/*
* Latched RB-trees
*
* Copyright (C) 2015 Intel Corp., Peter Zijlstra <peterz@infradead.org>
*
* Since RB-trees have non-atomic modifications they're not immediately suited
* for RCU/lockless queries. Even though we made RB-tree lookups non-fatal for
* lockless lookups; we cannot guarantee they return a correct result.
*
* The simplest solution is a seqlock + RB-tree, this will allow lockless
* lookups; but has the constraint (inherent to the seqlock) that read sides
* cannot nest in write sides.
*
* If we need to allow unconditional lookups (say as required for NMI context
* usage) we need a more complex setup; this data structure provides this by
* employing the latch technique -- see @raw_write_seqcount_latch -- to
* implement a latched RB-tree which does allow for unconditional lookups by
* virtue of always having (at least) one stable copy of the tree.
*
* However, while we have the guarantee that there is at all times one stable
* copy, this does not guarantee an iteration will not observe modifications.
* What might have been a stable copy at the start of the iteration, need not
* remain so for the duration of the iteration.
*
* Therefore, this does require a lockless RB-tree iteration to be non-fatal;
* see the comment in lib/rbtree.c. Note however that we only require the first
* condition -- not seeing partial stores -- because the latch thing isolates
* us from loops. If we were to interrupt a modification the lookup would be
* pointed at the stable tree and complete while the modification was halted.
*/
#ifndef RB_TREE_LATCH_H
#define RB_TREE_LATCH_H
#include <linux/rbtree.h>
#include <linux/seqlock.h>
struct latch_tree_node {
struct rb_node node[2];
};
struct latch_tree_root {
seqcount_t seq;
struct rb_root tree[2];
};
/**
* latch_tree_ops - operators to define the tree order
* @less: used for insertion; provides the (partial) order between two elements.
* @comp: used for lookups; provides the order between the search key and an element.
*
* The operators are related like:
*
* comp(a->key,b) < 0 := less(a,b)
* comp(a->key,b) > 0 := less(b,a)
* comp(a->key,b) == 0 := !less(a,b) && !less(b,a)
*
* If these operators define a partial order on the elements we make no
* guarantee on which of the elements matching the key is found. See
* latch_tree_find().
*/
struct latch_tree_ops {
bool (*less)(struct latch_tree_node *a, struct latch_tree_node *b);
int (*comp)(void *key, struct latch_tree_node *b);
};
static __always_inline struct latch_tree_node *
__lt_from_rb(struct rb_node *node, int idx)
{
return container_of(node, struct latch_tree_node, node[idx]);
}
static __always_inline void
__lt_insert(struct latch_tree_node *ltn, struct latch_tree_root *ltr, int idx,
bool (*less)(struct latch_tree_node *a, struct latch_tree_node *b))
{
struct rb_root *root = <r->tree[idx];
struct rb_node **link = &root->rb_node;
struct rb_node *node = <n->node[idx];
struct rb_node *parent = NULL;
struct latch_tree_node *ltp;
while (*link) {
parent = *link;
ltp = __lt_from_rb(parent, idx);
if (less(ltn, ltp))
link = &parent->rb_left;
else
link = &parent->rb_right;
}
rb_link_node_rcu(node, parent, link);
rb_insert_color(node, root);
}
static __always_inline void
__lt_erase(struct latch_tree_node *ltn, struct latch_tree_root *ltr, int idx)
{
rb_erase(<n->node[idx], <r->tree[idx]);
}
static __always_inline struct latch_tree_node *
__lt_find(void *key, struct latch_tree_root *ltr, int idx,
int (*comp)(void *key, struct latch_tree_node *node))
{
struct rb_node *node = rcu_dereference_raw(ltr->tree[idx].rb_node);
struct latch_tree_node *ltn;
int c;
while (node) {
ltn = __lt_from_rb(node, idx);
c = comp(key, ltn);
if (c < 0)
node = rcu_dereference_raw(node->rb_left);
else if (c > 0)
node = rcu_dereference_raw(node->rb_right);
else
return ltn;
}
return NULL;
}
/**
* latch_tree_insert() - insert @node into the trees @root
* @node: nodes to insert
* @root: trees to insert @node into
* @ops: operators defining the node order
*
* It inserts @node into @root in an ordered fashion such that we can always
* observe one complete tree. See the comment for raw_write_seqcount_latch().
*
* The inserts use rcu_assign_pointer() to publish the element such that the
* tree structure is stored before we can observe the new @node.
*
* All modifications (latch_tree_insert, latch_tree_remove) are assumed to be
* serialized.
*/
static __always_inline void
latch_tree_insert(struct latch_tree_node *node,
struct latch_tree_root *root,
const struct latch_tree_ops *ops)
{
raw_write_seqcount_latch(&root->seq);
__lt_insert(node, root, 0, ops->less);
raw_write_seqcount_latch(&root->seq);
__lt_insert(node, root, 1, ops->less);
}
/**
* latch_tree_erase() - removes @node from the trees @root
* @node: nodes to remote
* @root: trees to remove @node from
* @ops: operators defining the node order
*
* Removes @node from the trees @root in an ordered fashion such that we can
* always observe one complete tree. See the comment for
* raw_write_seqcount_latch().
*
* It is assumed that @node will observe one RCU quiescent state before being
* reused of freed.
*
* All modifications (latch_tree_insert, latch_tree_remove) are assumed to be
* serialized.
*/
static __always_inline void
latch_tree_erase(struct latch_tree_node *node,
struct latch_tree_root *root,
const struct latch_tree_ops *ops)
{
raw_write_seqcount_latch(&root->seq);
__lt_erase(node, root, 0);
raw_write_seqcount_latch(&root->seq);
__lt_erase(node, root, 1);
}
/**
* latch_tree_find() - find the node matching @key in the trees @root
* @key: search key
* @root: trees to search for @key
* @ops: operators defining the node order
*
* Does a lockless lookup in the trees @root for the node matching @key.
*
* It is assumed that this is called while holding the appropriate RCU read
* side lock.
*
* If the operators define a partial order on the elements (there are multiple
* elements which have the same key value) it is undefined which of these
* elements will be found. Nor is it possible to iterate the tree to find
* further elements with the same key value.
*
* Returns: a pointer to the node matching @key or NULL.
*/
static __always_inline struct latch_tree_node *
latch_tree_find(void *key, struct latch_tree_root *root,
const struct latch_tree_ops *ops)
{
struct latch_tree_node *node;
unsigned int seq;
do {
seq = raw_read_seqcount_latch(&root->seq);
node = __lt_find(key, root, seq & 1, ops->comp);
} while (read_seqcount_retry(&root->seq, seq));
return node;
}
#endif /* RB_TREE_LATCH_H */
