[IPV4] route cache: Introduce rt_genid for smooth cache invalidation

Current ip route cache implementation is not suited to large caches.

We can consume a lot of CPU when cache must be invalidated, since we
currently need to evict all cache entries, and this eviction is
sometimes asynchronous. min_delay & max_delay can somewhat control this
asynchronism behavior, but whole thing is a kludge, regularly triggering
infamous soft lockup messages. When entries are still in use, this also
consumes a lot of ram, filling dst_garbage.list.

A better scheme is to use a generation identifier on each entry,
so that cache invalidation can be performed by changing the table
identifier, without having to scan all entries.
No more delayed flushing, no more stalling when secret_interval expires.

Invalidated entries will then be freed at GC time (controled by
ip_rt_gc_timeout or stress), or when an invalidated entry is found
in a chain when an insert is done.
Thus we keep a normal equilibrium.

This patch :
- renames rt_hash_rnd to rt_genid (and makes it an atomic_t)
- Adds a new rt_genid field to 'struct rtable' (filling a hole on 64bit)
- Checks entry->rt_genid at appropriate places :

Documentation/filesystems/proc.txt

@@ -1919,11 +1919,6 @@ max_size
 Maximum size  of  the routing cache. Old entries will be purged once the cache
 reached has this size.
 
-max_delay, min_delay
---------------------
-
-Delays for flushing the routing cache.
-
 redirect_load, redirect_number
 ------------------------------
 

include/linux/sysctl.h

@@ -440,8 +440,8 @@ enum
 
 enum {
 	NET_IPV4_ROUTE_FLUSH=1,
-	NET_IPV4_ROUTE_MIN_DELAY=2,
-	NET_IPV4_ROUTE_MAX_DELAY=3,
+	NET_IPV4_ROUTE_MIN_DELAY=2, /* obsolete since 2.6.25 */
+	NET_IPV4_ROUTE_MAX_DELAY=3, /* obsolete since 2.6.25 */
 	NET_IPV4_ROUTE_GC_THRESH=4,
 	NET_IPV4_ROUTE_MAX_SIZE=5,
 	NET_IPV4_ROUTE_GC_MIN_INTERVAL=6,

include/net/route.h

@@ -62,6 +62,7 @@ struct rtable
 
 	struct in_device	*idev;
 
+	int			rt_genid;
 	unsigned		rt_flags;
 	__u16			rt_type;