From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Date: Mon, 7 May 2012 15:18:14 +0000 (-0400)
Subject: rculfhash: replace unneeded rcu_dereference by CMM_LOAD_SHARED
X-Git-Tag: v0.7.0~18
X-Git-Url: https://git.lttng.org./?a=commitdiff_plain;h=a85eff522c253434a9c2b53d6c3a702842fb1d5d;p=userspace-rcu.git

rculfhash: replace unneeded rcu_dereference by CMM_LOAD_SHARED

The difference between the two is that CMM_LOAD_SHARED() does not imply
a read barrier between the read and following uses of the data pointed
to by the pointer read.

All sites that only use the pointer load for its bits (never
dereference) don't need the read barrier implied by rcu_dereference.

Reviewed-by: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
---

diff --git a/rculfhash.c b/rculfhash.c
index b9f795f..b26a690 100644
--- a/rculfhash.c
+++ b/rculfhash.c
@@ -923,7 +923,7 @@ int _cds_lfht_replace(struct cds_lfht *ht, unsigned long size,
 	bucket = lookup_bucket(ht, size, bit_reverse_ulong(old_node->reverse_hash));
 	_cds_lfht_gc_bucket(bucket, new_node);
 
-	assert(is_removed(rcu_dereference(old_node->next)));
+	assert(is_removed(CMM_LOAD_SHARED(old_node->next)));
 	return 0;
 }
 
@@ -1061,7 +1061,7 @@ int _cds_lfht_del(struct cds_lfht *ht, unsigned long size,
 	 * logical removal flag). Return -ENOENT if the node had
 	 * previously been removed.
 	 */
-	next = rcu_dereference(node->next);
+	next = CMM_LOAD_SHARED(node->next);	/* next is not dereferenced */
 	if (caa_unlikely(is_removed(next)))
 		return -ENOENT;
 	assert(!is_bucket(next));
@@ -1082,7 +1082,7 @@ int _cds_lfht_del(struct cds_lfht *ht, unsigned long size,
 	bucket = lookup_bucket(ht, size, bit_reverse_ulong(node->reverse_hash));
 	_cds_lfht_gc_bucket(bucket, node);
 
-	assert(is_removed(rcu_dereference(node->next)));
+	assert(is_removed(CMM_LOAD_SHARED(node->next)));
 	/*
 	 * Last phase: atomically exchange node->next with a version
 	 * having "REMOVAL_OWNER_FLAG" set. If the returned node->next
@@ -1510,7 +1510,7 @@ void cds_lfht_lookup(struct cds_lfht *ht, unsigned long hash,
 		}
 		node = clear_flag(next);
 	}
-	assert(!node || !is_bucket(rcu_dereference(node->next)));
+	assert(!node || !is_bucket(CMM_LOAD_SHARED(node->next)));
 	iter->node = node;
 	iter->next = next;
 }
@@ -1543,7 +1543,7 @@ void cds_lfht_next_duplicate(struct cds_lfht *ht, cds_lfht_match_fct match,
 		}
 		node = clear_flag(next);
 	}
-	assert(!node || !is_bucket(rcu_dereference(node->next)));
+	assert(!node || !is_bucket(CMM_LOAD_SHARED(node->next)));
 	iter->node = node;
 	iter->next = next;
 }
@@ -1565,7 +1565,7 @@ void cds_lfht_next(struct cds_lfht *ht, struct cds_lfht_iter *iter)
 		}
 		node = clear_flag(next);
 	}
-	assert(!node || !is_bucket(rcu_dereference(node->next)));
+	assert(!node || !is_bucket(CMM_LOAD_SHARED(node->next)));
 	iter->node = node;
 	iter->next = next;
 }
@@ -1668,7 +1668,7 @@ int cds_lfht_del(struct cds_lfht *ht, struct cds_lfht_node *node)
 
 int cds_lfht_is_node_deleted(struct cds_lfht_node *node)
 {
-	return is_removed(rcu_dereference(node->next));
+	return is_removed(CMM_LOAD_SHARED(node->next));
 }
 
 static