2 * Copyright 2016 Facebook, 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.
18 * This module implements a Synchronized abstraction useful in
19 * mutex-based concurrency.
21 * The Synchronized<T, Mutex> class is the primary public API exposed by this
22 * module. See folly/docs/Synchronized.md for a more complete explanation of
23 * this class and its benefits.
28 #include <folly/Likely.h>
29 #include <folly/LockTraits.h>
30 #include <folly/Preprocessor.h>
31 #include <folly/SharedMutex.h>
32 #include <folly/Traits.h>
33 #include <glog/logging.h>
35 #include <type_traits>
39 template <class LockedType, class Mutex, class LockPolicy>
41 template <class LockedType, class LockPolicy>
43 template <class LockedType, class LockPolicy = LockPolicyExclusive>
47 * Public version of LockInterfaceDispatcher that contains the MutexLevel enum
48 * for the passed in mutex type
50 * This is decoupled from MutexLevelValueImpl in LockTraits.h because this
51 * ensures that a heterogenous mutex with a different API can be used. For
52 * example - if a mutex does not have a lock_shared() method but the
53 * LockTraits specialization for it supports a static non member
54 * lock_shared(Mutex&) it can be used as a shared mutex and will provide
55 * rlock() and wlock() functions.
57 template <class Mutex>
58 using MutexLevelValue = detail::MutexLevelValueImpl<
60 LockTraits<Mutex>::is_shared,
61 LockTraits<Mutex>::is_upgrade>;
64 * SynchronizedBase is a helper parent class for Synchronized<T>.
66 * It provides wlock() and rlock() methods for shared mutex types,
67 * or lock() methods for purely exclusive mutex types.
69 template <class Subclass, detail::MutexLevel level>
70 class SynchronizedBase;
73 * SynchronizedBase specialization for shared mutex types.
75 * This class provides wlock() and rlock() methods for acquiring the lock and
78 template <class Subclass>
79 class SynchronizedBase<Subclass, detail::MutexLevel::SHARED> {
81 using LockedPtr = ::folly::LockedPtr<Subclass, LockPolicyExclusive>;
82 using ConstWLockedPtr =
83 ::folly::LockedPtr<const Subclass, LockPolicyExclusive>;
84 using ConstLockedPtr = ::folly::LockedPtr<const Subclass, LockPolicyShared>;
87 * Acquire an exclusive lock, and return a LockedPtr that can be used to
88 * safely access the datum.
90 * LockedPtr offers operator -> and * to provide access to the datum.
91 * The lock will be released when the LockedPtr is destroyed.
94 return LockedPtr(static_cast<Subclass*>(this));
96 ConstWLockedPtr wlock() const {
97 return ConstWLockedPtr(static_cast<const Subclass*>(this));
101 * Acquire a read lock, and return a ConstLockedPtr that can be used to
102 * safely access the datum.
104 ConstLockedPtr rlock() const {
105 return ConstLockedPtr(static_cast<const Subclass*>(this));
109 * Attempts to acquire the lock, or fails if the timeout elapses first.
110 * If acquisition is unsuccessful, the returned LockedPtr will be null.
112 * (Use LockedPtr::isNull() to check for validity.)
114 template <class Rep, class Period>
115 LockedPtr wlock(const std::chrono::duration<Rep, Period>& timeout) {
116 return LockedPtr(static_cast<Subclass*>(this), timeout);
118 template <class Rep, class Period>
119 ConstWLockedPtr wlock(
120 const std::chrono::duration<Rep, Period>& timeout) const {
121 return ConstWLockedPtr(static_cast<const Subclass*>(this), timeout);
125 * Attempts to acquire the lock, or fails if the timeout elapses first.
126 * If acquisition is unsuccessful, the returned LockedPtr will be null.
128 * (Use LockedPtr::isNull() to check for validity.)
130 template <class Rep, class Period>
131 ConstLockedPtr rlock(
132 const std::chrono::duration<Rep, Period>& timeout) const {
133 return ConstLockedPtr(static_cast<const Subclass*>(this), timeout);
137 * Note: C++ 17 adds guaranteed copy elision. (http://wg21.link/P0135)
138 * Once compilers support this, it would be nice to add wguard() and rguard()
139 * methods that return LockedGuardPtr objects.
143 * Invoke a function while holding the lock exclusively.
145 * A reference to the datum will be passed into the function as its only
148 * This can be used with a lambda argument for easily defining small critical
149 * sections in the code. For example:
151 * auto value = obj.withWLock([](auto& data) {
153 * return data.getValue();
156 template <class Function>
157 auto withWLock(Function&& function) {
158 LockedGuardPtr<Subclass, LockPolicyExclusive> guardPtr(
159 static_cast<Subclass*>(this));
160 return function(*guardPtr);
162 template <class Function>
163 auto withWLock(Function&& function) const {
164 LockedGuardPtr<const Subclass, LockPolicyExclusive> guardPtr(
165 static_cast<const Subclass*>(this));
166 return function(*guardPtr);
170 * Invoke a function while holding the lock exclusively.
172 * This is similar to withWLock(), but the function will be passed a
173 * LockedPtr rather than a reference to the data itself.
175 * This allows scopedUnlock() to be called on the LockedPtr argument if
178 template <class Function>
179 auto withWLockPtr(Function&& function) {
180 return function(wlock());
182 template <class Function>
183 auto withWLockPtr(Function&& function) const {
184 return function(wlock());
188 * Invoke a function while holding an the lock in shared mode.
