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/* SPDX-License-Identifier: GPL-2.0 / / * 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> #include <linux/rcupdate.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 = &ltr->tree[idx]; struct rb_node link = &root->rb_node; struct rb_node node = &ltn->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(&ltn->node[idx], &ltr->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 */

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