/*
* Copyright (c) Meta Platforms, Inc. and affiliates.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
#pragma once
#include <atomic>
#include <cstdint>
#include <thread>
#include <folly/PackedSyncPtr.h>
#include <folly/concurrency/detail/AtomicSharedPtr-detail.h>
#include <folly/memory/SanitizeLeak.h>
#include <folly/synchronization/AtomicStruct.h>
#include <folly/synchronization/detail/AtomicUtils.h>
/*
* This is an implementation of the std::atomic_shared_ptr TS
* http://en.cppreference.com/w/cpp/experimental/atomic_shared_ptr
* https://isocpp.org/files/papers/N4162.pdf
*
* AFAIK, the only other implementation is Anthony Williams from
* Just::thread library:
*
* https://github.com/anthonywilliams/atomic_shared_ptr
*
* implementation details:
*
* Basically, three things need to be atomically exchanged to make this work:
* * the local count
* * the pointer to the control block
* * the aliased pointer, if any.
*
* The Williams version does it with DWcas: 32 bits for local count, 64
* bits for control block ptr, and he changes the shared_ptr
* implementation to also store the aliased pointers using a linked list
* like structure, and provides 32-bit index accessors to them (like
* IndexedMemPool trick).
*
* This version instead stores the 48 bits of address, plus 16 bits of
* local count in a single 8byte pointer. This avoids 'lock cmpxchg16b',
* which is much slower than 'lock xchg' in the normal 'store' case. In
* the less-common aliased pointer scenario, we just allocate it in a new
* block, and store a pointer to that instead.
*
* Note that even if we only want to use the 3-bits of pointer alignment,
* this trick should still work - Any more than 4 concurrent accesses
* will have to go to an external map count instead (slower, but lots of
* concurrent access will be slow anyway due to bouncing cachelines).
*
* As a perf optimization, we currently batch up local count and only
* move it global every once in a while. This means load() is usually
* only a single atomic operation, instead of 3. For this trick to work,
* we probably need at least 8 bits to make batching worth it.
*/
// A note on noexcept: If the pointer is an aliased pointer,
// store() will allocate. Otherwise is noexcept.
namespace folly {
template <
typename T,
template <typename> class Atom = std::atomic,
typename CountedDetail = detail::shared_ptr_internals>
class atomic_shared_ptr {
using SharedPtr = typename CountedDetail::template CountedPtr<T>;
using BasePtr = typename CountedDetail::counted_base;
using PackedPtr = folly::PackedSyncPtr<BasePtr>;
public:
atomic_shared_ptr() noexcept { init(); }
explicit atomic_shared_ptr(SharedPtr foo) /* noexcept */
: atomic_shared_ptr() {
store(std::move(foo));
}
atomic_shared_ptr(const atomic_shared_ptr<T>&) = delete;
~atomic_shared_ptr() { store(SharedPtr(nullptr)); }
void operator=(SharedPtr desired) /* noexcept */ {
store(std::move(desired));
}
void operator=(const atomic_shared_ptr<T>&) = delete;
bool is_lock_free() const noexcept {
// lock free unless more than EXTERNAL_OFFSET threads are
// contending and they all get unlucky and scheduled out during
// load().
//
// TODO: Could use a lock-free external map to fix this
// corner case.
