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2 * Copyright (c) 2014, 2016 Nicira, Inc.
4 * Licensed under the Apache License, Version 2.0 (the "License");
5 * you may not use this file except in compliance with the License.
6 * You may obtain a copy of the License at:
8 * http://www.apache.org/licenses/LICENSE-2.0
10 * Unless required by applicable law or agreed to in writing, software
11 * distributed under the License is distributed on an "AS IS" BASIS,
12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13 * See the License for the specific language governing permissions and
14 * limitations under the License.
26 COVERAGE_DEFINE(cmap_expand
);
27 COVERAGE_DEFINE(cmap_shrink
);
29 /* Optimistic Concurrent Cuckoo Hash
30 * =================================
32 * A "cuckoo hash" is an open addressing hash table schema, designed such that
33 * a given element can be in one of only a small number of buckets 'd', each of
34 * which holds up to a small number 'k' elements. Thus, the expected and
35 * worst-case lookup times are O(1) because they require comparing no more than
36 * a fixed number of elements (k * d). Inserting a new element can require
37 * moving around existing elements, but it is also O(1) amortized expected
40 * An optimistic concurrent hash table goes one step further, making it
41 * possible for a single writer to execute concurrently with any number of
42 * readers without requiring the readers to take any locks.
44 * This cuckoo hash implementation uses:
46 * - Two hash functions (d=2). More hash functions allow for a higher load
47 * factor, but increasing 'k' is easier and the benefits of increasing 'd'
48 * quickly fall off with the 'k' values used here. Also, the method of
49 * generating hashes used in this implementation is hard to reasonably
50 * extend beyond d=2. Finally, each additional hash function means that a
51 * lookup has to look at least one extra cache line.
53 * - 5 or 7 elements per bucket (k=5 or k=7), chosen to make buckets
54 * exactly one cache line in size.
56 * According to Erlingsson [4], these parameters suggest a maximum load factor
57 * of about 93%. The current implementation is conservative, expanding the
58 * hash table when it is over 85% full.
60 * When the load factor is below 20%, the hash table will be shrinked by half.
61 * This is to reduce the memory utilization of the hash table and to avoid
62 * the hash table occupying the top of heap chunk which prevents the trimming
68 * A cuckoo hash requires multiple hash functions. When reorganizing the hash
69 * becomes too difficult, it also requires the ability to change the hash
70 * functions. Requiring the client to provide multiple hashes and to be able
71 * to change them to new hashes upon insertion is inconvenient.
73 * This implementation takes another approach. The client provides a single,
74 * fixed hash. The cuckoo hash internally "rehashes" this hash against a
75 * randomly selected basis value (see rehash()). This rehashed value is one of
76 * the two hashes. The other hash is computed by 16-bit circular rotation of
77 * the rehashed value. Updating the basis changes the hash functions.
79 * To work properly, the hash functions used by a cuckoo hash must be
80 * independent. If one hash function is a function of the other (e.g. h2(x) =
81 * h1(x) + 1, or h2(x) = hash(h1(x))), then insertion will eventually fail
82 * catastrophically (loop forever) because of collisions. With this rehashing
83 * technique, the two hashes are completely independent for masks up to 16 bits
84 * wide. For masks wider than 16 bits, only 32-n bits are independent between
85 * the two hashes. Thus, it becomes risky to grow a cuckoo hash table beyond
86 * about 2**24 buckets (about 71 million elements with k=5 and maximum load
87 * 85%). Fortunately, Open vSwitch does not normally deal with hash tables
94 * This cuckoo hash table implementation deals with duplicate client-provided
95 * hash values by chaining: the second and subsequent cmap_nodes with a given
96 * hash are chained off the initially inserted node's 'next' member. The hash
97 * table maintains the invariant that a single client-provided hash value
98 * exists in only a single chain in a single bucket (even though that hash
99 * could be stored in two buckets).
105 * [1] D. Zhou, B. Fan, H. Lim, M. Kaminsky, D. G. Andersen, "Scalable, High
106 * Performance Ethernet Forwarding with CuckooSwitch". In Proc. 9th
109 * [2] B. Fan, D. G. Andersen, and M. Kaminsky. "MemC3: Compact and concurrent
110 * memcache with dumber caching and smarter hashing". In Proc. 10th USENIX
113 * [3] R. Pagh and F. Rodler. "Cuckoo hashing". Journal of Algorithms, 51(2):
116 * [4] U. Erlingsson, M. Manasse, F. McSherry, "A Cool and Practical
117 * Alternative to Traditional Hash Tables". In Proc. 7th Workshop on
118 * Distributed Data and Structures (WDAS'06), 2006.
