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Subject[PATCH] barriers in lockfree rtcache
Dave,

This patch fixes several things in ipv4 route cache that I missed in
the last patch -

1. All the memory barriers are SMP-only avoiding unnecessary overhead on UP.
2. My forward porting merge of the rt_rcu patch dropped two changes in
rt_intern_hash() in around 2.5.43 that ordered the writes while
inserting a dst entry at the start of a hash chain. The dst entry updates
must be visible to other (weakly ordered) CPUs before it is inserted.
The necessary smp_wmb()s are added.
3. Comments to go with the write ordering.

Please apply.

Thanks
Dipankar


diff -urN linux-2.5.54-base/net/ipv4/route.c linux-2.5.54-rt_barriers/net/ipv4/route.c
--- linux-2.5.54-base/net/ipv4/route.c 2003-01-02 08:53:15.000000000 +0530
+++ linux-2.5.54-rt_barriers/net/ipv4/route.c 2003-01-03 14:44:31.000000000 +0530
@@ -234,7 +234,7 @@
{
struct rt_cache_iter_state *st = seq->private;

- read_barrier_depends();
+ smp_read_barrier_depends();
r = r->u.rt_next;
while (!r) {
rcu_read_unlock();
@@ -718,7 +718,18 @@
if (compare_keys(&rth->fl, &rt->fl)) {
/* Put it first */
*rthp = rth->u.rt_next;
+ /*
+ * Since lookup is lockfree, the deletion
+ * must be visible to another weakly ordered CPU before
+ * the insertion at the start of the hash chain.
+ */
+ smp_wmb();
rth->u.rt_next = rt_hash_table[hash].chain;
+ /*
+ * Since lookup is lockfree, the update writes
+ * must be ordered for consistency on SMP.
+ */
+ smp_wmb();
rt_hash_table[hash].chain = rth;

rth->u.dst.__use++;
@@ -900,7 +911,7 @@
while ((rth = *rthp) != NULL) {
struct rtable *rt;

- read_barrier_depends();
+ smp_read_barrier_depends();
if (rth->fl.fl4_dst != daddr ||
rth->fl.fl4_src != skeys[i] ||
rth->fl.fl4_tos != tos ||
@@ -1148,7 +1159,7 @@
rcu_read_lock();
for (rth = rt_hash_table[hash].chain; rth;
rth = rth->u.rt_next) {
- read_barrier_depends();
+ smp_read_barrier_depends();
if (rth->fl.fl4_dst == daddr &&
rth->fl.fl4_src == skeys[i] &&
rth->rt_dst == daddr &&
@@ -1740,7 +1751,7 @@

rcu_read_lock();
for (rth = rt_hash_table[hash].chain; rth; rth = rth->u.rt_next) {
- read_barrier_depends();
+ smp_read_barrier_depends();
if (rth->fl.fl4_dst == daddr &&
rth->fl.fl4_src == saddr &&
rth->fl.iif == iif &&
@@ -2105,7 +2116,7 @@

rcu_read_lock();
for (rth = rt_hash_table[hash].chain; rth; rth = rth->u.rt_next) {
- read_barrier_depends();
+ smp_read_barrier_depends();
if (rth->fl.fl4_dst == flp->fl4_dst &&
rth->fl.fl4_src == flp->fl4_src &&
rth->fl.iif == 0 &&
@@ -2335,7 +2346,7 @@
rcu_read_lock();
for (rt = rt_hash_table[h].chain, idx = 0; rt;
rt = rt->u.rt_next, idx++) {
- read_barrier_depends();
+ smp_read_barrier_depends();
if (idx < s_idx)
continue;
skb->dst = dst_clone(&rt->u.dst);
-
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