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Release 4.7 include/linux/rbtree_latch.h

Directory: include/linux
/*
 * 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 = &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); }

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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]); }

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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); }

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/** * 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); }

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/** * 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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Directory: include/linux
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