[PATCH RFC 2/2] netfilter: conntrack: replace mutex with cmpxchg

[Benjamin Poirier <bpoirier@suse.de>]

This mutex protects a single pointer.
---
 net/netfilter/nf_conntrack_ecache.c |   38 +++++++++-------------------------
 1 files changed, 10 insertions(+), 28 deletions(-)

diff --git a/net/netfilter/nf_conntrack_ecache.c b/net/netfilter/nf_conntrack_ecache.c
index 0134009..603eb69 100644
--- a/net/netfilter/nf_conntrack_ecache.c
+++ b/net/netfilter/nf_conntrack_ecache.c
@@ -25,8 +25,6 @@
 #include <net/netfilter/nf_conntrack_core.h>
 #include <net/netfilter/nf_conntrack_extend.h>
 
-static DEFINE_MUTEX(nf_ct_ecache_mutex);
-
 /* deliver cached events and clear cache entry - must be called with locally
  * disabled softirqs */
 void nf_ct_deliver_cached_events(struct nf_conn *ct)
@@ -80,52 +78,36 @@ EXPORT_SYMBOL_GPL(nf_ct_deliver_cached_events);
 int nf_conntrack_register_notifier(struct net *net,
 				   struct nf_ct_event_notifier *new)
 {
-	int ret = 0;
-
-	mutex_lock(&nf_ct_ecache_mutex);
-	if (net->ct.nf_conntrack_event_cb != NULL)
-		ret = -EBUSY;
+	if (cmpxchg(&net->ct.nf_conntrack_event_cb, NULL, new) != NULL)
+		return -EBUSY;
 	else
-		net->ct.nf_conntrack_event_cb = new;
-	mutex_unlock(&nf_ct_ecache_mutex);
-
-	return ret;
+		return 0;
 }
 EXPORT_SYMBOL_GPL(nf_conntrack_register_notifier);
 
 void nf_conntrack_unregister_notifier(struct net *net,
 				      struct nf_ct_event_notifier *new)
 {
-	mutex_lock(&nf_ct_ecache_mutex);
-	BUG_ON(net->ct.nf_conntrack_event_cb != new);
-	net->ct.nf_conntrack_event_cb = NULL;
-	mutex_unlock(&nf_ct_ecache_mutex);
+	if (xchg(&net->ct.nf_conntrack_event_cb, NULL) != new)
+		BUG();
 }
 EXPORT_SYMBOL_GPL(nf_conntrack_unregister_notifier);
 
 int nf_ct_expect_register_notifier(struct net *net,
 				   struct nf_exp_event_notifier *new)
 {
-	int ret = 0;
-
-	mutex_lock(&nf_ct_ecache_mutex);
-	if (net->ct.nf_expect_event_cb != NULL)
-		ret = -EBUSY;
+	if (cmpxchg(&net->ct.nf_expect_event_cb, NULL, new) != NULL)
+		return -EBUSY;
 	else
-		net->ct.nf_expect_event_cb = new;
-	mutex_unlock(&nf_ct_ecache_mutex);
-
-	return ret;
+		return 0;
 }
 EXPORT_SYMBOL_GPL(nf_ct_expect_register_notifier);
 
 void nf_ct_expect_unregister_notifier(struct net *net,
 				      struct nf_exp_event_notifier *new)
 {
-	mutex_lock(&nf_ct_ecache_mutex);
-	BUG_ON(net->ct.nf_expect_event_cb != new);
-	net->ct.nf_expect_event_cb = NULL;
-	mutex_unlock(&nf_ct_ecache_mutex);
+	if (xchg(&net->ct.nf_expect_event_cb, NULL) != new)
+		BUG();
 }
 EXPORT_SYMBOL_GPL(nf_ct_expect_unregister_notifier);
 
-- 
1.7.7