190 * A const reference to the datum will be passed into the function as its
193 template <class Function>
194 auto withRLock(Function&& function) const {
195 LockedGuardPtr<const Subclass, LockPolicyShared> guardPtr(
196 static_cast<const Subclass*>(this));
197 return function(*guardPtr);
200 template <class Function>
201 auto withRLockPtr(Function&& function) const {
202 return function(rlock());
207 * SynchronizedBase specialization for upgrade mutex types.
209 * This class provides all the functionality provided by the SynchronizedBase
210 * specialization for shared mutexes and a ulock() method that returns an
211 * upgradable lock RAII proxy
213 template <class Subclass>
214 class SynchronizedBase<Subclass, detail::MutexLevel::UPGRADE>
215 : public SynchronizedBase<Subclass, detail::MutexLevel::SHARED> {
217 using UpgradeLockedPtr = ::folly::LockedPtr<Subclass, LockPolicyUpgrade>;
218 using ConstUpgradeLockedPtr =
219 ::folly::LockedPtr<const Subclass, LockPolicyUpgrade>;
220 using UpgradeLockedGuardPtr =
221 ::folly::LockedGuardPtr<Subclass, LockPolicyUpgrade>;
222 using ConstUpgradeLockedGuardPtr =
223 ::folly::LockedGuardPtr<const Subclass, LockPolicyUpgrade>;
226 * Acquire an upgrade lock and return a LockedPtr that can be used to safely
229 * And the const version
231 UpgradeLockedPtr ulock() {
232 return UpgradeLockedPtr(static_cast<Subclass*>(this));
234 ConstUpgradeLockedPtr ulock() const {
235 return ConstUpgradeLockedPtr(static_cast<const Subclass*>(this));
239 * Acquire an upgrade lock and return a LockedPtr that can be used to safely
242 * And the const version
244 template <class Rep, class Period>
245 UpgradeLockedPtr ulock(const std::chrono::duration<Rep, Period>& timeout) {
246 return UpgradeLockedPtr(static_cast<Subclass*>(this), timeout);
248 template <class Rep, class Period>
249 UpgradeLockedPtr ulock(
250 const std::chrono::duration<Rep, Period>& timeout) const {
251 return ConstUpgradeLockedPtr(static_cast<const Subclass*>(this), timeout);
255 * Invoke a function while holding the lock.
257 * A reference to the datum will be passed into the function as its only
260 * This can be used with a lambda argument for easily defining small critical
261 * sections in the code. For example:
263 * auto value = obj.withULock([](auto& data) {
265 * return data.getValue();
268 * This is probably not the function you want. If the intent is to read the
269 * data object and determine whether you should upgrade to a write lock then
270 * the withULockPtr() method should be called instead, since it gives access
271 * to the LockedPtr proxy (which can be upgraded via the
272 * moveFromUpgradeToWrite() method)
274 template <class Function>
275 auto withULock(Function&& function) const {
276 ConstUpgradeLockedGuardPtr guardPtr(static_cast<const Subclass*>(this));
277 return function(*guardPtr);
281 * Invoke a function while holding the lock exclusively.
283 * This is similar to withULock(), but the function will be passed a
284 * LockedPtr rather than a reference to the data itself.
286 * This allows scopedUnlock() and getUniqueLock() to be called on the
287 * LockedPtr argument.
289 * This also allows you to upgrade the LockedPtr proxy to a write state so
290 * that changes can be made to the underlying data
292 template <class Function>
293 auto withULockPtr(Function&& function) {
294 return function(ulock());
296 template <class Function>
297 auto withULockPtr(Function&& function) const {
298 return function(ulock());
303 * SynchronizedBase specialization for non-shared mutex types.
305 * This class provides lock() methods for acquiring the lock and accessing the
308 template <class Subclass>
309 class SynchronizedBase<Subclass, detail::MutexLevel::UNIQUE> {
311 using LockedPtr = ::folly::LockedPtr<Subclass, LockPolicyExclusive>;
312 using ConstLockedPtr =
313 ::folly::LockedPtr<const Subclass, LockPolicyExclusive>;
316 * Acquire a lock, and return a LockedPtr that can be used to safely access
320 return LockedPtr(static_cast<Subclass*>(this));
324 * Acquire a lock, and return a ConstLockedPtr that can be used to safely
327 ConstLockedPtr lock() const {
328 return ConstLockedPtr(static_cast<const Subclass*>(this));
332 * Attempts to acquire the lock, or fails if the timeout elapses first.
333 * If acquisition is unsuccessful, the returned LockedPtr will be null.
335 template <class Rep, class Period>
336 LockedPtr lock(const std::chrono::duration<Rep, Period>& timeout) {
337 return LockedPtr(static_cast<Subclass*>(this), timeout);
341 * Attempts to acquire the lock, or fails if the timeout elapses first.
342 * If acquisition is unsuccessful, the returned LockedPtr will be null.
344 template <class Rep, class Period>
345 ConstLockedPtr lock(const std::chrono::duration<Rep, Period>& timeout) const {
346 return ConstLockedPtr(static_cast<const Subclass*>(this), timeout);
350 * Note: C++ 17 adds guaranteed copy elision. (http://wg21.link/P0135)
351 * Once compilers support this, it would be nice to add guard() methods that
352 * return LockedGuardPtr objects.
356 * Invoke a function while holding the lock.
