xref: /openbmc/linux/include/linux/rbtree_latch.h (revision d16317de)
1 /* SPDX-License-Identifier: GPL-2.0 */
2 /*
3  * Latched RB-trees
4  *
5  * Copyright (C) 2015 Intel Corp., Peter Zijlstra <peterz@infradead.org>
6  *
7  * Since RB-trees have non-atomic modifications they're not immediately suited
8  * for RCU/lockless queries. Even though we made RB-tree lookups non-fatal for
9  * lockless lookups; we cannot guarantee they return a correct result.
10  *
11  * The simplest solution is a seqlock + RB-tree, this will allow lockless
12  * lookups; but has the constraint (inherent to the seqlock) that read sides
13  * cannot nest in write sides.
14  *
15  * If we need to allow unconditional lookups (say as required for NMI context
16  * usage) we need a more complex setup; this data structure provides this by
17  * employing the latch technique -- see @raw_write_seqcount_latch -- to
18  * implement a latched RB-tree which does allow for unconditional lookups by
19  * virtue of always having (at least) one stable copy of the tree.
20  *
21  * However, while we have the guarantee that there is at all times one stable
22  * copy, this does not guarantee an iteration will not observe modifications.
23  * What might have been a stable copy at the start of the iteration, need not
24  * remain so for the duration of the iteration.
25  *
26  * Therefore, this does require a lockless RB-tree iteration to be non-fatal;
27  * see the comment in lib/rbtree.c. Note however that we only require the first
28  * condition -- not seeing partial stores -- because the latch thing isolates
29  * us from loops. If we were to interrupt a modification the lookup would be
30  * pointed at the stable tree and complete while the modification was halted.
31  */
32 
33 #ifndef RB_TREE_LATCH_H
34 #define RB_TREE_LATCH_H
35 
36 #include <linux/rbtree.h>
37 #include <linux/seqlock.h>
38 #include <linux/rcupdate.h>
39 
40 struct latch_tree_node {
41 	struct rb_node node[2];
42 };
43 
44 struct latch_tree_root {
45 	seqcount_latch_t	seq;
46 	struct rb_root		tree[2];
47 };
48 
49 /**
50  * latch_tree_ops - operators to define the tree order
51  * @less: used for insertion; provides the (partial) order between two elements.
52  * @comp: used for lookups; provides the order between the search key and an element.
53  *
54  * The operators are related like:
55  *
56  *	comp(a->key,b) < 0  := less(a,b)
57  *	comp(a->key,b) > 0  := less(b,a)
58  *	comp(a->key,b) == 0 := !less(a,b) && !less(b,a)
59  *
60  * If these operators define a partial order on the elements we make no
61  * guarantee on which of the elements matching the key is found. See
62  * latch_tree_find().
63  */
64 struct latch_tree_ops {
65 	bool (*less)(struct latch_tree_node *a, struct latch_tree_node *b);
66 	int  (*comp)(void *key,                 struct latch_tree_node *b);
67 };
68 
69 static __always_inline struct latch_tree_node *
__lt_from_rb(struct rb_node * node,int idx)70 __lt_from_rb(struct rb_node *node, int idx)
71 {
72 	return container_of(node, struct latch_tree_node, node[idx]);
73 }
74 
75 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))76 __lt_insert(struct latch_tree_node *ltn, struct latch_tree_root *ltr, int idx,
77 	    bool (*less)(struct latch_tree_node *a, struct latch_tree_node *b))
78 {
79 	struct rb_root *root = &ltr->tree[idx];
80 	struct rb_node **link = &root->rb_node;
81 	struct rb_node *node = &ltn->node[idx];
82 	struct rb_node *parent = NULL;
83 	struct latch_tree_node *ltp;
84 
85 	while (*link) {
86 		parent = *link;
87 		ltp = __lt_from_rb(parent, idx);
88 
89 		if (less(ltn, ltp))
90 			link = &parent->rb_left;
91 		else
92 			link = &parent->rb_right;
93 	}
94 
95 	rb_link_node_rcu(node, parent, link);
96 	rb_insert_color(node, root);
97 }
98 
99 static __always_inline void
__lt_erase(struct latch_tree_node * ltn,struct latch_tree_root * ltr,int idx)100 __lt_erase(struct latch_tree_node *ltn, struct latch_tree_root *ltr, int idx)
101 {
102 	rb_erase(&ltn->node[idx], &ltr->tree[idx]);
103 }
104 
105 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))106 __lt_find(void *key, struct latch_tree_root *ltr, int idx,
107 	  int (*comp)(void *key, struct latch_tree_node *node))
108 {
109 	struct rb_node *node = rcu_dereference_raw(ltr->tree[idx].rb_node);
110 	struct latch_tree_node *ltn;
111 	int c;
112 
113 	while (node) {
114 		ltn = __lt_from_rb(node, idx);
115 		c = comp(key, ltn);
116 
117 		if (c < 0)
118 			node = rcu_dereference_raw(node->rb_left);
119 		else if (c > 0)
120 			node = rcu_dereference_raw(node->rb_right);
121 		else
122 			return ltn;
123 	}
124 
125 	return NULL;
126 }
127 
128 /**
129  * latch_tree_insert() - insert @node into the trees @root
130  * @node: nodes to insert
131  * @root: trees to insert @node into
132  * @ops: operators defining the node order
133  *
134  * It inserts @node into @root in an ordered fashion such that we can always
135  * observe one complete tree. See the comment for raw_write_seqcount_latch().
