1303567fa4
- Add write lock to add() method to prevent concurrent modifications - Remove insufficient locked() check in thread_for_each_nested() - Add vector snapshots in all iteration methods to prevent races - Update class documentation to reflect atomic operations Co-authored-by: mykaul <4655593+mykaul@users.noreply.github.com>
101 lines
3.9 KiB
C++
101 lines
3.9 KiB
C++
/*
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* Copyright (C) 2020-present ScyllaDB
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*/
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/*
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* SPDX-License-Identifier: LicenseRef-ScyllaDB-Source-Available-1.0
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*/
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#pragma once
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#include "utils/on_internal_error.hh"
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#include <seastar/core/rwlock.hh>
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#include <seastar/util/defer.hh>
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#include <seastar/util/noncopyable_function.hh>
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#include <vector>
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// This class supports atomic inserts, removes, and iteration.
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// All operations are synchronized using a read-write lock.
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template <typename T>
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class atomic_vector {
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std::vector<T> _vec;
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mutable seastar::rwlock _vec_lock;
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public:
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void add(const T& value) {
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auto lock = _vec_lock.for_write().lock().get();
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auto unlock = seastar::defer([this] {
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_vec_lock.for_write().unlock();
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});
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_vec.push_back(value);
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}
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seastar::future<> remove(const T& value) {
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return with_lock(_vec_lock.for_write(), [this, value] {
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_vec.erase(std::remove(_vec.begin(), _vec.end(), value), _vec.end());
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});
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}
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// This must be called on a thread. The callback function must not
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// call remove or thread_for_each. If the callback function needs to
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// call this, use thread_for_each_nested instead.
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//
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// We would take callbacks that take a T&, but we had bugs in the
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// past with some of those callbacks holding that reference past a
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// preemption.
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void thread_for_each(seastar::noncopyable_function<void(T)> func) const {
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_vec_lock.for_read().lock().get();
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auto unlock = seastar::defer([this] {
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_vec_lock.for_read().unlock();
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});
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// Take a snapshot of the current contents while holding the read lock,
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// so that concurrent add() calls and possible reallocations won't
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// affect our iteration.
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auto snapshot = _vec;
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// We grab locks in both add() and remove(), so we iterate using
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// indexes on the snapshot to avoid concurrent modifications.
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for (size_t i = 0, n = snapshot.size(); i < n; ++i) {
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func(snapshot[i]);
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}
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}
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// This must be called on a thread. This should be used only from
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// the context of a thread_for_each callback, when the read lock is
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// already held. The callback function must not call remove or
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// thread_for_each.
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void thread_for_each_nested(seastar::noncopyable_function<void(T)> func) const {
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// When called in the context of thread_for_each, the read lock is
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// already held, so we don't need to acquire it again. Acquiring it
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// again could lead to a deadlock. This function must only be called
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// while holding the read lock on _vec_lock.
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// Take a snapshot of the current contents while the read lock is held,
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// so that concurrent add() calls and possible reallocations won't
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// affect our iteration.
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auto snapshot = _vec;
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// We grab locks in both add() and remove(), so we iterate using
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// indexes on the snapshot to avoid concurrent modifications.
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for (size_t i = 0, n = snapshot.size(); i < n; ++i) {
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func(snapshot[i]);
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}
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}
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// The callback function must not call remove.
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//
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// We would take callbacks that take a T&, but we had bugs in the
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// past with some of those callbacks holding that reference past a
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// preemption.
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seastar::future<> for_each(seastar::noncopyable_function<seastar::future<>(T)> func) const {
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auto holder = co_await _vec_lock.hold_read_lock();
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// Take a snapshot of the current contents while holding the read lock,
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// so that concurrent add() calls and possible reallocations won't
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// affect our iteration.
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auto snapshot = _vec;
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// We grab locks in both add() and remove(), so we iterate using
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// indexes on the snapshot to avoid concurrent modifications.
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for (size_t i = 0, n = snapshot.size(); i < n; ++i) {
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co_await func(snapshot[i]);
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}
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}
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};
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