/*
* Copyright 2011-present Facebook, Inc.
*
* 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.
*/
/**
* This module implements a Synchronized abstraction useful in
* mutex-based concurrency.
*
* The Synchronized<T, Mutex> class is the primary public API exposed by this
* module. See folly/docs/Synchronized.md for a more complete explanation of
* this class and its benefits.
*/
#pragma once
#include <folly/Function.h>
#include <folly/Likely.h>
#include <folly/LockTraits.h>
#include <folly/Preprocessor.h>
#include <folly/SharedMutex.h>
#include <folly/Traits.h>
#include <folly/Utility.h>
#include <folly/container/Foreach.h>
#include <folly/functional/ApplyTuple.h>
#include <glog/logging.h>
#include <array>
#include <mutex>
#include <tuple>
#include <type_traits>
#include <utility>
namespace folly {
template <class LockedType, class Mutex, class LockPolicy>
class LockedPtrBase;
template <class LockedType, class LockPolicy>
class LockedPtr;
/**
* Public version of LockInterfaceDispatcher that contains the MutexLevel enum
* for the passed in mutex type
*
* This is decoupled from MutexLevelValueImpl in LockTraits.h because this
* ensures that a heterogenous mutex with a different API can be used. For
* example - if a mutex does not have a lock_shared() method but the
* LockTraits specialization for it supports a static non member
* lock_shared(Mutex&) it can be used as a shared mutex and will provide
* rlock() and wlock() functions.
*/
template <class Mutex>
using MutexLevelValue = detail::MutexLevelValueImpl<
true,
LockTraits<Mutex>::is_shared,
LockTraits<Mutex>::is_upgrade>;
/**
* SynchronizedBase is a helper parent class for Synchronized<T>.
*
* It provides wlock() and rlock() methods for shared mutex types,
* or lock() methods for purely exclusive mutex types.
*/
template <class Subclass, detail::MutexLevel level>
class SynchronizedBase;
/**
* SynchronizedBase specialization for shared mutex types.
*
* This class provides wlock() and rlock() methods for acquiring the lock and
* accessing the data.
*/
template <class Subclass>
class SynchronizedBase<Subclass, detail::MutexLevel::SHARED> {
public:
using LockedPtr = ::folly::LockedPtr<Subclass, LockPolicyExclusive>;
using ConstWLockedPtr =
::folly::LockedPtr<const Subclass, LockPolicyExclusive>;
using ConstLockedPtr = ::folly::LockedPtr<const Subclass, LockPolicyShared>;
using TryWLockedPtr = ::folly::LockedPtr<Subclass, LockPolicyTryExclusive>;
using ConstTryWLockedPtr =
::folly::LockedPtr<const Subclass, LockPolicyTryExclusive>;
using TryRLockedPtr = ::folly::LockedPtr<const Subclass, LockPolicyTryShared>;
/**
* Acquire an exclusive lock, and return a LockedPtr that can be used to
* safely access the datum.
*
* LockedPtr offers operator -> and * to provide access to the datum.
* The lock will be released when the LockedPtr is destroyed.
*/
LockedPtr wlock() {
return LockedPtr(static_cast<Subclass*>(this));
}
/**
* Attempts to acquire the lock in exclusive mode. If acquisition is
* unsuccessful, the returned LockedPtr will be null.
*
* (Use LockedPtr::operator bool() or LockedPtr::isNull() to check for
* validity.)
*/
TryWLockedPtr tryWLock() {
return TryWLockedPtr{static_cast<Subclass*>(this)};
}
/**
* Acquire a read lock, and return a ConstLockedPtr that can be used to
* safely access the datum.
*/
ConstLockedPtr rlock() const {
return ConstLockedPtr(static_cast<const Subclass*>(this));
}
/**
* Attempts to acquire the lock in shared mode. If acquisition is
* unsuccessful, the returned LockedPtr will be null.
*
* (Use LockedPtr::operator bool() or LockedPtr::isNull() to check for
* validity.)
*/
TryRLockedPtr tryRLock() const {
return TryRLockedPtr{static_cast<const Subclass*>(this)};
}
/**
* Attempts to acquire the lock, or fails if the timeout elapses first.
* If acquisition is unsuccessful, the returned LockedPtr will be null.
*
* (Use LockedPtr::operator bool() or LockedPtr::isNull() to check for
* validity.)
*/
template <class Rep, class Period>
LockedPtr wlock(const std::chrono::duration<Rep, Period>& timeout) {
return LockedPtr(static_cast<Subclass*>(this), timeout);
}
/**
* Attempts to acquire the lock, or fails if the timeout elapses first.