358 * A reference to the datum will be passed into the function as its only
361 * This can be used with a lambda argument for easily defining small critical
362 * sections in the code. For example:
364 * auto value = obj.withLock([](auto& data) {
366 * return data.getValue();
369 template <class Function>
370 auto withLock(Function&& function) {
371 LockedGuardPtr<Subclass, LockPolicyExclusive> guardPtr(
372 static_cast<Subclass*>(this));
373 return function(*guardPtr);
375 template <class Function>
376 auto withLock(Function&& function) const {
377 LockedGuardPtr<const Subclass, LockPolicyExclusive> guardPtr(
378 static_cast<const Subclass*>(this));
379 return function(*guardPtr);
383 * Invoke a function while holding the lock exclusively.
385 * This is similar to withWLock(), but the function will be passed a
386 * LockedPtr rather than a reference to the data itself.
388 * This allows scopedUnlock() and getUniqueLock() to be called on the
389 * LockedPtr argument.
391 template <class Function>
392 auto withLockPtr(Function&& function) {
393 return function(lock());
395 template <class Function>
396 auto withLockPtr(Function&& function) const {
397 return function(lock());
402 * Synchronized<T> encapsulates an object of type T (a "datum") paired
403 * with a mutex. The only way to access the datum is while the mutex
404 * is locked, and Synchronized makes it virtually impossible to do
405 * otherwise. The code that would access the datum in unsafe ways
406 * would look odd and convoluted, thus readily alerting the human
407 * reviewer. In contrast, the code that uses Synchronized<T> correctly
408 * looks simple and intuitive.
410 * The second parameter must be a mutex type. Any mutex type supported by
411 * LockTraits<Mutex> can be used. By default any class with lock() and
412 * unlock() methods will work automatically. LockTraits can be specialized to
413 * teach Synchronized how to use other custom mutex types. See the
414 * documentation in LockTraits.h for additional details.
416 * Supported mutexes that work by default include std::mutex,
417 * std::recursive_mutex, std::timed_mutex, std::recursive_timed_mutex,
418 * folly::SharedMutex, folly::RWSpinLock, and folly::SpinLock.
419 * Include LockTraitsBoost.h to get additional LockTraits specializations to
420 * support the following boost mutex types: boost::mutex,
421 * boost::recursive_mutex, boost::shared_mutex, boost::timed_mutex, and
422 * boost::recursive_timed_mutex.
424 template <class T, class Mutex = SharedMutex>
425 struct Synchronized : public SynchronizedBase<
426 Synchronized<T, Mutex>,
427 MutexLevelValue<Mutex>::value> {
430 SynchronizedBase<Synchronized<T, Mutex>, MutexLevelValue<Mutex>::value>;
431 static constexpr bool nxCopyCtor{
432 std::is_nothrow_copy_constructible<T>::value};
433 static constexpr bool nxMoveCtor{
434 std::is_nothrow_move_constructible<T>::value};
437 using LockedPtr = typename Base::LockedPtr;
438 using ConstLockedPtr = typename Base::ConstLockedPtr;
440 using MutexType = Mutex;
443 * Default constructor leaves both members call their own default
446 Synchronized() = default;
449 * Copy constructor copies the data (with locking the source and
450 * all) but does NOT copy the mutex. Doing so would result in
453 * Note that the copy constructor may throw because it acquires a lock in
454 * the contextualRLock() method
456 Synchronized(const Synchronized& rhs) /* may throw */
457 : Synchronized(rhs, rhs.contextualRLock()) {}
460 * Move constructor moves the data (with locking the source and all)
461 * but does not move the mutex.
463 * Note that the move constructor may throw because it acquires a lock.
464 * Since the move constructor is not declared noexcept, when objects of this
465 * class are used as elements in a vector or a similar container. The
466 * elements might not be moved around when resizing. They might be copied
467 * instead. You have been warned.
469 Synchronized(Synchronized&& rhs) /* may throw */
470 : Synchronized(std::move(rhs), rhs.contextualLock()) {}
473 * Constructor taking a datum as argument copies it. There is no
474 * need to lock the constructing object.
476 explicit Synchronized(const T& rhs) noexcept(nxCopyCtor) : datum_(rhs) {}
479 * Constructor taking a datum rvalue as argument moves it. Again,
480 * there is no need to lock the constructing object.
482 explicit Synchronized(T&& rhs) noexcept(nxMoveCtor)
483 : datum_(std::move(rhs)) {}
486 * Lets you construct non-movable types in-place. Use the constexpr
487 * instance `construct_in_place` as the first argument.
489 template <typename... Args>
490 explicit Synchronized(construct_in_place_t, Args&&... args)
491 : datum_(std::forward<Args>(args)...) {}
494 * The canonical assignment operator only assigns the data, NOT the
495 * mutex. It locks the two objects in ascending order of their
498 Synchronized& operator=(const Synchronized& rhs) {
500 // Self-assignment, pass.
501 } else if (this < &rhs) {
502 auto guard1 = operator->();
503 auto guard2 = rhs.operator->();
506 auto guard1 = rhs.operator->();
507 auto guard2 = operator->();
514 * Move assignment operator, only assigns the data, NOT the
515 * mutex. It locks the two objects in ascending order of their
518 Synchronized& operator=(Synchronized&& rhs) {
520 // Self-assignment, pass.
521 } else if (this < &rhs) {
522 auto guard1 = operator->();
523 auto guard2 = rhs.operator->();
524 datum_ = std::move(rhs.datum_);
526 auto guard1 = rhs.operator->();
527 auto guard2 = operator->();
528 datum_ = std::move(rhs.datum_);
534 * Lock object, assign datum.