120 /* An entry is an int and a pointer: 8 bytes on 32-bit, 12 bytes on 64-bit. */
121 #define CMAP_ENTRY_SIZE (4 + (UINTPTR_MAX == UINT32_MAX ? 4 : 8))
123 /* Number of entries per bucket: 7 on 32-bit, 5 on 64-bit for 64B cacheline. */
124 #define CMAP_K ((CACHE_LINE_SIZE - 4) / CMAP_ENTRY_SIZE)
126 /* A cuckoo hash bucket. Designed to be cache-aligned and exactly one cache
129 /* Padding to make cmap_bucket exactly one cache line long. */
130 PADDED_MEMBERS(CACHE_LINE_SIZE
,
131 /* Allows readers to track in-progress changes. Initially zero, each
132 * writer increments this value just before and just after each change
133 * (see cmap_set_bucket()). Thus, a reader can ensure that it gets a
134 * consistent snapshot by waiting for the counter to become even (see
135 * read_even_counter()), then checking that its value does not change
136 * while examining the bucket (see cmap_find()). */
137 atomic_uint32_t counter
;
139 /* (hash, node) slots. They are parallel arrays instead of an array of
140 * structs to reduce the amount of space lost to padding.
142 * The slots are in no particular order. A null pointer indicates that
143 * a pair is unused. In-use slots are not necessarily in the earliest
145 uint32_t hashes
[CMAP_K
];
146 struct cmap_node nodes
[CMAP_K
];
149 BUILD_ASSERT_DECL(sizeof(struct cmap_bucket
) == CACHE_LINE_SIZE
);
151 /* Default maximum load factor (as a fraction of UINT32_MAX + 1) before
152 * enlarging a cmap. Reasonable values lie between about 75% and 93%. Smaller
153 * values waste memory; larger values increase the average insertion time. */
154 #define CMAP_MAX_LOAD ((uint32_t) (UINT32_MAX * .85))
156 /* Default minimum load factor (as a fraction of UINT32_MAX + 1) before
157 * shrinking a cmap. Currently, the value is chosen to be 20%, this
158 * means cmap will have a 40% load factor after shrink. */
159 #define CMAP_MIN_LOAD ((uint32_t) (UINT32_MAX * .20))
161 /* The implementation of a concurrent hash map. */
163 PADDED_MEMBERS_CACHELINE_MARKER(CACHE_LINE_SIZE
, cacheline0
,
164 unsigned int n
; /* Number of in-use elements. */
165 unsigned int max_n
; /* Max elements before enlarging. */
166 unsigned int min_n
; /* Min elements before shrinking. */
167 uint32_t mask
; /* Number of 'buckets', minus one. */
168 uint32_t basis
; /* Basis for rehashing client's
172 PADDED_MEMBERS_CACHELINE_MARKER(CACHE_LINE_SIZE
, cacheline1
,
173 struct cmap_bucket buckets
[1];
176 BUILD_ASSERT_DECL(sizeof(struct cmap_impl
) == CACHE_LINE_SIZE
* 2);
179 OVS_ALIGNED_VAR(CACHE_LINE_SIZE
) const struct cmap_impl empty_cmap
;
181 static struct cmap_impl
*cmap_rehash(struct cmap
*, uint32_t mask
);
183 /* Explicit inline keywords in utility functions seem to be necessary
184 * to prevent performance regression on cmap_find(). */
186 /* Given a rehashed value 'hash', returns the other hash for that rehashed
187 * value. This is symmetric: other_hash(other_hash(x)) == x. (See also "Hash
188 * Functions" at the top of this file.) */
189 static inline uint32_t
190 other_hash(uint32_t hash
)
192 return (hash
<< 16) | (hash
>> 16);
195 /* Returns the rehashed value for 'hash' within 'impl'. (See also "Hash
196 * Functions" at the top of this file.) */
197 static inline uint32_t
198 rehash(const struct cmap_impl
*impl
, uint32_t hash
)
200 return hash_finish(impl
->basis
, hash
);
203 /* Not always without the inline keyword. */
204 static inline struct cmap_impl
*
205 cmap_get_impl(const struct cmap
*cmap
)
207 return ovsrcu_get(struct cmap_impl
