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authorDavid S. Miller <davem@sunset.davemloft.net>2007-03-24 23:36:25 -0400
committerDavid S. Miller <davem@sunset.davemloft.net>2007-03-25 21:48:05 -0400
commitf11e6659ce9058928d73ff440f9b40a818d628ab (patch)
tree00b7b33eec4c8e5ade0be1d7a6fc8a8f74b383da /net/ipv6/ip6_fib.c
parenta979101106f549f4ed80d6dcbc35077be34d4346 (diff)
[IPV6]: Fix routing round-robin locking.
As per RFC2461, section 6.3.6, item #2, when no routers on the matching list are known to be reachable or probably reachable we do round robin on those available routes so that we make sure to probe as many of them as possible to detect when one becomes reachable faster. Each routing table has a rwlock protecting the tree and the linked list of routes at each leaf. The round robin code executes during lookup and thus with the rwlock taken as a reader. A small local spinlock tries to provide protection but this does not work at all for two reasons: 1) The round-robin list manipulation, as coded, goes like this (with read lock held): walk routes finding head and tail spin_lock(); rotate list using head and tail spin_unlock(); While one thread is rotating the list, another thread can end up with stale values of head and tail and then proceed to corrupt the list when it gets the lock. This ends up causing the OOPS in fib6_add() later onthat many people have been hitting. 2) All the other code paths that run with the rwlock held as a reader do not expect the list to change on them, they expect it to remain completely fixed while they hold the lock in that way. So, simply stated, it is impossible to implement this correctly using a manipulation of the list without violating the rwlock locking semantics. Reimplement using a per-fib6_node round-robin pointer. This way we don't need to manipulate the list at all, and since the round-robin pointer can only ever point to real existing entries we don't need to perform any locking on the changing of the round-robin pointer itself. We only need to reset the round-robin pointer to NULL when the entry it is pointing to is removed. The idea is from Thomas Graf and it is very similar to how this was implemented before the advanced router selection code when in. Signed-off-by: David S. Miller <davem@davemloft.net>
Diffstat (limited to 'net/ipv6/ip6_fib.c')
-rw-r--r--net/ipv6/ip6_fib.c8
1 files changed, 8 insertions, 0 deletions
diff --git a/net/ipv6/ip6_fib.c b/net/ipv6/ip6_fib.c
index f4d7be77eb0f..268f476ef3db 100644
--- a/net/ipv6/ip6_fib.c
+++ b/net/ipv6/ip6_fib.c
@@ -658,6 +658,10 @@ static int fib6_add_rt2node(struct fib6_node *fn, struct rt6_info *rt,
658 ins = &iter->u.dst.rt6_next; 658 ins = &iter->u.dst.rt6_next;
659 } 659 }
660 660
661 /* Reset round-robin state, if necessary */
662 if (ins == &fn->leaf)
663 fn->rr_ptr = NULL;
664
661 /* 665 /*
662 * insert node 666 * insert node
663 */ 667 */
@@ -1109,6 +1113,10 @@ static void fib6_del_route(struct fib6_node *fn, struct rt6_info **rtp,
1109 rt6_stats.fib_rt_entries--; 1113 rt6_stats.fib_rt_entries--;
1110 rt6_stats.fib_discarded_routes++; 1114 rt6_stats.fib_discarded_routes++;
1111 1115
1116 /* Reset round-robin state, if necessary */
1117 if (fn->rr_ptr == rt)
1118 fn->rr_ptr = NULL;
1119
1112 /* Adjust walkers */ 1120 /* Adjust walkers */
1113 read_lock(&fib6_walker_lock); 1121 read_lock(&fib6_walker_lock);
1114 FOR_WALKERS(w) { 1122 FOR_WALKERS(w) {