536 Synchronized& operator=(const T& rhs) {
537 auto guard = operator->();
543 * Lock object, move-assign datum.
545 Synchronized& operator=(T&& rhs) {
546 auto guard = operator->();
547 datum_ = std::move(rhs);
552 * Acquire an appropriate lock based on the context.
554 * If the mutex is a shared mutex, and the Synchronized instance is const,
555 * this acquires a shared lock. Otherwise this acquires an exclusive lock.
557 * In general, prefer using the explicit rlock() and wlock() methods
558 * for read-write locks, and lock() for purely exclusive locks.
560 * contextualLock() is primarily intended for use in other template functions
561 * that do not necessarily know the lock type.
563 LockedPtr contextualLock() {
564 return LockedPtr(this);
566 ConstLockedPtr contextualLock() const {
567 return ConstLockedPtr(this);
569 template <class Rep, class Period>
570 LockedPtr contextualLock(const std::chrono::duration<Rep, Period>& timeout) {
571 return LockedPtr(this, timeout);
573 template <class Rep, class Period>
574 ConstLockedPtr contextualLock(
575 const std::chrono::duration<Rep, Period>& timeout) const {
576 return ConstLockedPtr(this, timeout);
579 * contextualRLock() acquires a read lock if the mutex type is shared,
580 * or a regular exclusive lock for non-shared mutex types.
582 * contextualRLock() when you know that you prefer a read lock (if
583 * available), even if the Synchronized<T> object itself is non-const.
585 ConstLockedPtr contextualRLock() const {
586 return ConstLockedPtr(this);
588 template <class Rep, class Period>
589 ConstLockedPtr contextualRLock(
590 const std::chrono::duration<Rep, Period>& timeout) const {
591 return ConstLockedPtr(this, timeout);
595 * This accessor offers a LockedPtr. In turn, LockedPtr offers
596 * operator-> returning a pointer to T. The operator-> keeps
597 * expanding until it reaches a pointer, so syncobj->foo() will lock
598 * the object and call foo() against it.
600 * NOTE: This API is planned to be deprecated in an upcoming diff.
601 * Prefer using lock(), wlock(), or rlock() instead.
603 LockedPtr operator->() {
604 return LockedPtr(this);
608 * Obtain a ConstLockedPtr.
610 * NOTE: This API is planned to be deprecated in an upcoming diff.
611 * Prefer using lock(), wlock(), or rlock() instead.
613 ConstLockedPtr operator->() const {
614 return ConstLockedPtr(this);
618 * Attempts to acquire for a given number of milliseconds. If
619 * acquisition is unsuccessful, the returned LockedPtr is NULL.
621 * NOTE: This API is deprecated. Use lock(), wlock(), or rlock() instead.
622 * In the future it will be marked with a deprecation attribute to emit
623 * build-time warnings, and then it will be removed entirely.
625 LockedPtr timedAcquire(unsigned int milliseconds) {
626 return LockedPtr(this, std::chrono::milliseconds(milliseconds));
630 * Attempts to acquire for a given number of milliseconds. If
631 * acquisition is unsuccessful, the returned ConstLockedPtr is NULL.
633 * NOTE: This API is deprecated. Use lock(), wlock(), or rlock() instead.
634 * In the future it will be marked with a deprecation attribute to emit
635 * build-time warnings, and then it will be removed entirely.
637 ConstLockedPtr timedAcquire(unsigned int milliseconds) const {
638 return ConstLockedPtr(this, std::chrono::milliseconds(milliseconds));
642 * Sometimes, although you have a mutable object, you only want to
643 * call a const method against it. The most efficient way to achieve
644 * that is by using a read lock. You get to do so by using
645 * obj.asConst()->method() instead of obj->method().
647 * NOTE: This API is planned to be deprecated in an upcoming diff.
648 * Use rlock() instead.
650 const Synchronized& asConst() const {
655 * Swaps with another Synchronized. Protected against
656 * self-swap. Only data is swapped. Locks are acquired in increasing
659 void swap(Synchronized& rhs) {
664 return rhs.swap(*this);
666 auto guard1 = operator->();
667 auto guard2 = rhs.operator->();
670 swap(datum_, rhs.datum_);
674 * Swap with another datum. Recommended because it keeps the mutex
678 LockedPtr guard(this);
685 * Copies datum to a given target.
687 void copy(T* target) const {
688 ConstLockedPtr guard(this);
693 * Returns a fresh copy of the datum.
696 ConstLockedPtr guard(this);
701 template <class LockedType, class MutexType, class LockPolicy>
702 friend class folly::LockedPtrBase;
703 template <class LockedType, class LockPolicy>
704 friend class folly::LockedPtr;
705 template <class LockedType, class LockPolicy>
706 friend class folly::LockedGuardPtr;
709 * Helper constructors to enable Synchronized for
710 * non-default constructible types T.
711 * Guards are created in actual public constructors and are alive
712 * for the time required to construct the object
715 const Synchronized& rhs,
716 const ConstLockedPtr& /*guard*/) noexcept(nxCopyCtor)
717 : datum_(rhs.datum_) {}
719 Synchronized(Synchronized&& rhs, const LockedPtr& /*guard*/) noexcept(
721 : datum_(std::move(rhs.datum_)) {}
723 // Synchronized data members
725 mutable Mutex mutex_;
728 template <class SynchronizedType, class LockPolicy>
729 class ScopedUnlocker;
733 * A helper alias that resolves to "const T" if the template parameter
734 * is a const Synchronized<T>, or "T" if the parameter is not const.