*, &cmap
->impl
);
211 calc_max_n(uint32_t mask
)
213 return ((uint64_t) (mask
+ 1) * CMAP_K
* CMAP_MAX_LOAD
) >> 32;
217 calc_min_n(uint32_t mask
)
219 return ((uint64_t) (mask
+ 1) * CMAP_K
* CMAP_MIN_LOAD
) >> 32;
222 static struct cmap_impl
*
223 cmap_impl_create(uint32_t mask
)
225 struct cmap_impl
*impl
;
227 ovs_assert(is_pow2(mask
+ 1));
229 /* There are 'mask + 1' buckets but struct cmap_impl has one bucket built
230 * in, so we only need to add space for the extra 'mask' buckets. */
231 impl
= xzalloc_cacheline(sizeof *impl
+ mask
* sizeof *impl
->buckets
);
233 impl
->max_n
= calc_max_n(mask
);
234 impl
->min_n
= calc_min_n(mask
);
236 impl
->basis
= random_uint32();
241 /* Initializes 'cmap' as an empty concurrent hash map. */
243 cmap_init(struct cmap
*cmap
)
245 ovsrcu_set(&cmap
->impl
, CONST_CAST(struct cmap_impl
*, &empty_cmap
));
250 * The client is responsible for destroying any data previously held in
253 cmap_destroy(struct cmap
*cmap
)
256 struct cmap_impl
*impl
= cmap_get_impl(cmap
);
257 if (impl
!= &empty_cmap
) {
258 ovsrcu_postpone(free_cacheline
, impl
);
263 /* Returns the number of elements in 'cmap'. */
265 cmap_count(const struct cmap
*cmap
)
267 return cmap_get_impl(cmap
)->n
;
270 /* Returns true if 'cmap' is empty, false otherwise. */
272 cmap_is_empty(const struct cmap
*cmap
)
274 return cmap_count(cmap
) == 0;
277 static inline uint32_t
278 read_counter(const struct cmap_bucket
*bucket_
)
280 struct cmap_bucket
*bucket
= CONST_CAST(struct cmap_bucket
*, bucket_
);
283 atomic_read_explicit(&bucket
->counter
, &counter
, memory_order_acquire
);
288 static inline uint32_t
289 read_even_counter(const struct cmap_bucket
*bucket
)
294 counter
= read_counter(bucket
);
295 } while (OVS_UNLIKELY(counter
& 1));
301 counter_changed(const struct cmap_bucket
*b_
, uint32_t c
)
303 struct cmap_bucket
*b
= CONST_CAST(struct cmap_bucket
*, b_
);
306 /* Need to make sure the counter read is not moved up, before the hash and
307 * cmap_node_next(). Using atomic_read_explicit with memory_order_acquire
308 * would allow prior reads to be moved after the barrier.
309 * atomic_thread_fence prevents all following memory accesses from moving
310 * prior to preceding loads. */
311 atomic_thread_fence(memory_order_acquire
);
312 atomic_read_relaxed(&b
->counter
, &counter
);
314 return OVS_UNLIKELY(counter
!= c
);
317 static inline const struct cmap_node
*
318 cmap_find_in_bucket(const struct cmap_bucket
*bucket
, uint32_t hash
)
320 for (int i
= 0; i
< CMAP_K
; i
++) {
321 if (bucket
->hashes
[i
] == hash
) {
322 return cmap_node_next(&bucket
->nodes
[i
]);
328 static inline const struct cmap_node
*
329 cmap_find__(const struct cmap_bucket
*b1
, const struct cmap_bucket
*b2
,
333 const struct cmap_node
*node
;
337 c1
= read_even_counter(b1
);
338 node
= cmap_find_in_bucket(b1
, hash
);
339 } while (OVS_UNLIKELY(counter_changed(b1
, c1
)));
344 c2
= read_even_counter(b2
);
345 node
= cmap_find_in_bucket(b2
, hash
);
346 } while (OVS_UNLIKELY(counter_changed(b2
, c2
)));
350 } while (OVS_UNLIKELY(counter_changed(b1
, c1
)));
355 /* Searches 'cmap' for an element with the specified 'hash'. If one or more is
356 * found, returns a pointer to the first one, otherwise a null pointer. All of
357 * the nodes on the returned list are guaranteed to have exactly the given
360 * This function works even if 'cmap' is changing concurrently. If 'cmap' is
361 * not changing, then cmap_find_protected() is slightly faster.
