| Message ID | 587.1467304611@warthog.procyon.org.uk |
|---|---|
| State | RFC, archived |
| Delegated to: | David Miller |
| Headers | show |
On Thu, Jun 30, 2016 at 05:36:51PM +0100, David Howells wrote: > David Howells <dhowells@redhat.com> wrote: > > > > 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. > 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. > 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.
diff --git a/include/linux/rbtree_augmented.h b/include/linux/rbtree_augmented.h index 14d7b831b63a..d076183e49be 100644 --- a/include/linux/rbtree_augmented.h +++ b/include/linux/rbtree_augmented.h @@ -130,6 +130,19 @@ __rb_change_child(struct rb_node *old, struct rb_node *new, WRITE_ONCE(root->rb_node, new); } +static inline void +__rb_change_child_rcu(struct rb_node *old, struct rb_node *new, + struct rb_node *parent, struct rb_root *root) +{ + if (parent) { + if (parent->rb_left == old) + rcu_assign_pointer(parent->rb_left, new); + else + rcu_assign_pointer(parent->rb_right, new); + } else + rcu_assign_pointer(root->rb_node, new); +} + extern void __rb_erase_color(struct rb_node *parent, struct rb_root *root, void (*augment_rotate)(struct rb_node *old, struct rb_node *new)); 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); 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);