Re: [PATCH net-next 19/19] rxrpc: Use RCU to access a peer's service connection tree
From: Peter Zijlstra <peterz@infradead.org>
Date: 2016-06-30 20:21:06
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On Thu, Jun 30, 2016 at 05:36:51PM +0100, David Howells wrote:
David Howells [off-list ref] wrote:quoted
quoted
You want rb_link_node_rcu() here.Should there be an rb_replace_node_rcu() also?Or I could make rb_replace_node() RCU friendly. What do you think of the attached changes (split into appropriate patches)? It's a case of changing the order in which pointers are set in the rbtree code and inserting a barrier.
quoted hunk ↗ jump to hunk
diff --git a/lib/rbtree.c b/lib/rbtree.c index 1356454e36de..2b1a190c737c 100644 --- a/lib/rbtree.c +++ b/lib/rbtree.c@@ -539,15 +539,17 @@ void rb_replace_node(struct rb_node *victim, struct rb_node *new, { struct rb_node *parent = rb_parent(victim); + /* Copy the pointers/colour from the victim to the replacement */ + *new = *victim; + /* Set the surrounding nodes to point to the replacement */ - __rb_change_child(victim, new, parent, root); if (victim->rb_left) rb_set_parent(victim->rb_left, new); if (victim->rb_right) rb_set_parent(victim->rb_right, new); - /* Copy the pointers/colour from the victim to the replacement */ - *new = *victim; + /* Set the onward pointer last with an RCU barrier */ + __rb_change_child_rcu(victim, new, parent, root); } EXPORT_SYMBOL(rb_replace_node);
So back when I did this work there was resistance to making the regular RB-tree primitives more expensive for the rare RCU user. And I suspect that this is still so. Now, rb_replace_node() isn't a widely used primitive, so it might go unnoticed, but since we already have rb_link_node_rcu() adding rb_replace_node_rcu() is the consistent thing to do.
quoted hunk ↗ jump to hunk
diff --git a/net/rxrpc/conn_service.c b/net/rxrpc/conn_service.c index dc64211c5ee8..298ec300cfcc 100644 --- a/net/rxrpc/conn_service.c +++ b/net/rxrpc/conn_service.c@@ -41,14 +41,14 @@ struct rxrpc_connection *rxrpc_find_service_conn_rcu(struct rxrpc_peer *peer, */ read_seqbegin_or_lock(&peer->service_conn_lock, &seq); - p = peer->service_conns.rb_node; + p = rcu_dereference(peer->service_conns.rb_node); while (p) { conn = rb_entry(p, struct rxrpc_connection, service_node); if (conn->proto.index_key < k.index_key) - p = p->rb_left; + p = rcu_dereference(p->rb_left); else if (conn->proto.index_key > k.index_key) - p = p->rb_right; + p = rcu_dereference(p->rb_right); else goto done; conn = NULL;@@ -90,7 +90,7 @@ rxrpc_publish_service_conn(struct rxrpc_peer *peer, goto found_extant_conn; } - rb_link_node(&conn->service_node, parent, pp); + rb_link_node_rcu(&conn->service_node, parent, pp); rb_insert_color(&conn->service_node, &peer->service_conns); conn_published: set_bit(RXRPC_CONN_IN_SERVICE_CONNS, &conn->flags);
Yep, that's about right.