* If acquisition is unsuccessful, the returned LockedPtr will be null.
*
* (Use LockedPtr::operator bool() or LockedPtr::isNull() to check for
* validity.)
*/
template <class Rep, class Period>
ConstLockedPtr rlock(
const std::chrono::duration<Rep, Period>& timeout) const {
return ConstLockedPtr(static_cast<const Subclass*>(this), timeout);
}
/**
* Invoke a function while holding the lock exclusively.
*
* A reference to the datum will be passed into the function as its only
* argument.
*
* This can be used with a lambda argument for easily defining small critical
* sections in the code. For example:
*
* auto value = obj.withWLock([](auto& data) {
* data.doStuff();
* return data.getValue();
* });
*/
template <class Function>
auto withWLock(Function&& function) {
return function(*wlock());
}
/**
* Invoke a function while holding the lock exclusively.
*
* This is similar to withWLock(), but the function will be passed a
* LockedPtr rather than a reference to the data itself.
*
* This allows scopedUnlock() to be called on the LockedPtr argument if
* desired.
*/
template <class Function>
auto withWLockPtr(Function&& function) {
return function(wlock());
}
/**
* Invoke a function while holding an the lock in shared mode.
*
* A const reference to the datum will be passed into the function as its
* only argument.
*/
template <class Function>
auto withRLock(Function&& function) const {
return function(*rlock());
}
template <class Function>
auto withRLockPtr(Function&& function) const {
return function(rlock());
}
};
/**
* SynchronizedBase specialization for upgrade mutex types.
*
* This class provides all the functionality provided by the SynchronizedBase
* specialization for shared mutexes and a ulock() method that returns an
* upgradable lock RAII proxy
*/
template <class Subclass>
class SynchronizedBase<Subclass, detail::MutexLevel::UPGRADE>
: public SynchronizedBase<Subclass, detail::MutexLevel::SHARED> {
public:
using UpgradeLockedPtr = ::folly::LockedPtr<Subclass, LockPolicyUpgrade>;
using ConstUpgradeLockedPtr =
::folly::LockedPtr<const Subclass, LockPolicyUpgrade>;
using TryUpgradeLockedPtr =
::folly::LockedPtr<Subclass, LockPolicyTryUpgrade>;
using ConstTryUpgradeLockedPtr =
::folly::LockedPtr<const Subclass, LockPolicyTryUpgrade>;
/**
* Acquire an upgrade lock and return a LockedPtr that can be used to safely
* access the datum
*
* And the const version
*/
UpgradeLockedPtr ulock() {
return UpgradeLockedPtr(static_cast<Subclass*>(this));
}
/**
* Attempts to acquire the lock in upgrade mode. If acquisition is
* unsuccessful, the returned LockedPtr will be null.
*
* (Use LockedPtr::operator bool() or LockedPtr::isNull() to check for
* validity.)
*/
TryUpgradeLockedPtr tryULock() {
return TryUpgradeLockedPtr{static_cast<Subclass*>(this)};
}
/**
* Acquire an upgrade lock and return a LockedPtr that can be used to safely
* access the datum
*
* And the const version
*/
template <class Rep, class Period>
UpgradeLockedPtr ulock(const std::chrono::duration<Rep, Period>& timeout) {
return UpgradeLockedPtr(static_cast<Subclass*>(this), timeout);
}
/**
* Invoke a function while holding the lock.
*
* A reference to the datum will be passed into the function as its only
* argument.
*
* This can be used with a lambda argument for easily defining small critical
* sections in the code. For example:
*
* auto value = obj.withULock([](auto& data) {
* data.doStuff();
* return data.getValue();
* });
*
* This is probably not the function you want. If the intent is to read the
* data object and determine whether you should upgrade to a write lock then
* the withULockPtr() method should be called instead, since it gives access
* to the LockedPtr proxy (which can be upgraded via the
* moveFromUpgradeToWrite() method)
*/
template <class Function>
auto withULock(Function&& function) {
return function(*ulock());
}
/**
* Invoke a function while holding the lock exclusively.
*
* This is similar to withULock(), but the function will be passed a
* LockedPtr rather than a reference to the data itself.
*
* This allows scopedUnlock() and getUniqueLock() to be called on the
* LockedPtr argument.
*
* This also allows you to upgrade the LockedPtr proxy to a write state so
* that changes can be made to the underlying data
*/
template <class Function>
auto withULockPtr(Function&& function) {
return function(ulock());
}
};
/**
* SynchronizedBase specialization for non-shared mutex types.