363 * CMAP_FOR_EACH_WITH_HASH is usually more convenient. */
364 const struct cmap_node
*
365 cmap_find(const struct cmap
*cmap
, uint32_t hash
)
367 const struct cmap_impl
*impl
= cmap_get_impl(cmap
);
368 uint32_t h1
= rehash(impl
, hash
);
369 uint32_t h2
= other_hash(h1
);
371 return cmap_find__(&impl
->buckets
[h1
& impl
->mask
],
372 &impl
->buckets
[h2
& impl
->mask
],
376 /* Looks up multiple 'hashes', when the corresponding bit in 'map' is 1,
377 * and sets the corresponding pointer in 'nodes', if the hash value was
378 * found from the 'cmap'. In other cases the 'nodes' values are not changed,
379 * i.e., no NULL pointers are stored there.
380 * Returns a map where a bit is set to 1 if the corresponding 'nodes' pointer
381 * was stored, 0 otherwise.
382 * Generally, the caller wants to use CMAP_NODE_FOR_EACH to verify for
383 * hash collisions. */
385 cmap_find_batch(const struct cmap
*cmap
, unsigned long map
,
386 uint32_t hashes
[], const struct cmap_node
*nodes
[])
388 const struct cmap_impl
*impl
= cmap_get_impl(cmap
);
389 unsigned long result
= map
;
391 uint32_t h1s
[sizeof map
* CHAR_BIT
];
392 const struct cmap_bucket
*b1s
[sizeof map
* CHAR_BIT
];
393 const struct cmap_bucket
*b2s
[sizeof map
* CHAR_BIT
];
394 uint32_t c1s
[sizeof map
* CHAR_BIT
];
396 /* Compute hashes and prefetch 1st buckets. */
397 ULLONG_FOR_EACH_1(i
, map
) {
398 h1s
[i
] = rehash(impl
, hashes
[i
]);
399 b1s
[i
] = &impl
->buckets
[h1s
[i
] & impl
->mask
];
400 OVS_PREFETCH(b1s
[i
]);
402 /* Lookups, Round 1. Only look up at the first bucket. */
403 ULLONG_FOR_EACH_1(i
, map
) {
405 const struct cmap_bucket
*b1
= b1s
[i
];
406 const struct cmap_node
*node
;
409 c1
= read_even_counter(b1
);
410 node
= cmap_find_in_bucket(b1
, hashes
[i
]);
411 } while (OVS_UNLIKELY(counter_changed(b1
, c1
)));
414 /* Not found (yet); Prefetch the 2nd bucket. */
415 b2s
[i
] = &impl
->buckets
[other_hash(h1s
[i
]) & impl
->mask
];
416 OVS_PREFETCH(b2s
[i
]);
417 c1s
[i
] = c1
; /* We may need to check this after Round 2. */
421 ULLONG_SET0(map
, i
); /* Ignore this on round 2. */
425 /* Round 2. Look into the 2nd bucket, if needed. */
426 ULLONG_FOR_EACH_1(i
, map
) {
428 const struct cmap_bucket
*b2
= b2s
[i
];
429 const struct cmap_node
*node
;
432 c2
= read_even_counter(b2
);
433 node
= cmap_find_in_bucket(b2
, hashes
[i
]);
434 } while (OVS_UNLIKELY(counter_changed(b2
, c2
)));
437 /* Not found, but the node may have been moved from b2 to b1 right
438 * after we finished with b1 earlier. We just got a clean reading
439 * of the 2nd bucket, so we check the counter of the 1st bucket
440 * only. However, we need to check both buckets again, as the
441 * entry may be moved again to the 2nd bucket. Basically, we
442 * need to loop as long as it takes to get stable readings of
443 * both buckets. cmap_find__() does that, and now that we have
444 * fetched both buckets we can just use it. */
445 if (OVS_UNLIKELY(counter_changed(b1s
[i
], c1s
[i
]))) {
446 node
= cmap_find__(b1s
[i
], b2s
[i
], hashes
[i
]);
452 ULLONG_SET0(result
, i
); /* Fix the result. */
463 cmap_find_slot_protected(struct cmap_bucket
*b
, uint32_t hash
)
467 for (i
= 0; i
< CMAP_K
; i
++) {
468 if (b
->hashes
[i
] == hash
&& cmap_node_next_protected(&b
->nodes
[i
])) {
475 static struct cmap_node
*
476 cmap_find_bucket_protected(struct cmap_impl
*impl
, uint32_t hash
, uint32_t h
)
478 struct cmap_bucket
*b
= &impl
->buckets
[h
& impl
->mask
];
481 for (i
= 0; i
< CMAP_K
; i
++) {
482 if (b
->hashes
[i
] == hash
) {
483 return cmap_node_next_protected(&b
->nodes
[i
]);
489 /* Like cmap_find(), but only for use if 'cmap' cannot change concurrently.