136  *
137  * The inserts use rcu_assign_pointer() to publish the element such that the
138  * tree structure is stored before we can observe the new @node.
139  *
140  * All modifications (latch_tree_insert, latch_tree_remove) are assumed to be
141  * serialized.
142  */
143 static __always_inline void
latch_tree_insert(struct latch_tree_node * node,struct latch_tree_root * root,const struct latch_tree_ops * ops)144 latch_tree_insert(struct latch_tree_node *node,
145 		  struct latch_tree_root *root,
146 		  const struct latch_tree_ops *ops)
147 {
148 	raw_write_seqcount_latch(&root->seq);
149 	__lt_insert(node, root, 0, ops->less);
150 	raw_write_seqcount_latch(&root->seq);
151 	__lt_insert(node, root, 1, ops->less);
152 }
153 
154 /**
155  * latch_tree_erase() - removes @node from the trees @root
156  * @node: nodes to remote
157  * @root: trees to remove @node from
158  * @ops: operators defining the node order
159  *
160  * Removes @node from the trees @root in an ordered fashion such that we can
161  * always observe one complete tree. See the comment for
162  * raw_write_seqcount_latch().
163  *
164  * It is assumed that @node will observe one RCU quiescent state before being
165  * reused of freed.
166  *
167  * All modifications (latch_tree_insert, latch_tree_remove) are assumed to be
168  * serialized.
169  */
170 static __always_inline void
latch_tree_erase(struct latch_tree_node * node,struct latch_tree_root * root,const struct latch_tree_ops * ops)171 latch_tree_erase(struct latch_tree_node *node,
172 		 struct latch_tree_root *root,
173 		 const struct latch_tree_ops *ops)
174 {
175 	raw_write_seqcount_latch(&root->seq);
176 	__lt_erase(node, root, 0);
177 	raw_write_seqcount_latch(&root->seq);
178 	__lt_erase(node, root, 1);
179 }
180 
181 /**
182  * latch_tree_find() - find the node matching @key in the trees @root
183  * @key: search key
184  * @root: trees to search for @key
185  * @ops: operators defining the node order
186  *
187  * Does a lockless lookup in the trees @root for the node matching @key.
188  *
189  * It is assumed that this is called while holding the appropriate RCU read
190  * side lock.
191  *
192  * If the operators define a partial order on the elements (there are multiple
193  * elements which have the same key value) it is undefined which of these
194  * elements will be found. Nor is it possible to iterate the tree to find
195  * further elements with the same key value.
196  *
197  * Returns: a pointer to the node matching @key or NULL.
198  */
199 static __always_inline struct latch_tree_node *
latch_tree_find(void * key,struct latch_tree_root * root,const struct latch_tree_ops * ops)200 latch_tree_find(void *key, struct latch_tree_root *root,
201 		const struct latch_tree_ops *ops)
202 {
203 	struct latch_tree_node *node;
204 	unsigned int seq;
205 
206 	do {
207 		seq = raw_read_seqcount_latch(&root->seq);
208 		node = __lt_find(key, root, seq & 1, ops->comp);
209 	} while (raw_read_seqcount_latch_retry(&root->seq, seq));
210 
211 	return node;
212 }
213 
214 #endif /* RB_TREE_LATCH_H */
215