*
* This class provides lock() methods for acquiring the lock and accessing the
* data.
*/
template <class Subclass>
class SynchronizedBase<Subclass, detail::MutexLevel::UNIQUE> {
public:
using LockedPtr = ::folly::LockedPtr<Subclass, LockPolicyExclusive>;
using ConstLockedPtr =
::folly::LockedPtr<const Subclass, LockPolicyExclusive>;
using TryLockedPtr = ::folly::LockedPtr<Subclass, LockPolicyTryExclusive>;
using ConstTryLockedPtr =
::folly::LockedPtr<const Subclass, LockPolicyTryExclusive>;
/**
* Acquire a lock, and return a LockedPtr that can be used to safely access
* the datum.
*/
LockedPtr lock() {
return LockedPtr(static_cast<Subclass*>(this));
}
/**
* Acquire a lock, and return a ConstLockedPtr that can be used to safely
* access the datum.
*/
ConstLockedPtr lock() const {
return ConstLockedPtr(static_cast<const Subclass*>(this));
}
/**
* Attempts to acquire the lock in exclusive mode. If acquisition is
* unsuccessful, the returned LockedPtr will be null.
*
* (Use LockedPtr::operator bool() or LockedPtr::isNull() to check for
* validity.)
*/
TryLockedPtr tryLock() {
return TryLockedPtr{static_cast<Subclass*>(this)};
}
ConstTryLockedPtr tryLock() const {
return ConstTryLockedPtr{static_cast<const Subclass*>(this)};
}
/**
* Attempts to acquire the lock, or fails if the timeout elapses first.
* If acquisition is unsuccessful, the returned LockedPtr will be null.
*/
template <class Rep, class Period>
LockedPtr lock(const std::chrono::duration<Rep, Period>& timeout) {
return LockedPtr(static_cast<Subclass*>(this), timeout);
}
/**
* Attempts to acquire the lock, or fails if the timeout elapses first.
* If acquisition is unsuccessful, the returned LockedPtr will be null.
*/
template <class Rep, class Period>
ConstLockedPtr lock(const std::chrono::duration<Rep, Period>& timeout) const {
return ConstLockedPtr(static_cast<const Subclass*>(this), timeout);
}
/**
* Invoke a function while holding the lock.
*
* A reference to the datum will be passed into the function as its only
* argument.
*
* This can be used with a lambda argument for easily defining small critical
* sections in the code. For example:
*
* auto value = obj.withLock([](auto& data) {
* data.doStuff();
* return data.getValue();
* });
*/
template <class Function>
auto withLock(Function&& function) {
return function(*lock());
}
template <class Function>
auto withLock(Function&& function) const {
return function(*lock());
}
/**
* Invoke a function while holding the lock exclusively.
*
* This is similar to withWLock(), but the function will be passed a
* LockedPtr rather than a reference to the data itself.
*
* This allows scopedUnlock() and getUniqueLock() to be called on the
* LockedPtr argument.
*/
template <class Function>
auto withLockPtr(Function&& function) {
return function(lock());
}
template <class Function>
auto withLockPtr(Function&& function) const {
return function(lock());
}
};
/**
* Synchronized<T> encapsulates an object of type T (a "datum") paired
* with a mutex. The only way to access the datum is while the mutex
* is locked, and Synchronized makes it virtually impossible to do
* otherwise. The code that would access the datum in unsafe ways
* would look odd and convoluted, thus readily alerting the human
* reviewer. In contrast, the code that uses Synchronized<T> correctly
* looks simple and intuitive.
*
* The second parameter must be a mutex type. Any mutex type supported by
* LockTraits<Mutex> can be used. By default any class with lock() and
* unlock() methods will work automatically. LockTraits can be specialized to
* teach Synchronized how to use other custom mutex types. See the
* documentation in LockTraits.h for additional details.
*
* Supported mutexes that work by default include std::mutex,
* std::recursive_mutex, std::timed_mutex, std::recursive_timed_mutex,
* folly::SharedMutex, folly::RWSpinLock, and folly::SpinLock.
* Include LockTraitsBoost.h to get additional LockTraits specializations to
* support the following boost mutex types: boost::mutex,
* boost::recursive_mutex, boost::shared_mutex, boost::timed_mutex, and
* boost::recursive_timed_mutex.