return true;
}
SharedPtr load(
std::memory_order order = std::memory_order_seq_cst) const noexcept {
auto local = takeOwnedBase(order);
return get_shared_ptr(local, false);
}
/* implicit */ operator SharedPtr() const { return load(); }
void store(
SharedPtr n,
std::memory_order order = std::memory_order_seq_cst) /* noexcept */ {
auto newptr = get_newptr(std::move(n));
auto old = ptr_.exchange(newptr, order);
release_external(old);
}
SharedPtr exchange(
SharedPtr n,
std::memory_order order = std::memory_order_seq_cst) /* noexcept */ {
auto newptr = get_newptr(std::move(n));
auto old = ptr_.exchange(newptr, order);
SharedPtr old_ptr;
if (old.get()) {
old_ptr = get_shared_ptr(old);
release_external(old);
}
return old_ptr;
}
bool compare_exchange_weak(
SharedPtr& expected,
const SharedPtr& n,
std::memory_order mo = std::memory_order_seq_cst) noexcept {
return compare_exchange_weak(
expected, n, mo, detail::default_failure_memory_order(mo));
}
bool compare_exchange_weak(
SharedPtr& expected,
const SharedPtr& n,
std::memory_order success,
std::memory_order failure) /* noexcept */ {
auto newptr = get_newptr(n);
PackedPtr oldptr, expectedptr;
oldptr = takeOwnedBase(success);
if (!owners_eq(oldptr, CountedDetail::get_counted_base(expected))) {
expected = get_shared_ptr(oldptr, false);
release_external(newptr);
return false;
}
expectedptr = oldptr; // Need oldptr to release if failed
if (ptr_.compare_exchange_weak(expectedptr, newptr, success, failure)) {
if (oldptr.get()) {
release_external(oldptr, -1);
}
return true;
} else {
if (oldptr.get()) {
expected = get_shared_ptr(oldptr, false);
} else {
expected = SharedPtr(nullptr);
}
release_external(newptr);
return false;
}
}
bool compare_exchange_weak(
SharedPtr& expected,
SharedPtr&& desired,
std::memory_order mo = std::memory_order_seq_cst) noexcept {
return compare_exchange_weak(
expected, desired, mo, detail::default_failure_memory_order(mo));
}
bool compare_exchange_weak(
SharedPtr& expected,
SharedPtr&& desired,
std::memory_order success,
std::memory_order failure) /* noexcept */ {
return compare_exchange_weak(expected, desired, success, failure);
}
bool compare_exchange_strong(
SharedPtr& expected,
const SharedPtr& n,
std::memory_order mo = std::memory_order_seq_cst) noexcept {
return compare_exchange_strong(
expected, n, mo, detail::default_failure_memory_order(mo));
}
bool compare_exchange_strong(
SharedPtr& expected,
const SharedPtr& n,
std::memory_order success,
std::memory_order failure) /* noexcept */ {
auto local_expected = expected;
do {
if (compare_exchange_weak(expected, n, success, failure)) {
return true;
}
} while (local_expected == expected);
return false;
}
bool compare_exchange_strong(
SharedPtr& expected,
SharedPtr&& desired,
std::memory_order mo = std::memory_order_seq_cst) noexcept {
return compare_exchange_strong(
expected, desired, mo, detail::default_failure_memory_order(mo));
}
bool compare_exchange_strong(
SharedPtr& expected,
SharedPtr&& desired,
std::memory_order success,
std::memory_order failure) /* noexcept */ {
return compare_exchange_strong(expected, desired, success, failure);
}
private:
// Matches packed_sync_pointer. Must be > max number of local
// counts. This is the max number of threads that can access this
// atomic_shared_ptr at once before we start blocking.
static constexpr std::uint16_t EXTERNAL_OFFSET{0x2000};
// Bit signifying aliased constructor
static constexpr std::uint16_t ALIASED_PTR{0x4000};
static std::uint16_t get_local_count(const PackedPtr& p) {
return p.extra() & ~ALIASED_PTR;
}
static void add_external(BasePtr* res, int64_t c = 0) {
assert(res);
CountedDetail::inc_shared_count(res, EXTERNAL_OFFSET + c);
annotate_object_leaked(res);
}
static void release_external(PackedPtr& res, int64_t c = 0) {
if (!res.get()) {
return;
}
annotate_object_collected(res.get());
int64_t count = get_local_count(res) + c;
int64_t diff = EXTERNAL_OFFSET - count;
assert(diff >= 0);
CountedDetail::template release_shared<T>(res.get(), diff);
}
static PackedPtr get_newptr(const SharedPtr& n) {
return get_newptr_impl<false>(n);
}
static PackedPtr get_newptr(SharedPtr&& n) {
return get_newptr_impl<true>(std::move(n));
}
template <bool kOwn, class S>
static PackedPtr get_newptr_impl(S&& n) {
std::uint16_t count = 0;
BasePtr* newval = CountedDetail::get_counted_base(n);
if (!n && newval == nullptr) {
// n is default-constructed, nothing to do.