736 template <class SynchronizedType>
737 using SynchronizedDataType = typename std::conditional<
738 std::is_const<SynchronizedType>::value,
739 typename SynchronizedType::DataType const,
740 typename SynchronizedType::DataType>::type;
742 * A helper alias that resolves to a ConstLockedPtr if the template parameter
743 * is a const Synchronized<T>, or a LockedPtr if the parameter is not const.
745 template <class SynchronizedType>
746 using LockedPtrType = typename std::conditional<
747 std::is_const<SynchronizedType>::value,
748 typename SynchronizedType::ConstLockedPtr,
749 typename SynchronizedType::LockedPtr>::type;
753 * A helper base class for implementing LockedPtr.
755 * The main reason for having this as a separate class is so we can specialize
756 * it for std::mutex, so we can expose a std::unique_lock to the caller
757 * when std::mutex is being used. This allows callers to use a
758 * std::condition_variable with the mutex from a Synchronized<T, std::mutex>.
760 * We don't use std::unique_lock with other Mutex types since it makes the
761 * LockedPtr class slightly larger, and it makes the logic to support
762 * ScopedUnlocker slightly more complicated. std::mutex is the only one that
763 * really seems to benefit from the unique_lock. std::condition_variable
764 * itself only supports std::unique_lock<std::mutex>, so there doesn't seem to
765 * be any real benefit to exposing the unique_lock with other mutex types.
767 * Note that the SynchronizedType template parameter may or may not be const
770 template <class SynchronizedType, class Mutex, class LockPolicy>
771 class LockedPtrBase {
773 using MutexType = Mutex;
774 friend class folly::ScopedUnlocker<SynchronizedType, LockPolicy>;
777 * Destructor releases.
781 LockPolicy::unlock(parent_->mutex_);
786 * Unlock the synchronized data.
788 * The LockedPtr can no longer be dereferenced after unlock() has been
789 * called. isValid() will return false on an unlocked LockedPtr.
791 * unlock() can only be called on a LockedPtr that is valid.
794 DCHECK(parent_ != nullptr);
795 LockPolicy::unlock(parent_->mutex_);
801 explicit LockedPtrBase(SynchronizedType* parent) : parent_(parent) {
802 LockPolicy::lock(parent_->mutex_);
804 template <class Rep, class Period>
806 SynchronizedType* parent,
807 const std::chrono::duration<Rep, Period>& timeout) {
808 if (LockPolicy::try_lock_for(parent->mutex_, timeout)) {
809 this->parent_ = parent;
812 LockedPtrBase(LockedPtrBase&& rhs) noexcept : parent_(rhs.parent_) {
813 rhs.parent_ = nullptr;
815 LockedPtrBase& operator=(LockedPtrBase&& rhs) noexcept {
817 LockPolicy::unlock(parent_->mutex_);
820 parent_ = rhs.parent_;
821 rhs.parent_ = nullptr;
825 using UnlockerData = SynchronizedType*;
828 * Get a pointer to the Synchronized object from the UnlockerData.
830 * In the generic case UnlockerData is just the Synchronized pointer,
831 * so we return it as is. (This function is more interesting in the
832 * std::mutex specialization below.)
834 static SynchronizedType* getSynchronized(UnlockerData data) {
838 UnlockerData releaseLock() {
839 DCHECK(parent_ != nullptr);
840 auto current = parent_;
842 LockPolicy::unlock(current->mutex_);
845 void reacquireLock(UnlockerData&& data) {
846 DCHECK(parent_ == nullptr);
848 LockPolicy::lock(parent_->mutex_);
851 SynchronizedType* parent_ = nullptr;
855 * LockedPtrBase specialization for use with std::mutex.
857 * When std::mutex is used we use a std::unique_lock to hold the mutex.
858 * This makes it possible to use std::condition_variable with a
859 * Synchronized<T, std::mutex>.
861 template <class SynchronizedType, class LockPolicy>
862 class LockedPtrBase<SynchronizedType, std::mutex, LockPolicy> {
864 using MutexType = std::mutex;
865 friend class folly::ScopedUnlocker<SynchronizedType, LockPolicy>;
868 * Destructor releases.
871 // The std::unique_lock will automatically release the lock when it is
872 // destroyed, so we don't need to do anything extra here.
875 LockedPtrBase(LockedPtrBase&& rhs) noexcept
876 : lock_(std::move(rhs.lock_)), parent_(rhs.parent_) {
877 rhs.parent_ = nullptr;
879 LockedPtrBase& operator=(LockedPtrBase&& rhs) noexcept {
880 lock_ = std::move(rhs.lock_);
881 parent_ = rhs.parent_;
882 rhs.parent_ = nullptr;
887 * Get a reference to the std::unique_lock.
889 * This is provided so that callers can use Synchronized<T, std::mutex>
890 * with a std::condition_variable.
892 * While this API could be used to bypass the normal Synchronized APIs and
893 * manually interact with the underlying unique_lock, this is strongly
896 std::unique_lock<std::mutex>& getUniqueLock() {
901 * Unlock the synchronized data.
903 * The LockedPtr can no longer be dereferenced after unlock() has been
904 * called. isValid() will return false on an unlocked LockedPtr.
906 * unlock() can only be called on a LockedPtr that is valid.