*/
template <class T, class Mutex = SharedMutex>
struct Synchronized : public SynchronizedBase<
Synchronized<T, Mutex>,
MutexLevelValue<Mutex>::value> {
private:
using Base =
SynchronizedBase<Synchronized<T, Mutex>, MutexLevelValue<Mutex>::value>;
static constexpr bool nxCopyCtor{
std::is_nothrow_copy_constructible<T>::value};
static constexpr bool nxMoveCtor{
std::is_nothrow_move_constructible<T>::value};
// used to disable copy construction and assignment
class NonImplementedType;
public:
using LockedPtr = typename Base::LockedPtr;
using ConstLockedPtr = typename Base::ConstLockedPtr;
using DataType = T;
using MutexType = Mutex;
/**
* Default constructor leaves both members call their own default
* constructor.
*/
Synchronized() = default;
public:
/**
* Copy constructor; deprecated
*
* Enabled only when the data type is copy-constructible.
*
* Takes a shared-or-exclusive lock on the source mutex while performing the
* copy-construction of the destination data from the source data. No lock is
* taken on the destination mutex.
*
* May throw even when the data type is is nothrow-copy-constructible because
* acquiring a lock may throw.
*/
/* implicit */ Synchronized(typename std::conditional<
std::is_copy_constructible<T>::value,
const Synchronized&,
NonImplementedType>::type rhs) /* may throw */
: Synchronized(rhs.copy()) {}
/**
* Move constructor; deprecated
*
* Move-constructs from the source data without locking either the source or
* the destination mutex.
*
* Semantically, assumes that the source object is a true rvalue and therefore
* that no synchronization is required for accessing it.
*/
Synchronized(Synchronized&& rhs) noexcept(nxMoveCtor)
: Synchronized(std::move(rhs.datum_)) {}
/**
* Constructor taking a datum as argument copies it. There is no
* need to lock the constructing object.
*/
explicit Synchronized(const T& rhs) noexcept(nxCopyCtor) : datum_(rhs) {}
/**
* Constructor taking a datum rvalue as argument moves it. Again,
* there is no need to lock the constructing object.
*/
explicit Synchronized(T&& rhs) noexcept(nxMoveCtor)
: datum_(std::move(rhs)) {}
/**
* Lets you construct non-movable types in-place. Use the constexpr
* instance `in_place` as the first argument.
*/
template <typename... Args>
explicit Synchronized(in_place_t, Args&&... args)
: datum_(std::forward<Args>(args)...) {}
/**
* Lets you construct the synchronized object and also pass construction
* parameters to the underlying mutex if desired
*/
template <typename... DatumArgs, typename... MutexArgs>
Synchronized(
std::piecewise_construct_t,
std::tuple<DatumArgs...> datumArgs,
std::tuple<MutexArgs...> mutexArgs)
: Synchronized{std::piecewise_construct,
std::move(datumArgs),
std::move(mutexArgs),
make_index_sequence<sizeof...(DatumArgs)>{},
make_index_sequence<sizeof...(MutexArgs)>{}} {}
/**
* Copy assignment operator; deprecated
*
* Enabled only when the data type is copy-constructible and move-assignable.
*
* Move-assigns from a copy of the source data.
*
* Takes a shared-or-exclusive lock on the source mutex while copying the
* source data to a temporary. Takes an exclusive lock on the destination
* mutex while move-assigning from the temporary.
*
* This technique consts an extra temporary but avoids the need to take locks
* on both mutexes together.
*/
Synchronized& operator=(typename std::conditional<
std::is_copy_constructible<T>::value &&
std::is_move_assignable<T>::value,
const Synchronized&,
NonImplementedType>::type rhs) {
return *this = rhs.copy();
}
/**
* Move assignment operator; deprecated
*
* Takes an exclusive lock on the destination mutex while move-assigning the
* destination data from the source data. The source mutex is not locked or
* otherwise accessed.
*
* Semantically, assumes that the source object is a true rvalue and therefore
* that no synchronization is required for accessing it.
*/
Synchronized& operator=(Synchronized&& rhs) {
return *this = std::move(rhs.datum_);
}
/**
* Lock object, assign datum.
*/
Synchronized& operator=(const T& rhs) {
if (&datum_ != &rhs) {
auto guard = operator->();
datum_ = rhs;
}
return *this;
}
/**
* Lock object, move-assign datum.
*/
Synchronized& operator=(T&& rhs) {
if (&datum_ != &rhs) {
auto guard = operator->();
datum_ = std::move(rhs);
}
return *this;
}
/**
* Acquire an appropriate lock based on the context.
*
* If the mutex is a shared mutex, and the Synchronized instance is const,
* this acquires a shared lock. Otherwise this acquires an exclusive lock.
*
* In general, prefer using the explicit rlock() and wlock() methods
* for read-write locks, and lock() for purely exclusive locks.
*
* contextualLock() is primarily intended for use in other template functions
* that do not necessarily know the lock type.