491 * CMAP_FOR_EACH_WITH_HASH_PROTECTED is usually more convenient. */
493 cmap_find_protected(const struct cmap
*cmap
, uint32_t hash
)
495 struct cmap_impl
*impl
= cmap_get_impl(cmap
);
496 uint32_t h1
= rehash(impl
, hash
);
497 uint32_t h2
= other_hash(hash
);
498 struct cmap_node
*node
;
500 node
= cmap_find_bucket_protected(impl
, hash
, h1
);
504 return cmap_find_bucket_protected(impl
, hash
, h2
);
508 cmap_find_empty_slot_protected(const struct cmap_bucket
*b
)
512 for (i
= 0; i
< CMAP_K
; i
++) {
513 if (!cmap_node_next_protected(&b
->nodes
[i
])) {
521 cmap_set_bucket(struct cmap_bucket
*b
, int i
,
522 struct cmap_node
*node
, uint32_t hash
)
526 atomic_read_explicit(&b
->counter
, &c
, memory_order_acquire
);
527 atomic_store_explicit(&b
->counter
, c
+ 1, memory_order_release
);
528 ovsrcu_set(&b
->nodes
[i
].next
, node
); /* Also atomic. */
530 atomic_store_explicit(&b
->counter
, c
+ 2, memory_order_release
);
533 /* Searches 'b' for a node with the given 'hash'. If it finds one, adds
534 * 'new_node' to the node's linked list and returns true. If it does not find
535 * one, returns false. */
537 cmap_insert_dup(struct cmap_node
*new_node
, uint32_t hash
,
538 struct cmap_bucket
*b
)
542 for (i
= 0; i
< CMAP_K
; i
++) {
543 if (b
->hashes
[i
] == hash
) {
544 struct cmap_node
*node
= cmap_node_next_protected(&b
->nodes
[i
]);
549 /* The common case is that 'new_node' is a singleton,
550 * with a null 'next' pointer. Rehashing can add a
551 * longer chain, but due to our invariant of always
552 * having all nodes with the same (user) hash value at
553 * a single chain, rehashing will always insert the
554 * chain to an empty node. The only way we can end up
555 * here is by the user inserting a chain of nodes at
556 * once. Find the end of the chain starting at
557 * 'new_node', then splice 'node' to the end of that
561 struct cmap_node
*next
= cmap_node_next_protected(p
);
568 ovsrcu_set_hidden(&p
->next
, node
);
570 /* The hash value is there from some previous insertion, but
571 * the associated node has been removed. We're not really
572 * inserting a duplicate, but we can still reuse the slot.
