215 lines
6.7 KiB
C
215 lines
6.7 KiB
C
/* SPDX-License-Identifier: GPL-2.0 */
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/*
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* Latched RB-trees
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*
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* Copyright (C) 2015 Intel Corp., Peter Zijlstra <peterz@infradead.org>
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*
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* Since RB-trees have non-atomic modifications they're not immediately suited
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* for RCU/lockless queries. Even though we made RB-tree lookups non-fatal for
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* lockless lookups; we cannot guarantee they return a correct result.
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*
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* The simplest solution is a seqlock + RB-tree, this will allow lockless
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* lookups; but has the constraint (inherent to the seqlock) that read sides
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* cannot nest in write sides.
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*
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* If we need to allow unconditional lookups (say as required for NMI context
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* usage) we need a more complex setup; this data structure provides this by
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* employing the latch technique -- see @raw_write_seqcount_latch -- to
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* implement a latched RB-tree which does allow for unconditional lookups by
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* virtue of always having (at least) one stable copy of the tree.
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*
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* However, while we have the guarantee that there is at all times one stable
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* copy, this does not guarantee an iteration will not observe modifications.
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* What might have been a stable copy at the start of the iteration, need not
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* remain so for the duration of the iteration.
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*
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* Therefore, this does require a lockless RB-tree iteration to be non-fatal;
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* see the comment in lib/rbtree.c. Note however that we only require the first
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* condition -- not seeing partial stores -- because the latch thing isolates
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* us from loops. If we were to interrupt a modification the lookup would be
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* pointed at the stable tree and complete while the modification was halted.
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*/
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#ifndef RB_TREE_LATCH_H
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#define RB_TREE_LATCH_H
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#include <linux/rbtree.h>
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#include <linux/seqlock.h>
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#include <linux/rcupdate.h>
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struct latch_tree_node {
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struct rb_node node[2];
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};
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struct latch_tree_root {
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seqcount_latch_t seq;
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struct rb_root tree[2];
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};
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/**
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* latch_tree_ops - operators to define the tree order
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* @less: used for insertion; provides the (partial) order between two elements.
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* @comp: used for lookups; provides the order between the search key and an element.
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*
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* The operators are related like:
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*
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* comp(a->key,b) < 0 := less(a,b)
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* comp(a->key,b) > 0 := less(b,a)
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* comp(a->key,b) == 0 := !less(a,b) && !less(b,a)
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*
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* If these operators define a partial order on the elements we make no
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* guarantee on which of the elements matching the key is found. See
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* latch_tree_find().
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*/
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struct latch_tree_ops {
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bool (*less)(struct latch_tree_node *a, struct latch_tree_node *b);
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int (*comp)(void *key, struct latch_tree_node *b);
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};
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static __always_inline struct latch_tree_node *
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__lt_from_rb(struct rb_node *node, int idx)
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{
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return container_of(node, struct latch_tree_node, node[idx]);
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}
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static __always_inline void
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__lt_insert(struct latch_tree_node *ltn, struct latch_tree_root *ltr, int idx,
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bool (*less)(struct latch_tree_node *a, struct latch_tree_node *b))
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{
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struct rb_root *root = <r->tree[idx];
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struct rb_node **link = &root->rb_node;
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struct rb_node *node = <n->node[idx];
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struct rb_node *parent = NULL;
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struct latch_tree_node *ltp;
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while (*link) {
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parent = *link;
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ltp = __lt_from_rb(parent, idx);
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if (less(ltn, ltp))
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link = &parent->rb_left;
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else
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link = &parent->rb_right;
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}
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rb_link_node_rcu(node, parent, link);
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rb_insert_color(node, root);
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}
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static __always_inline void
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__lt_erase(struct latch_tree_node *ltn, struct latch_tree_root *ltr, int idx)
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{
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rb_erase(<n->node[idx], <r->tree[idx]);
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}
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static __always_inline struct latch_tree_node *
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__lt_find(void *key, struct latch_tree_root *ltr, int idx,
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int (*comp)(void *key, struct latch_tree_node *node))
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{
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struct rb_node *node = rcu_dereference_raw(ltr->tree[idx].rb_node);
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struct latch_tree_node *ltn;
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int c;
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while (node) {
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ltn = __lt_from_rb(node, idx);
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c = comp(key, ltn);
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if (c < 0)
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node = rcu_dereference_raw(node->rb_left);
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else if (c > 0)
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node = rcu_dereference_raw(node->rb_right);
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else
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return ltn;
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}
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return NULL;
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}
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/**
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* latch_tree_insert() - insert @node into the trees @root
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* @node: nodes to insert
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* @root: trees to insert @node into
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* @ops: operators defining the node order
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*
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* It inserts @node into @root in an ordered fashion such that we can always
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* observe one complete tree. See the comment for raw_write_seqcount_latch().
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*
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* The inserts use rcu_assign_pointer() to publish the element such that the
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* tree structure is stored before we can observe the new @node.
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*
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* All modifications (latch_tree_insert, latch_tree_remove) are assumed to be
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* serialized.
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*/
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static __always_inline void
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latch_tree_insert(struct latch_tree_node *node,
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struct latch_tree_root *root,
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const struct latch_tree_ops *ops)
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{
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raw_write_seqcount_latch(&root->seq);
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__lt_insert(node, root, 0, ops->less);
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raw_write_seqcount_latch(&root->seq);
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__lt_insert(node, root, 1, ops->less);
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}
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/**
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* latch_tree_erase() - removes @node from the trees @root
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* @node: nodes to remote
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* @root: trees to remove @node from
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* @ops: operators defining the node order
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*
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* Removes @node from the trees @root in an ordered fashion such that we can
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* always observe one complete tree. See the comment for
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* raw_write_seqcount_latch().
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*
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* It is assumed that @node will observe one RCU quiescent state before being
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* reused of freed.
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*
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* All modifications (latch_tree_insert, latch_tree_remove) are assumed to be
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* serialized.
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*/
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static __always_inline void
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latch_tree_erase(struct latch_tree_node *node,
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struct latch_tree_root *root,
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const struct latch_tree_ops *ops)
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{
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raw_write_seqcount_latch(&root->seq);
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__lt_erase(node, root, 0);
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raw_write_seqcount_latch(&root->seq);
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__lt_erase(node, root, 1);
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}
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/**
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* latch_tree_find() - find the node matching @key in the trees @root
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* @key: search key
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* @root: trees to search for @key
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* @ops: operators defining the node order
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*
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* Does a lockless lookup in the trees @root for the node matching @key.
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*
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* It is assumed that this is called while holding the appropriate RCU read
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* side lock.
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*
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* If the operators define a partial order on the elements (there are multiple
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* elements which have the same key value) it is undefined which of these
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* elements will be found. Nor is it possible to iterate the tree to find
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* further elements with the same key value.
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*
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* Returns: a pointer to the node matching @key or NULL.
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*/
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static __always_inline struct latch_tree_node *
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latch_tree_find(void *key, struct latch_tree_root *root,
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const struct latch_tree_ops *ops)
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{
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struct latch_tree_node *node;
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unsigned int seq;
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do {
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seq = raw_read_seqcount_latch(&root->seq);
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node = __lt_find(key, root, seq & 1, ops->comp);
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} while (read_seqcount_latch_retry(&root->seq, seq));
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return node;
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}
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#endif /* RB_TREE_LATCH_H */
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