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// -*- mode:C++; tab-width:8; c-basic-offset:2; indent-tabs-mode:t -*-
// vim: ts=8 sw=2 smarttab
#pragma once
#include <atomic>
// What and why
// ============
//
// ceph::atomic – thin wrapper to differentiate behavior of atomics.
//
// Not all users of the common truly need costly atomic operations to
// synchronize data between CPUs and threads. Some, like crimson-osd,
// stick to shared-nothing approach. Enforcing issue of atomics in
// such cases is wasteful – on x86 any locked instruction works actually
// like a full memory barrier stalling execution till CPU's store and
// load buffers are drained.
#if defined(WITH_SEASTAR) && !defined(WITH_BLUESTORE)
#include <type_traits>
namespace ceph {
template <class T>
class dummy_atomic {
T value;
public:
dummy_atomic() = default;
dummy_atomic(const dummy_atomic&) = delete;
dummy_atomic(T value) : value(std::move(value)) {
}
bool is_lock_free() const noexcept {
return true;
}
void store(T desired, std::memory_order) noexcept {
value = std::move(desired);
}
T load(std::memory_order = std::memory_order_seq_cst) const noexcept {
return value;
}
T operator=(T desired) noexcept {
value = std::move(desired);
}
operator T() const noexcept {
return value;
}
// We need to differentiate with SFINAE as std::atomic offers beefier
// interface for integral types.
std::enable_if_t<std::is_integral_v<T>, T> operator++() {
return ++value;
}
std::enable_if_t<std::is_integral_v<T>, T> operator++(int) {
return value++;
}
std::enable_if_t<std::is_integral_v<T>, T> operator--() {
return --value;
}
std::enable_if_t<std::is_integral_v<T>, T> operator--(int) {
return value--;
}
std::enable_if_t<std::is_integral_v<T>, T> operator+=(const dummy_atomic& b) {
value += b;
return value;
}
std::enable_if_t<std::is_integral_v<T>, T> operator-=(const dummy_atomic& b) {
value -= b;
return value;
}
static constexpr bool is_always_lock_free = true;
};
template <class T> using atomic = dummy_atomic<T>;
} // namespace ceph
#else // WITH_SEASTAR
namespace ceph {
template <class T> using atomic = ::std::atomic<T>;
} // namespace ceph
#endif // WITH_SEASTAR
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