576 /* Change the bucket to point to 'new_node'. This is a degenerate
577 * form of cmap_set_bucket() that doesn't update the counter since
578 * we're only touching one field and in a way that doesn't change
579 * the bucket's meaning for readers. */
580 ovsrcu_set(&b
->nodes
[i
].next
, new_node
);
588 /* Searches 'b' for an empty slot. If successful, stores 'node' and 'hash' in
589 * the slot and returns true. Otherwise, returns false. */
591 cmap_insert_bucket(struct cmap_node
*node
, uint32_t hash
,
592 struct cmap_bucket
*b
)
596 for (i
= 0; i
< CMAP_K
; i
++) {
597 if (!cmap_node_next_protected(&b
->nodes
[i
])) {
598 cmap_set_bucket(b
, i
, node
, hash
);
605 /* Returns the other bucket that b->nodes[slot] could occupy in 'impl'. (This
606 * might be the same as 'b'.) */
607 static struct cmap_bucket
*
608 other_bucket_protected(struct cmap_impl
*impl
, struct cmap_bucket
*b
, int slot
)
610 uint32_t h1
= rehash(impl
, b
->hashes
[slot
]);
611 uint32_t h2
= other_hash(h1
);
612 uint32_t b_idx
= b
- impl
->buckets
;
613 uint32_t other_h
= (h1
& impl
->mask
) == b_idx
? h2
: h1
;
615 return &impl
->buckets
[other_h
& impl
->mask
];
618 /* 'new_node' is to be inserted into 'impl', but both candidate buckets 'b1'
619 * and 'b2' are full. This function attempts to rearrange buckets within
620 * 'impl' to make room for 'new_node'.
622 * The implementation is a general-purpose breadth-first search. At first
623 * glance, this is more complex than a random walk through 'impl' (suggested by
624 * some references), but random walks have a tendency to loop back through a
625 * single bucket. We have to move nodes backward along the path that we find,
626 * so that no node actually disappears from the hash table, which means a
627 * random walk would have to be careful to deal with loops. By contrast, a
628 * successful breadth-first search always finds a *shortest* path through the
629 * hash table, and a shortest path will never contain loops, so it avoids that
633 cmap_insert_bfs(struct cmap_impl
*impl
, struct cmap_node
*new_node
,
634 uint32_t hash
, struct cmap_bucket
*b1
, struct cmap_bucket
*b2
)
636 enum { MAX_DEPTH
= 4 };
638 /* A path from 'start' to 'end' via the 'n' steps in 'slots[]'.
640 * One can follow the path via:
642 * struct cmap_bucket *b;
646 * for (i = 0; i < path->n; i++) {
647 * b = other_bucket_protected(impl, b, path->slots[i]);
649 * ovs_assert(b == path->end);
652 struct cmap_bucket
*start
; /* First bucket along the path. */
653 struct cmap_bucket
*end
; /* Last bucket on the path. */
654 uint8_t slots
[MAX_DEPTH
]; /* Slots used for each hop. */
655 int n
; /* Number of slots[]. */
658 /* We need to limit the amount of work we do trying to find a path. It
659 * might actually be impossible to rearrange the cmap, and after some time
660 * it is likely to be easier to rehash the entire cmap.
662 * This value of MAX_QUEUE is an arbitrary limit suggested by one of the
663 * references. Empirically, it seems to work OK. */
664 enum { MAX_QUEUE
= 500 };
665 struct cmap_path queue
[MAX_QUEUE
];
669 /* Add 'b1' and 'b2' as starting points for the search. */
670 queue
[head
].start
= b1
;
671 queue
[head
].end
= b1
;
675 queue
[head
].start
= b2
;
676 queue
[head
].end
= b2
;
681 while (tail
< head
) {
682 const struct cmap_path
*path
= &queue
[tail
++];
683 struct cmap_bucket
*this = path
->end
;
686 for (i
= 0; i
< CMAP_K
; i
++) {
687 struct cmap_bucket
*next
= other_bucket_protected(impl
, this, i
);
694 j
= cmap_find_empty_slot_protected(next
);
696 /* We've found a path along which we can rearrange the hash
697 * table: Start at path->start, follow all the slots in
698 * path->slots[], then follow slot 'i', then the bucket you
699 * arrive at has slot 'j' empty. */
700 struct cmap_bucket
*buckets
[MAX_DEPTH
+ 2];
701 int slots
[MAX_DEPTH
+ 2];
704 /* Figure out the full sequence of slots. */
705 for (k
= 0; k
< path
->n
; k
++) {
706 slots
[k
] = path
->slots
[k
];
709 slots
[path
->n
+ 1] = j
;
711 /* Figure out the full sequence of buckets. */
712 buckets
[0] = path
->start
;
713 for (k
= 0; k
<= path
->n
; k
++) {
714 buckets
[k
+ 1] = other_bucket_protected(impl
, buckets
[k
], slots
[k
]);
717 /* Now the path is fully expressed. One can start from
718 * buckets[0], go via slots[0] to buckets[1], via slots[1] to
719 * buckets[2], and so on.
