123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960616263646566676869707172737475767778798081828384858687888990919293949596979899100101102103104105106107108109110111112113114115116117118119120121122123124125126127128129130131132133134135136137 |
- // Copyright 2018 The Chromium Authors. All rights reserved.
- // Use of this source code is governed by a BSD-style license that can be
- // found in the LICENSE file.
- #ifndef BASE_SAMPLING_HEAP_PROFILER_LOCK_FREE_ADDRESS_HASH_SET_H_
- #define BASE_SAMPLING_HEAP_PROFILER_LOCK_FREE_ADDRESS_HASH_SET_H_
- #include <atomic>
- #include <cstdint>
- #include <vector>
- #include "base/base_export.h"
- #include "base/check_op.h"
- #include "base/compiler_specific.h"
- namespace base {
- // A hash set container that provides lock-free version of |Contains| operation.
- // It does not support concurrent write operations |Insert| and |Remove|.
- // All write operations if performed from multiple threads must be properly
- // guarded with a lock.
- // |Contains| method can be executed concurrently with other |Insert|, |Remove|,
- // or |Contains| even over the same key.
- // However, please note the result of concurrent execution of |Contains|
- // with |Insert| or |Remove| over the same key is racy.
- //
- // The hash set never rehashes, so the number of buckets stays the same
- // for the lifetime of the set.
- //
- // Internally the hashset is implemented as a vector of N buckets
- // (N has to be a power of 2). Each bucket holds a single-linked list of
- // nodes each corresponding to a key.
- // It is not possible to really delete nodes from the list as there might
- // be concurrent reads being executed over the node. The |Remove| operation
- // just marks the node as empty by placing nullptr into its key field.
- // Consequent |Insert| operations may reuse empty nodes when possible.
- //
- // The structure of the hashset for N buckets is the following:
- // 0: {*}--> {key1,*}--> {key2,*}--> NULL
- // 1: {*}--> NULL
- // 2: {*}--> {NULL,*}--> {key3,*}--> {key4,*}--> NULL
- // ...
- // N-1: {*}--> {keyM,*}--> NULL
- class BASE_EXPORT LockFreeAddressHashSet {
- public:
- explicit LockFreeAddressHashSet(size_t buckets_count);
- ~LockFreeAddressHashSet();
- // Checks if the |key| is in the set. Can be executed concurrently with
- // |Insert|, |Remove|, and |Contains| operations.
- ALWAYS_INLINE bool Contains(void* key) const;
- // Removes the |key| from the set. The key must be present in the set before
- // the invocation.
- // Concurrent execution of |Insert|, |Remove|, or |Copy| is not supported.
- ALWAYS_INLINE void Remove(void* key);
- // Inserts the |key| into the set. The key must not be present in the set
- // before the invocation.
- // Concurrent execution of |Insert|, |Remove|, or |Copy| is not supported.
- void Insert(void* key);
- // Copies contents of |other| set into the current set. The current set
- // must be empty before the call.
- // Concurrent execution of |Insert|, |Remove|, or |Copy| is not supported.
- void Copy(const LockFreeAddressHashSet& other);
- size_t buckets_count() const { return buckets_.size(); }
- size_t size() const { return size_; }
- // Returns the average bucket utilization.
- float load_factor() const { return 1.f * size() / buckets_.size(); }
- private:
- friend class LockFreeAddressHashSetTest;
- struct Node {
- ALWAYS_INLINE Node(void* key, Node* next);
- std::atomic<void*> key;
- Node* next;
- };
- ALWAYS_INLINE static uint32_t Hash(void* key);
- ALWAYS_INLINE Node* FindNode(void* key) const;
- std::vector<std::atomic<Node*>> buckets_;
- size_t size_ = 0;
- const size_t bucket_mask_;
- };
- ALWAYS_INLINE LockFreeAddressHashSet::Node::Node(void* key, Node* next)
- : next(next) {
- this->key.store(key, std::memory_order_relaxed);
- }
- ALWAYS_INLINE bool LockFreeAddressHashSet::Contains(void* key) const {
- return FindNode(key) != nullptr;
- }
- ALWAYS_INLINE void LockFreeAddressHashSet::Remove(void* key) {
- Node* node = FindNode(key);
- DCHECK_NE(node, nullptr);
- // We can never delete the node, nor detach it from the current bucket
- // as there may always be another thread currently iterating over it.
- // Instead we just mark it as empty, so |Insert| can reuse it later.
- node->key.store(nullptr, std::memory_order_relaxed);
- --size_;
- }
- ALWAYS_INLINE LockFreeAddressHashSet::Node* LockFreeAddressHashSet::FindNode(
- void* key) const {
- DCHECK_NE(key, nullptr);
- const std::atomic<Node*>& bucket = buckets_[Hash(key) & bucket_mask_];
- // It's enough to use std::memory_order_consume ordering here, as the
- // node->next->...->next loads form dependency chain.
- // However std::memory_order_consume is temporary deprecated in C++17.
- // See https://isocpp.org/files/papers/p0636r0.html#removed
- // Make use of more strong std::memory_order_acquire for now.
- for (Node* node = bucket.load(std::memory_order_acquire); node != nullptr;
- node = node->next) {
- if (node->key.load(std::memory_order_relaxed) == key)
- return node;
- }
- return nullptr;
- }
- // static
- ALWAYS_INLINE uint32_t LockFreeAddressHashSet::Hash(void* key) {
- // A simple fast hash function for addresses.
- constexpr uintptr_t random_bits = static_cast<uintptr_t>(0x4bfdb9df5a6f243b);
- uint64_t k = reinterpret_cast<uintptr_t>(key);
- return static_cast<uint32_t>((k * random_bits) >> 32);
- }
- } // namespace base
- #endif // BASE_SAMPLING_HEAP_PROFILER_LOCK_FREE_ADDRESS_HASH_SET_H_
|