rbtree: Make lockless searches non-fatal
Change the insert and erase code such that lockless searches are
non-fatal.
In and of itself an rbtree cannot be correctly searched while
in-modification, we can however provide weaker guarantees that will
allow the rbtree to be used in conjunction with other techniques, such
as latches; see 9b0fd802e8c0 ("seqcount: Add raw_write_seqcount_latch()").
For this to work we need the following guarantees from the rbtree
code:
1) a lockless reader must not see partial stores, this would allow it
to observe nodes that are invalid memory.
2) there must not be (temporary) loops in the tree structure in the
modifier's program order, this would cause a lookup which
interrupts the modifier to get stuck indefinitely.
For 1) we must use WRITE_ONCE() for all updates to the tree structure;
in particular this patch only does rb_{left,right} as those are the
only element required for simple searches.
It generates slightly worse code, probably because volatile. But in
pointer chasing heavy code a few instructions more should not matter.
For 2) I have carefully audited the code and drawn every intermediate
link state and not found a loop.
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: David Woodhouse <David.Woodhouse@intel.com>
Cc: Rik van Riel <riel@redhat.com>
Reviewed-by: Michel Lespinasse <walken@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
diff --git a/include/linux/rbtree.h b/include/linux/rbtree.h
index fb31765..830c499 100644
--- a/include/linux/rbtree.h
+++ b/include/linux/rbtree.h
@@ -31,6 +31,7 @@
#include <linux/kernel.h>
#include <linux/stddef.h>
+#include <linux/rcupdate.h>
struct rb_node {
unsigned long __rb_parent_color;
@@ -73,11 +74,11 @@
extern struct rb_node *rb_next_postorder(const struct rb_node *);
/* Fast replacement of a single node without remove/rebalance/add/rebalance */
-extern void rb_replace_node(struct rb_node *victim, struct rb_node *new,
+extern void rb_replace_node(struct rb_node *victim, struct rb_node *new,
struct rb_root *root);
-static inline void rb_link_node(struct rb_node * node, struct rb_node * parent,
- struct rb_node ** rb_link)
+static inline void rb_link_node(struct rb_node *node, struct rb_node *parent,
+ struct rb_node **rb_link)
{
node->__rb_parent_color = (unsigned long)parent;
node->rb_left = node->rb_right = NULL;
@@ -85,6 +86,15 @@
*rb_link = node;
}
+static inline void rb_link_node_rcu(struct rb_node *node, struct rb_node *parent,
+ struct rb_node **rb_link)
+{
+ node->__rb_parent_color = (unsigned long)parent;
+ node->rb_left = node->rb_right = NULL;
+
+ rcu_assign_pointer(*rb_link, node);
+}
+
#define rb_entry_safe(ptr, type, member) \
({ typeof(ptr) ____ptr = (ptr); \
____ptr ? rb_entry(____ptr, type, member) : NULL; \
diff --git a/include/linux/rbtree_augmented.h b/include/linux/rbtree_augmented.h
index 378c5ee..14d7b83 100644
--- a/include/linux/rbtree_augmented.h
+++ b/include/linux/rbtree_augmented.h
@@ -123,11 +123,11 @@
{
if (parent) {
if (parent->rb_left == old)
- parent->rb_left = new;
+ WRITE_ONCE(parent->rb_left, new);
else
- parent->rb_right = new;
+ WRITE_ONCE(parent->rb_right, new);
} else
- root->rb_node = new;
+ WRITE_ONCE(root->rb_node, new);
}
extern void __rb_erase_color(struct rb_node *parent, struct rb_root *root,
@@ -137,7 +137,8 @@
__rb_erase_augmented(struct rb_node *node, struct rb_root *root,
const struct rb_augment_callbacks *augment)
{
- struct rb_node *child = node->rb_right, *tmp = node->rb_left;
+ struct rb_node *child = node->rb_right;
+ struct rb_node *tmp = node->rb_left;
struct rb_node *parent, *rebalance;
unsigned long pc;
@@ -167,6 +168,7 @@
tmp = parent;
} else {
struct rb_node *successor = child, *child2;
+
tmp = child->rb_left;
if (!tmp) {
/*
@@ -180,6 +182,7 @@
*/
parent = successor;
child2 = successor->rb_right;
+
augment->copy(node, successor);
} else {
/*
@@ -201,19 +204,23 @@
successor = tmp;
tmp = tmp->rb_left;
} while (tmp);
- parent->rb_left = child2 = successor->rb_right;
- successor->rb_right = child;
+ child2 = successor->rb_right;
+ WRITE_ONCE(parent->rb_left, child2);
+ WRITE_ONCE(successor->rb_right, child);
rb_set_parent(child, successor);
+
augment->copy(node, successor);
augment->propagate(parent, successor);
}
- successor->rb_left = tmp = node->rb_left;
+ tmp = node->rb_left;
+ WRITE_ONCE(successor->rb_left, tmp);
rb_set_parent(tmp, successor);
pc = node->__rb_parent_color;
tmp = __rb_parent(pc);
__rb_change_child(node, successor, tmp, root);
+
if (child2) {
successor->__rb_parent_color = pc;
rb_set_parent_color(child2, parent, RB_BLACK);