909 DCHECK(parent_ != nullptr);
916 explicit LockedPtrBase(SynchronizedType* parent)
917 : lock_(parent->mutex_), parent_(parent) {}
920 std::pair<std::unique_lock<std::mutex>, SynchronizedType*>;
922 static SynchronizedType* getSynchronized(const UnlockerData& data) {
926 UnlockerData releaseLock() {
927 DCHECK(parent_ != nullptr);
928 UnlockerData data(std::move(lock_), parent_);
933 void reacquireLock(UnlockerData&& data) {
934 lock_ = std::move(data.first);
936 parent_ = data.second;
939 // The specialization for std::mutex does have to store slightly more
940 // state than the default implementation.
941 std::unique_lock<std::mutex> lock_;
942 SynchronizedType* parent_ = nullptr;
946 * This class temporarily unlocks a LockedPtr in a scoped manner.
948 template <class SynchronizedType, class LockPolicy>
949 class ScopedUnlocker {
951 explicit ScopedUnlocker(LockedPtr<SynchronizedType, LockPolicy>* p)
952 : ptr_(p), data_(ptr_->releaseLock()) {}
953 ScopedUnlocker(const ScopedUnlocker&) = delete;
954 ScopedUnlocker& operator=(const ScopedUnlocker&) = delete;
955 ScopedUnlocker(ScopedUnlocker&& other) noexcept
956 : ptr_(other.ptr_), data_(std::move(other.data_)) {
957 other.ptr_ = nullptr;
959 ScopedUnlocker& operator=(ScopedUnlocker&& other) = delete;
963 ptr_->reacquireLock(std::move(data_));
968 * Return a pointer to the Synchronized object used by this ScopedUnlocker.
970 SynchronizedType* getSynchronized() const {
971 return LockedPtr<SynchronizedType, LockPolicy>::getSynchronized(data_);
975 using Data = typename LockedPtr<SynchronizedType, LockPolicy>::UnlockerData;
976 LockedPtr<SynchronizedType, LockPolicy>* ptr_{nullptr};
981 * A LockedPtr keeps a Synchronized<T> object locked for the duration of
982 * LockedPtr's existence.
984 * It provides access the datum's members directly by using operator->() and
987 * The LockPolicy parameter controls whether or not the lock is acquired in
988 * exclusive or shared mode.
990 template <class SynchronizedType, class LockPolicy>
991 class LockedPtr : public LockedPtrBase<
993 typename SynchronizedType::MutexType,
996 using Base = LockedPtrBase<
998 typename SynchronizedType::MutexType,
1000 using UnlockerData = typename Base::UnlockerData;
1001 // CDataType is the DataType with the appropriate const-qualification
1002 using CDataType = detail::SynchronizedDataType<SynchronizedType>;
1005 using DataType = typename SynchronizedType::DataType;
1006 using MutexType = typename SynchronizedType::MutexType;
1007 using Synchronized = typename std::remove_const<SynchronizedType>::type;
1008 friend class ScopedUnlocker<SynchronizedType, LockPolicy>;
1011 * Creates an uninitialized LockedPtr.
1013 * Dereferencing an uninitialized LockedPtr is not allowed.
1018 * Takes a Synchronized<T> and locks it.
1020 explicit LockedPtr(SynchronizedType* parent) : Base(parent) {}
1023 * Takes a Synchronized<T> and attempts to lock it, within the specified
1026 * Blocks until the lock is acquired or until the specified timeout expires.
1027 * If the timeout expired without acquiring the lock, the LockedPtr will be
1028 * null, and LockedPtr::isNull() will return true.
1030 template <class Rep, class Period>
1032 SynchronizedType* parent,
1033 const std::chrono::duration<Rep, Period>& timeout)
1034 : Base(parent, timeout) {}
1039 LockedPtr(LockedPtr&& rhs) noexcept = default;
1042 * Move assignment operator.
1044 LockedPtr& operator=(LockedPtr&& rhs) noexcept = default;
1047 * Copy constructor and assignment operator are deleted.
1049 LockedPtr(const LockedPtr& rhs) = delete;
1050 LockedPtr& operator=(const LockedPtr& rhs) = delete;
1053 * Destructor releases.
1058 * Check if this LockedPtr is uninitialized, or points to valid locked data.
1060 * This method can be used to check if a timed-acquire operation succeeded.
1061 * If an acquire operation times out it will result in a null LockedPtr.
1063 * A LockedPtr is always either null, or holds a lock to valid data.
1064 * Methods such as scopedUnlock() reset the LockedPtr to null for the
1065 * duration of the unlock.
1067 bool isNull() const {
1068 return this->parent_ == nullptr;
1072 * Explicit boolean conversion.
1076 explicit operator bool() const {
1077 return this->parent_ != nullptr;
1081 * Access the locked data.
1083 * This method should only be used if the LockedPtr is valid.
1085 CDataType* operator->() const {
1086 return &this->parent_->datum_;
1090 * Access the locked data.
1092 * This method should only be used if the LockedPtr is valid.
1094 CDataType& operator*() const {
1095 return this->parent_->datum_;
1099 * Temporarily unlock the LockedPtr, and reset it to null.
1101 * Returns an helper object that will re-lock and restore the LockedPtr when
1102 * the helper is destroyed. The LockedPtr may not be dereferenced for as
1103 * long as this helper object exists.
1105 ScopedUnlocker<SynchronizedType, LockPolicy> scopedUnlock() {
1106 return ScopedUnlocker<SynchronizedType, LockPolicy>(this);
1109 /***************************************************************************
1110 * Upgradable lock methods.
