updated n->state to allow applications to
+			 * react to the event
+			 */
+			if (time_before(jiffies,
+					n->updated + CONFIG_ARP_GC_GUARDBAND)) {
+				goto next_elt;
+			}
+#endif
+
 			if (atomic_read(&n->refcnt) == 1 &&
 			    (state == NUD_FAILED ||
 			     time_after(jiffies, n->used + NEIGH_VAR(n->parms, GC_STALETIME)))) {
@@ -802,9 +812,8 @@ static void neigh_periodic_work(struct work_struct *work)
 				neigh_cleanup_and_release(n);
 				continue;
 			}
-			write_unlock(&n->lock);
-
 next_elt:
+			write_unlock(&n->lock);
 			np = &n->next;
 		}
 		/*
-- 
1.7.9.5