721 * Move all the nodes across the path "backward". After each
722 * step some node appears in two buckets. Thus, every node is
723 * always visible to a concurrent search. */
724 for (k
= path
->n
+ 1; k
> 0; k
--) {
725 int slot
= slots
[k
- 1];
728 buckets
[k
], slots
[k
],
729 cmap_node_next_protected(&buckets
[k
- 1]->nodes
[slot
]),
730 buckets
[k
- 1]->hashes
[slot
]);
733 /* Finally, replace the first node on the path by
735 cmap_set_bucket(buckets
[0], slots
[0], new_node
, hash
);
740 if (path
->n
< MAX_DEPTH
&& head
< MAX_QUEUE
) {
741 struct cmap_path
*new_path
= &queue
[head
++];
744 new_path
->end
= next
;
745 new_path
->slots
[new_path
->n
++] = i
;
753 /* Adds 'node', with the given 'hash', to 'impl'.
755 * 'node' is ordinarily a single node, with a null 'next' pointer. When
756 * rehashing, however, it may be a longer chain of nodes. */
758 cmap_try_insert(struct cmap_impl
*impl
, struct cmap_node
*node
, uint32_t hash
)
760 uint32_t h1
= rehash(impl
, hash
);
761 uint32_t h2
= other_hash(h1
);
762 struct cmap_bucket
*b1
= &impl
->buckets
[h1
& impl
->mask
];
763 struct cmap_bucket
*b2
= &impl
->buckets
[h2
& impl
->mask
];
765 return (OVS_UNLIKELY(cmap_insert_dup(node
, hash
, b1
) ||
766 cmap_insert_dup(node
, hash
, b2
)) ||
767 OVS_LIKELY(cmap_insert_bucket(node
, hash
, b1
) ||
768 cmap_insert_bucket(node
, hash
, b2
)) ||
769 cmap_insert_bfs(impl
, node
, hash
, b1
, b2
));
772 /* Inserts 'node', with the given 'hash', into 'cmap'. The caller must ensure
773 * that 'cmap' cannot change concurrently (from another thread). If duplicates
774 * are undesirable, the caller must have already verified that 'cmap' does not
775 * contain a duplicate of 'node'.
777 * Returns the current number of nodes in the cmap after the insertion. */
779 cmap_insert(struct cmap
*cmap
, struct cmap_node
*node
, uint32_t hash
)
781 struct cmap_impl
*impl
= cmap_get_impl(cmap
);
783 ovsrcu_set_hidden(&node
->next
, NULL
);
785 if (OVS_UNLIKELY(impl
->n
>= impl
->max_n
)) {
786 COVERAGE_INC(cmap_expand
);
787 impl
= cmap_rehash(cmap
, (impl
->mask
<< 1) | 1);
790 while (OVS_UNLIKELY(!cmap_try_insert(impl
, node
, hash
))) {
791 impl
= cmap_rehash(cmap
, impl
->mask
);
797 cmap_replace__(struct cmap_impl
*impl
, struct cmap_node
*node
,
798 struct cmap_node
*replacement
, uint32_t hash
, uint32_t h
)
800 struct cmap_bucket
*b
= &impl
->buckets
[h
& impl
->mask
];
803 slot
= cmap_find_slot_protected(b
, hash
);
808 /* The pointer to 'node' is changed to point to 'replacement',
809 * which is the next node if no replacement node is given. */
811 replacement
= cmap_node_next_protected(node
);
813 /* 'replacement' takes the position of 'node' in the list. */
814 ovsrcu_set_hidden(&replacement
->next
, cmap_node_next_protected(node
));
817 struct cmap_node
*iter
= &b
->nodes
[slot
];
819 struct cmap_node
*next
= cmap_node_next_protected(iter
);
822 ovsrcu_set(&iter
->next
, replacement
);
829 /* Replaces 'old_node' in 'cmap' with 'new_node'. The caller must
830 * ensure that 'cmap' cannot change concurrently (from another thread).