1111 * These are disabled via SFINAE when the mutex is not upgradable
1112 **************************************************************************/
1114 * Move the locked ptr from an upgrade state to an exclusive state. The
1115 * current lock is left in a null state.
1118 typename SyncType = SynchronizedType,
1119 typename = typename std::enable_if<
1120 LockTraits<typename SyncType::MutexType>::is_upgrade>::type>
1121 LockedPtr<SynchronizedType, LockPolicyFromUpgradeToExclusive>
1122 moveFromUpgradeToWrite() {
1123 auto* parent_to_pass_on = this->parent_;
1124 this->parent_ = nullptr;
1125 return LockedPtr<SynchronizedType, LockPolicyFromUpgradeToExclusive>(
1130 * Move the locked ptr from an exclusive state to an upgrade state. The
1131 * current lock is left in a null state.
1134 typename SyncType = SynchronizedType,
1135 typename = typename std::enable_if<
1136 LockTraits<typename SyncType::MutexType>::is_upgrade>::type>
1137 LockedPtr<SynchronizedType, LockPolicyFromExclusiveToUpgrade>
1138 moveFromWriteToUpgrade() {
1139 auto* parent_to_pass_on = this->parent_;
1140 this->parent_ = nullptr;
1141 return LockedPtr<SynchronizedType, LockPolicyFromExclusiveToUpgrade>(
1146 * Move the locked ptr from an upgrade state to a shared state. The
1147 * current lock is left in a null state.
1150 typename SyncType = SynchronizedType,
1151 typename = typename std::enable_if<
1152 LockTraits<typename SyncType::MutexType>::is_upgrade>::type>
1153 LockedPtr<SynchronizedType, LockPolicyFromUpgradeToShared>
1154 moveFromUpgradeToRead() {
1155 auto* parent_to_pass_on = this->parent_;
1156 this->parent_ = nullptr;
1157 return LockedPtr<SynchronizedType, LockPolicyFromUpgradeToShared>(
1162 * Move the locked ptr from an exclusive state to a shared state. The
1163 * current lock is left in a null state.
1166 typename SyncType = SynchronizedType,
1167 typename = typename std::enable_if<
1168 LockTraits<typename SyncType::MutexType>::is_upgrade>::type>
1169 LockedPtr<SynchronizedType, LockPolicyFromExclusiveToShared>
1170 moveFromWriteToRead() {
1171 auto* parent_to_pass_on = this->parent_;
1172 this->parent_ = nullptr;
1173 return LockedPtr<SynchronizedType, LockPolicyFromExclusiveToShared>(
1179 * LockedGuardPtr is a simplified version of LockedPtr.
1181 * It is non-movable, and supports fewer features than LockedPtr. However, it
1182 * is ever-so-slightly more performant than LockedPtr. (The destructor can
1183 * unconditionally release the lock, without requiring a conditional branch.)
1185 * The relationship between LockedGuardPtr and LockedPtr is similar to that
1186 * between std::lock_guard and std::unique_lock.
1188 template <class SynchronizedType, class LockPolicy>
1189 class LockedGuardPtr {
1191 // CDataType is the DataType with the appropriate const-qualification
1192 using CDataType = detail::SynchronizedDataType<SynchronizedType>;
1195 using DataType = typename SynchronizedType::DataType;
1196 using MutexType = typename SynchronizedType::MutexType;
1197 using Synchronized = typename std::remove_const<SynchronizedType>::type;
1199 LockedGuardPtr() = delete;
1202 * Takes a Synchronized<T> and locks it.
1204 explicit LockedGuardPtr(SynchronizedType* parent) : parent_(parent) {
1205 LockPolicy::lock(parent_->mutex_);
1209 * Destructor releases.
1212 LockPolicy::unlock(parent_->mutex_);
1216 * Access the locked data.
1218 CDataType* operator->() const {
1219 return &parent_->datum_;
1223 * Access the locked data.
1225 CDataType& operator*() const {
1226 return parent_->datum_;
1230 // This is the entire state of LockedGuardPtr.
1231 SynchronizedType* const parent_{nullptr};
1235 * Acquire locks for multiple Synchronized<T> objects, in a deadlock-safe
1238 * The locks are acquired in order from lowest address to highest address.
1239 * (Note that this is not necessarily the same algorithm used by std::lock().)
1241 * For parameters that are const and support shared locks, a read lock is
1242 * acquired. Otherwise an exclusive lock is acquired.
1244 * TODO: Extend acquireLocked() with variadic template versions that
1245 * allow for more than 2 Synchronized arguments. (I haven't given too much
1246 * thought about how to implement this. It seems like it would be rather
1247 * complicated, but I think it should be possible.)
1249 template <class Sync1, class Sync2>
1250 std::tuple<detail::LockedPtrType<Sync1>, detail::LockedPtrType<Sync2>>
1251 acquireLocked(Sync1& l1, Sync2& l2) {
1252 if (static_cast<const void*>(&l1) < static_cast<const void*>(&l2)) {
1253 auto p1 = l1.contextualLock();
1254 auto p2 = l2.contextualLock();
1255 return std::make_tuple(std::move(p1), std::move(p2));
1257 auto p2 = l2.contextualLock();
1258 auto p1 = l1.contextualLock();
1259 return std::make_tuple(std::move(p1), std::move(p2));
1264 * A version of acquireLocked() that returns a std::pair rather than a
1265 * std::tuple, which is easier to use in many places.