*/
LockedPtr contextualLock() {
return LockedPtr(this);
}
ConstLockedPtr contextualLock() const {
return ConstLockedPtr(this);
}
template <class Rep, class Period>
LockedPtr contextualLock(const std::chrono::duration<Rep, Period>& timeout) {
return LockedPtr(this, timeout);
}
template <class Rep, class Period>
ConstLockedPtr contextualLock(
const std::chrono::duration<Rep, Period>& timeout) const {
return ConstLockedPtr(this, timeout);
}
/**
* contextualRLock() acquires a read lock if the mutex type is shared,
* or a regular exclusive lock for non-shared mutex types.
*
* contextualRLock() when you know that you prefer a read lock (if
* available), even if the Synchronized<T> object itself is non-const.
*/
ConstLockedPtr contextualRLock() const {
return ConstLockedPtr(this);
}
template <class Rep, class Period>
ConstLockedPtr contextualRLock(
const std::chrono::duration<Rep, Period>& timeout) const {
return ConstLockedPtr(this, timeout);
}
/**
* This accessor offers a LockedPtr. In turn, LockedPtr offers
* operator-> returning a pointer to T. The operator-> keeps
* expanding until it reaches a pointer, so syncobj->foo() will lock
* the object and call foo() against it.
*
* NOTE: This API is planned to be deprecated in an upcoming diff.
* Prefer using lock(), wlock(), or rlock() instead.
*/
LockedPtr operator->() {
return LockedPtr(this);
}
/**
* Obtain a ConstLockedPtr.
*
* NOTE: This API is planned to be deprecated in an upcoming diff.
* Prefer using lock(), wlock(), or rlock() instead.
*/
ConstLockedPtr operator->() const {
return ConstLockedPtr(this);
}
/**
* Attempts to acquire for a given number of milliseconds. If
* acquisition is unsuccessful, the returned LockedPtr is nullptr.
*
* NOTE: This API is deprecated. Use lock(), wlock(), or rlock() instead.
* In the future it will be marked with a deprecation attribute to emit
* build-time warnings, and then it will be removed entirely.
*/
LockedPtr timedAcquire(unsigned int milliseconds) {
return LockedPtr(this, std::chrono::milliseconds(milliseconds));
}
/**
* Attempts to acquire for a given number of milliseconds. If
* acquisition is unsuccessful, the returned ConstLockedPtr is nullptr.
*
* NOTE: This API is deprecated. Use lock(), wlock(), or rlock() instead.
* In the future it will be marked with a deprecation attribute to emit
* build-time warnings, and then it will be removed entirely.
*/
ConstLockedPtr timedAcquire(unsigned int milliseconds) const {
return ConstLockedPtr(this, std::chrono::milliseconds(milliseconds));
}
/**
* Swaps with another Synchronized. Protected against
* self-swap. Only data is swapped. Locks are acquired in increasing
* address order.
*/
void swap(Synchronized& rhs) {
if (this == &rhs) {
return;
}
if (this > &rhs) {
return rhs.swap(*this);
}
auto guard1 = operator->();
auto guard2 = rhs.operator->();
using std::swap;
swap(datum_, rhs.datum_);
}
/**
* Swap with another datum. Recommended because it keeps the mutex
* held only briefly.
*/
void swap(T& rhs) {
LockedPtr guard(this);
using std::swap;
swap(datum_, rhs);
}
/**
* Assign another datum and return the original value. Recommended
* because it keeps the mutex held only briefly.
*/
T exchange(T&& rhs) {
swap(rhs);
return std::move(rhs);
}
/**
* Copies datum to a given target.
*/
void copy(T* target) const {
ConstLockedPtr guard(this);
*target = datum_;
}
/**
* Returns a fresh copy of the datum.
*/
T copy() const {
ConstLockedPtr guard(this);
return datum_;
}
private:
template <class LockedType, class MutexType, class LockPolicy>
friend class folly::LockedPtrBase;
template <class LockedType, class LockPolicy>
friend class folly::LockedPtr;
/**
* Helper constructors to enable Synchronized for
* non-default constructible types T.