832 * 'old_node' must not be destroyed or modified or inserted back into 'cmap' or
833 * into any other concurrent hash map while any other thread might be accessing
834 * it. One correct way to do this is to free it from an RCU callback with
837 * Returns the current number of nodes in the cmap after the replacement. The
838 * number of nodes decreases by one if 'new_node' is NULL. */
840 cmap_replace(struct cmap
*cmap
, struct cmap_node
*old_node
,
841 struct cmap_node
*new_node
, uint32_t hash
)
843 struct cmap_impl
*impl
= cmap_get_impl(cmap
);
844 uint32_t h1
= rehash(impl
, hash
);
845 uint32_t h2
= other_hash(h1
);
847 ovs_assert(cmap_replace__(impl
, old_node
, new_node
, hash
, h1
) ||
848 cmap_replace__(impl
, old_node
, new_node
, hash
, h2
));
852 if (OVS_UNLIKELY(impl
->n
< impl
->min_n
)) {
853 COVERAGE_INC(cmap_shrink
);
854 impl
= cmap_rehash(cmap
, impl
->mask
>> 1);
861 cmap_try_rehash(const struct cmap_impl
*old
, struct cmap_impl
*new)
863 const struct cmap_bucket
*b
;
865 for (b
= old
->buckets
; b
<= &old
->buckets
[old
->mask
]; b
++) {
868 for (i
= 0; i
< CMAP_K
; i
++) {
869 /* possible optimization here because we know the hashes are
871 struct cmap_node
*node
= cmap_node_next_protected(&b
->nodes
[i
]);
873 if (node
&& !cmap_try_insert(new, node
, b
->hashes
[i
])) {
881 static struct cmap_impl
*
882 cmap_rehash(struct cmap
*cmap
, uint32_t mask
)
884 struct cmap_impl
*old
= cmap_get_impl(cmap
);
885 struct cmap_impl
*new;
887 new = cmap_impl_create(mask
);
888 ovs_assert(old
->n
< new->max_n
);
890 while (!cmap_try_rehash(old
, new)) {
891 memset(new->buckets
, 0, (mask
+ 1) * sizeof *new->buckets
);
892 new->basis
= random_uint32();
896 ovsrcu_set(&cmap
->impl
, new);
897 if (old
!= &empty_cmap
) {
898 ovsrcu_postpone(free_cacheline
, old
);
905 cmap_cursor_start(const struct cmap
*cmap
)
907 struct cmap_cursor cursor
;
909 cursor
.impl
= cmap_get_impl(cmap
);
910 cursor
.bucket_idx
= 0;
911 cursor
.entry_idx
= 0;
913 cmap_cursor_advance(&cursor
);
919 cmap_cursor_advance(struct cmap_cursor
*cursor
)
921 const struct cmap_impl
*impl
= cursor
->impl
;
924 cursor
->node
= cmap_node_next(cursor
->node
);
930 while (cursor
->bucket_idx
<= impl
->mask
) {
931 const struct cmap_bucket
*b
= &impl
->buckets
[cursor
->bucket_idx
];
933 while (cursor
->entry_idx
< CMAP_K
) {
934 cursor
->node
= cmap_node_next(&b
->nodes
[cursor
->entry_idx
++]);
940 cursor
->bucket_idx
++;
941 cursor
->entry_idx
= 0;
945 /* Returns the next node in 'cmap' in hash order, or NULL if no nodes remain in
946 * 'cmap'. Uses '*pos' to determine where to begin iteration, and updates
947 * '*pos' to pass on the next iteration into them before returning.
949 * It's better to use plain CMAP_FOR_EACH and related functions, since they are
950 * faster and better at dealing with cmaps that change during iteration.
952 * Before beginning iteration, set '*pos' to all zeros. */
954 cmap_next_position(const struct cmap
*cmap
,
955 struct cmap_position
*pos
)
957 struct cmap_impl
*impl
= cmap_get_impl(cmap
);
958 unsigned int bucket
= pos
->bucket
;
959 unsigned int entry
= pos
->entry
;
960 unsigned int offset
= pos
->offset
;
962 while (bucket
<= impl
->mask
) {
963 const struct cmap_bucket
*b
= &impl
->buckets
[bucket
];
965 while (entry
< CMAP_K
) {
966 const struct cmap_node
*node
= cmap_node_next(&b
->nodes
[entry
]);
969 for (i
= 0; node
; i
++, node
= cmap_node_next(node
)) {
971 if (cmap_node_next(node
)) {
977 pos
->bucket
= bucket
;
979 pos
->offset
= offset
;
980 return CONST_CAST(struct cmap_node
*, node
);
992 pos
->bucket
= pos
->entry
= pos
->offset
= 0;