1267 template <class Sync1, class Sync2>
1268 std::pair<detail::LockedPtrType<Sync1>, detail::LockedPtrType<Sync2>>
1269 acquireLockedPair(Sync1& l1, Sync2& l2) {
1270 auto lockedPtrs = acquireLocked(l1, l2);
1271 return {std::move(std::get<0>(lockedPtrs)),
1272 std::move(std::get<1>(lockedPtrs))};
1275 /************************************************************************
1276 * NOTE: All APIs below this line will be deprecated in upcoming diffs.
1277 ************************************************************************/
1279 // Non-member swap primitive
1280 template <class T, class M>
1281 void swap(Synchronized<T, M>& lhs, Synchronized<T, M>& rhs) {
1286 * SYNCHRONIZED is the main facility that makes Synchronized<T>
1287 * helpful. It is a pseudo-statement that introduces a scope where the
1288 * object is locked. Inside that scope you get to access the unadorned
1293 * Synchronized<vector<int>> svector;
1295 * SYNCHRONIZED (svector) { ... use svector as a vector<int> ... }
1297 * SYNCHRONIZED (v, svector) { ... use v as a vector<int> ... }
1299 * Refer to folly/docs/Synchronized.md for a detailed explanation and more
1302 #define SYNCHRONIZED(...) \
1303 FOLLY_PUSH_WARNING \
1304 FOLLY_GCC_DISABLE_WARNING(shadow) \
1305 FOLLY_GCC_DISABLE_NEW_SHADOW_WARNINGS \
1306 if (bool SYNCHRONIZED_state = false) { \
1308 for (auto SYNCHRONIZED_lockedPtr = \
1309 (FB_VA_GLUE(FB_ARG_2_OR_1, (__VA_ARGS__))).operator->(); \
1310 !SYNCHRONIZED_state; \
1311 SYNCHRONIZED_state = true) \
1312 for (auto& FB_VA_GLUE(FB_ARG_1, (__VA_ARGS__)) = \
1313 *SYNCHRONIZED_lockedPtr.operator->(); \
1314 !SYNCHRONIZED_state; \
1315 SYNCHRONIZED_state = true) \
1318 #define TIMED_SYNCHRONIZED(timeout, ...) \
1319 if (bool SYNCHRONIZED_state = false) { \
1321 for (auto SYNCHRONIZED_lockedPtr = \
1322 (FB_VA_GLUE(FB_ARG_2_OR_1, (__VA_ARGS__))).timedAcquire(timeout); \
1323 !SYNCHRONIZED_state; \
1324 SYNCHRONIZED_state = true) \
1325 for (auto FB_VA_GLUE(FB_ARG_1, (__VA_ARGS__)) = \
1326 (!SYNCHRONIZED_lockedPtr \
1328 : SYNCHRONIZED_lockedPtr.operator->()); \
1329 !SYNCHRONIZED_state; \
1330 SYNCHRONIZED_state = true)
1333 * Similar to SYNCHRONIZED, but only uses a read lock.
1335 #define SYNCHRONIZED_CONST(...) \
1337 FB_VA_GLUE(FB_ARG_1, (__VA_ARGS__)), \
1338 (FB_VA_GLUE(FB_ARG_2_OR_1, (__VA_ARGS__))).asConst())
1341 * Similar to TIMED_SYNCHRONIZED, but only uses a read lock.
1343 #define TIMED_SYNCHRONIZED_CONST(timeout, ...) \
1344 TIMED_SYNCHRONIZED( \
1346 FB_VA_GLUE(FB_ARG_1, (__VA_ARGS__)), \
1347 (FB_VA_GLUE(FB_ARG_2_OR_1, (__VA_ARGS__))).asConst())
1350 * Temporarily disables synchronization inside a SYNCHRONIZED block.
1352 * Note: This macro is deprecated, and kind of broken. The input parameter
1353 * does not control what it unlocks--it always unlocks the lock acquired by the
1354 * most recent SYNCHRONIZED scope. If you have two nested SYNCHRONIZED blocks,
1355 * UNSYNCHRONIZED always unlocks the inner-most, even if you pass in the
1356 * variable name used in the outer SYNCHRONIZED block.
1358 * This macro will be removed soon in a subsequent diff.
1360 #define UNSYNCHRONIZED(name) \
1361 for (auto SYNCHRONIZED_state3 = SYNCHRONIZED_lockedPtr.scopedUnlock(); \
1362 !SYNCHRONIZED_state; \
1363 SYNCHRONIZED_state = true) \
1364 for (auto& name = *SYNCHRONIZED_state3.getSynchronized(); \
1365 !SYNCHRONIZED_state; \
1366 SYNCHRONIZED_state = true)
1369 * Synchronizes two Synchronized objects (they may encapsulate
1370 * different data). Synchronization is done in increasing address of
1371 * object order, so there is no deadlock risk.
1373 #define SYNCHRONIZED_DUAL(n1, e1, n2, e2) \
1374 if (bool SYNCHRONIZED_state = false) { \
1376 for (auto SYNCHRONIZED_ptrs = acquireLockedPair(e1, e2); \
1377 !SYNCHRONIZED_state; \
1378 SYNCHRONIZED_state = true) \
1379 for (auto& n1 = *SYNCHRONIZED_ptrs.first; !SYNCHRONIZED_state; \
1380 SYNCHRONIZED_state = true) \
1381 for (auto& n2 = *SYNCHRONIZED_ptrs.second; !SYNCHRONIZED_state; \
1382 SYNCHRONIZED_state = true)
1384 } /* namespace folly */