* Guards are created in actual public constructors and are alive
* for the time required to construct the object
*/
Synchronized(
const Synchronized& rhs,
const ConstLockedPtr& /*guard*/) noexcept(nxCopyCtor)
: datum_(rhs.datum_) {}
Synchronized(Synchronized&& rhs, const LockedPtr& /*guard*/) noexcept(
nxMoveCtor)
: datum_(std::move(rhs.datum_)) {}
template <
typename... DatumArgs,
typename... MutexArgs,
std::size_t... IndicesOne,
std::size_t... IndicesTwo>
Synchronized(
std::piecewise_construct_t,
std::tuple<DatumArgs...> datumArgs,
std::tuple<MutexArgs...> mutexArgs,
index_sequence<IndicesOne...>,
index_sequence<IndicesTwo...>)
: datum_{std::get<IndicesOne>(std::move(datumArgs))...},
mutex_{std::get<IndicesTwo>(std::move(mutexArgs))...} {}
// Synchronized data members
T datum_;
mutable Mutex mutex_;
};
template <class SynchronizedType, class LockPolicy>
class ScopedUnlocker;
namespace detail {
/*
* A helper alias that resolves to "const T" if the template parameter
* is a const Synchronized<T>, or "T" if the parameter is not const.
*/
template <class SynchronizedType>
using SynchronizedDataType = typename std::conditional<
std::is_const<SynchronizedType>::value,
typename SynchronizedType::DataType const,
typename SynchronizedType::DataType>::type;
/*
* A helper alias that resolves to a ConstLockedPtr if the template parameter
* is a const Synchronized<T>, or a LockedPtr if the parameter is not const.
*/
template <class SynchronizedType>
using LockedPtrType = typename std::conditional<
std::is_const<SynchronizedType>::value,
typename SynchronizedType::ConstLockedPtr,
typename SynchronizedType::LockedPtr>::type;
template <
typename Synchronized,
typename LockFunc,
typename TryLockFunc,
typename... Args>
class SynchronizedLocker {
public:
using LockedPtr = invoke_result_t<LockFunc&, Synchronized&, const Args&...>;
template <typename LockFuncType, typename TryLockFuncType, typename... As>
SynchronizedLocker(
Synchronized& sync,
LockFuncType&& lockFunc,
TryLockFuncType tryLockFunc,
As&&... as)
: synchronized{sync},
lockFunc_{std::forward<LockFuncType>(lockFunc)},
tryLockFunc_{std::forward<TryLockFuncType>(tryLockFunc)},
args_{std::forward<As>(as)...} {}
auto lock() const {
auto args = std::tuple<const Args&...>{args_};
return apply(lockFunc_, std::tuple_cat(std::tie(synchronized), args));
}
auto tryLock() const {
return tryLockFunc_(synchronized);
}
private:
Synchronized& synchronized;
LockFunc lockFunc_;
TryLockFunc tryLockFunc_;
std::tuple<Args...> args_;
};
template <
typename Synchronized,
typename LockFunc,
typename TryLockFunc,
typename... Args>
auto makeSynchronizedLocker(
Synchronized& synchronized,
LockFunc&& lockFunc,
TryLockFunc&& tryLockFunc,
Args&&... args) {
using LockFuncType = std::decay_t<LockFunc>;
using TryLockFuncType = std::decay_t<TryLockFunc>;
return SynchronizedLocker<
Synchronized,
LockFuncType,
TryLockFuncType,
std::decay_t<Args>...>{synchronized,
std::forward<LockFunc>(lockFunc),
std::forward<TryLockFunc>(tryLockFunc),
std::forward<Args>(args)...};
}
/**
* Acquire locks for multiple Synchronized<T> objects, in a deadlock-safe
* manner.
*
* The function uses the "smart and polite" algorithm from this link
* http://howardhinnant.github.io/dining_philosophers.html#Polite
*
* The gist of the algorithm is that it locks a mutex, then tries to lock the
* other mutexes in a non-blocking manner. If all the locks succeed, we are
* done, if not, we release the locks we have held, yield to allow other
* threads to continue and then block on the mutex that we failed to acquire.
*
* This allows dynamically yielding ownership of all the mutexes but one, so
* that other threads can continue doing work and locking the other mutexes.
* See the benchmarks in folly/test/SynchronizedBenchmark.cpp for more.