} else if (
newval == nullptr ||
n.get() != CountedDetail::template get_shared_ptr<T>(newval)) {
// This is an aliased sharedptr. Make an un-aliased one by wrapping in
// *another* shared_ptr.
auto data =
CountedDetail::template make_ptr<SharedPtr>(std::forward<S>(n));
newval = CountedDetail::get_counted_base(data);
count = ALIASED_PTR;
CountedDetail::release_ptr(data);
add_external(newval, -1);
} else {
if constexpr (kOwn) {
CountedDetail::release_ptr(n);
}
add_external(newval, kOwn ? -1 : 0);
}
PackedPtr newptr;
newptr.init(newval, count);
return newptr;
}
void init() {
PackedPtr data;
data.init();
ptr_.store(data);
}
// Check pointer equality considering wrapped aliased pointers.
bool owners_eq(PackedPtr& p1, BasePtr* p2) {
bool aliased1 = p1.extra() & ALIASED_PTR;
if (aliased1) {
auto p1a = CountedDetail::template get_shared_ptr_from_counted_base<T>(
p1.get(), false);
return CountedDetail::get_counted_base(p1a) == p2;
}
return p1.get() == p2;
}
SharedPtr get_shared_ptr(const PackedPtr& p, bool inc = true) const {
bool aliased = p.extra() & ALIASED_PTR;
auto res = CountedDetail::template get_shared_ptr_from_counted_base<T>(
p.get(), inc);
if (aliased) {
auto aliasedp =
CountedDetail::template get_shared_ptr_from_counted_base<SharedPtr>(
p.get());
res = *aliasedp;
}
return res;
}
/* Get a reference to the pointer, either from the local batch or
* from the global count.
*
* return is the base ptr, and the previous local count, if it is
* needed for compare_and_swap later.
*/
PackedPtr takeOwnedBase(std::memory_order order) const noexcept {
PackedPtr local, newlocal;
local = ptr_.load(std::memory_order_acquire);
while (true) {
if (!local.get()) {
return local;
}
newlocal = local;
if (get_local_count(newlocal) + 1 > EXTERNAL_OFFSET) {
// spinlock in the rare case we have more than
// EXTERNAL_OFFSET threads trying to access at once.
std::this_thread::yield();
// Force DeterministicSchedule to choose a different thread
local = ptr_.load(std::memory_order_acquire);
} else {
newlocal.setExtra(newlocal.extra() + 1);
assert(get_local_count(newlocal) > 0);
if (ptr_.compare_exchange_weak(local, newlocal, order)) {
break;
}
}
}
// Check if we need to push a batch from local -> global
std::uint16_t batchcount = EXTERNAL_OFFSET / 2;
if (get_local_count(newlocal) > batchcount) {
CountedDetail::inc_shared_count(newlocal.get(), batchcount);
putOwnedBase(newlocal.get(), batchcount, order);
}
return newlocal;
}
void putOwnedBase(
BasePtr* p, std::uint16_t count, std::memory_order mo) const noexcept {
PackedPtr local = ptr_.load(std::memory_order_acquire);
while (true) {
if (local.get() != p) {
break;
}
auto newlocal = local;
if (get_local_count(local) > count) {
newlocal.setExtra(local.extra() - count);
} else {
// Otherwise it may be the same pointer, but someone else won
// the compare_exchange below, local count was already made
// global. We decrement the global count directly instead of
// the local one.
break;
}
if (ptr_.compare_exchange_weak(local, newlocal, mo)) {
return;
}
}
CountedDetail::template release_shared<T>(p, count);
}
mutable AtomicStruct<PackedPtr, Atom> ptr_;
};
} // namespace folly
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