*/
template <typename... SynchronizedLocker>
auto lock(SynchronizedLocker... lockersIn)
-> std::tuple<typename SynchronizedLocker::LockedPtr...> {
// capture the list of lockers as a tuple
auto lockers = std::forward_as_tuple(lockersIn...);
// make a list of null LockedPtr instances that we will return to the caller
auto lockedPtrs = std::tuple<typename SynchronizedLocker::LockedPtr...>{};
// start by locking the first thing in the list
std::get<0>(lockedPtrs) = std::get<0>(lockers).lock();
auto indexLocked = 0;
while (true) {
auto couldLockAll = true;
for_each(lockers, [&](auto& locker, auto index) {
// if we should try_lock on the current locker then do so
if (index != indexLocked) {
auto lockedPtr = locker.tryLock();
// if we were unable to lock this mutex,
//
// 1. release all the locks,
// 2. yield control to another thread to be nice
// 3. block on the mutex we failed to lock, acquire the lock
// 4. break out and set the index of the current mutex to indicate
// which mutex we have locked
if (!lockedPtr) {
// writing lockedPtrs = decltype(lockedPtrs){} does not compile on
// gcc, I believe this is a bug D7676798
lockedPtrs = std::tuple<typename SynchronizedLocker::LockedPtr...>{};
std::this_thread::yield();
fetch(lockedPtrs, index) = locker.lock();
indexLocked = index;
couldLockAll = false;
return loop_break;
}
// else store the locked mutex in the list we return
fetch(lockedPtrs, index) = std::move(lockedPtr);
}
return loop_continue;
});
if (couldLockAll) {
return lockedPtrs;
}
}
}
template <typename Synchronized, typename... Args>
auto wlock(Synchronized& synchronized, Args&&... args) {
return detail::makeSynchronizedLocker(
synchronized,
[](auto& s, auto&&... a) {
return s.wlock(std::forward<decltype(a)>(a)...);
},
[](auto& s) { return s.tryWLock(); },
std::forward<Args>(args)...);
}
template <typename Synchronized, typename... Args>
auto rlock(Synchronized& synchronized, Args&&... args) {
return detail::makeSynchronizedLocker(
synchronized,
[](auto& s, auto&&... a) {
return s.rlock(std::forward<decltype(a)>(a)...);
},
[](auto& s) { return s.tryRLock(); },
std::forward<Args>(args)...);
}
template <typename Synchronized, typename... Args>
auto ulock(Synchronized& synchronized, Args&&... args) {
return detail::makeSynchronizedLocker(
synchronized,
[](auto& s, auto&&... a) {
return s.ulock(std::forward<decltype(a)>(a)...);
},
[](auto& s) { return s.tryULock(); },
std::forward<Args>(args)...);
}
template <typename Synchronized, typename... Args>
auto lock(Synchronized& synchronized, Args&&... args) {
return detail::makeSynchronizedLocker(
synchronized,
[](auto& s, auto&&... a) {
return s.lock(std::forward<decltype(a)>(a)...);
},
[](auto& s) { return s.tryLock(); },
std::forward<Args>(args)...);
}
} // namespace detail
/**
* A helper base class for implementing LockedPtr.
*
* The main reason for having this as a separate class is so we can specialize
* it for std::mutex, so we can expose a std::unique_lock to the caller
* when std::mutex is being used. This allows callers to use a
* std::condition_variable with the mutex from a Synchronized<T, std::mutex>.
*
* We don't use std::unique_lock with other Mutex types since it makes the
* LockedPtr class slightly larger, and it makes the logic to support
* ScopedUnlocker slightly more complicated. std::mutex is the only one that
* really seems to benefit from the unique_lock. std::condition_variable
* itself only supports std::unique_lock<std::mutex>, so there doesn't seem to
* be any real benefit to exposing the unique_lock with other mutex types.
*
* Note that the SynchronizedType template parameter may or may not be const
* qualified.
*/
template <class SynchronizedType, class Mutex, class LockPolicy>
class LockedPtrBase {
public:
using MutexType = Mutex;
friend class folly::ScopedUnlocker<SynchronizedType, LockPolicy>;
/**
* Friend all instantiations of LockedPtr and LockedPtrBase
*/
template <typename S, typename L>
friend class folly::LockedPtr;
template <typename S, typename M, typename L>
friend class LockedPtrBase;
/**
* Destructor releases.
*/
~LockedPtrBase() {
if (parent_) {
LockPolicy::unlock(parent_->mutex_);
}
}
/**
* Unlock the synchronized data.
*
* The LockedPtr can no longer be dereferenced after unlock() has been
* called. isValid() will return false on an unlocked LockedPtr.
*
* unlock() can only be called on a LockedPtr that is valid.
*/
void unlock() {
DCHECK(parent_ != nullptr);
LockPolicy::unlock(parent_->mutex_);
parent_ = nullptr;
}
protected:
LockedPtrBase() {}
explicit LockedPtrBase(SynchronizedType* parent) : parent_(parent) {
DCHECK(parent);
if (!LockPolicy::lock(parent_->mutex_)) {
parent_ = nullptr;
}
}
template <class Rep, class Period>
LockedPtrBase(
SynchronizedType* parent,
const std::chrono::duration<Rep, Period>& timeout) {
if (LockPolicy::try_lock_for(parent->mutex_, timeout)) {
this->parent_ = parent;
}
}
LockedPtrBase(LockedPtrBase&& rhs) noexcept
: parent_{exchange(rhs.parent_, nullptr)} {}
LockedPtrBase& operator=(LockedPtrBase&& rhs) noexcept {
assignImpl(*this, rhs);
return *this;
}
/**
* Templated move construct and assignment operators
*
* These allow converting LockedPtr types that have the same unlocking
* policy to each other. This allows us to write code like
*
* auto wlock = sync.wlock();
* wlock.unlock();
*
* auto ulock = sync.ulock();
* wlock = ulock.moveFromUpgradeToWrite();
*/
template <typename LockPolicyType>
LockedPtrBase(
LockedPtrBase<SynchronizedType, Mutex, LockPolicyType>&& rhs) noexcept
: parent_{exchange(rhs.parent_, nullptr)} {}
template <typename LockPolicyType>
LockedPtrBase& operator=(
LockedPtrBase<SynchronizedType, Mutex, LockPolicyType>&& rhs) noexcept {
assignImpl(*this, rhs);
return *this;
}
/**
* Implementation for the assignment operator
*/
template <typename LockPolicyLhs, typename LockPolicyRhs>
void assignImpl(
LockedPtrBase<SynchronizedType, Mutex, LockPolicyLhs>& lhs,
LockedPtrBase<SynchronizedType, Mutex, LockPolicyRhs>& rhs) noexcept {
if (lhs.parent_) {
LockPolicy::unlock(lhs.parent_->mutex_);
}
lhs.parent_ = exchange(rhs.parent_, nullptr);
}
using UnlockerData = SynchronizedType*;
/**
* Get a pointer to the Synchronized object from the UnlockerData.
*
* In the generic case UnlockerData is just the Synchronized pointer,
* so we return it as is. (This function is more interesting in the
* std::mutex specialization below.)
*/
static SynchronizedType* getSynchronized(UnlockerData data) {
return data;
}
UnlockerData releaseLock() {
DCHECK(parent_ != nullptr);
auto current = parent_;
parent_ = nullptr;
LockPolicy::unlock(current->mutex_);
return current;
}
void reacquireLock(UnlockerData&& data) {
DCHECK(parent_ == nullptr);
parent_ = data;
LockPolicy::lock(parent_->mutex_);
}
SynchronizedType* parent_ = nullptr;
};
/**
* LockedPtrBase specialization for use with std::mutex.
*
* When std::mutex is used we use a std::unique_lock to hold the mutex.
* This makes it possible to use std::condition_variable with a
* Synchronized<T, std::mutex>.
*/
template <class SynchronizedType, class LockPolicy>
class LockedPtrBase<SynchronizedType, std::mutex, LockPolicy> {
public:
using MutexType = std::mutex;
friend class folly::ScopedUnlocker<SynchronizedType, LockPolicy>;
/**
* Friend all instantiations of LockedPtr and LockedPtrBase
*/
template <typename S, typename L>
friend class folly::LockedPtr;
template <typename S, typename M, typename L>
friend class LockedPtrBase;
/**
* Destructor releases.
*/
~LockedPtrBase() {
// The std::unique_lock will automatically release the lock when it is
// destroyed, so we don't need to do anything extra here.
}
LockedPtrBase(LockedPtrBase&& rhs) noexcept
: lock_{std::move(rhs.lock_)}, parent_{exchange(rhs.parent_, nullptr)} {}
LockedPtrBase& operator=(LockedPtrBase&& rhs) noexcept {
assignImpl(*this, rhs);
return *this;
}
/**
* Templated move construct and assignment operators
*
* These allow converting LockedPtr types that have the same unlocking
* policy to each other.
*/
template <typename LockPolicyType>
LockedPtrBase(LockedPtrBase<SynchronizedType, std::mutex, LockPolicyType>&&
other) noexcept
: lock_{std::move(other.lock_)},
parent_{exchange(other.parent_, nullptr)} {}
template <typename LockPolicyType>
LockedPtrBase& operator=(
LockedPtrBase<SynchronizedType, std::mutex, LockPolicyType>&&
rhs) noexcept {
assignImpl(*this, rhs);
return *this;
}
/**
* Implementation for the assignment operator
*/
template <typename LockPolicyLhs, typename LockPolicyRhs>
void assignImpl(
LockedPtrBase<SynchronizedType, std::mutex, LockPolicyLhs>& lhs,
LockedPtrBase<SynchronizedType, std::mutex, LockPolicyRhs>&
rhs) noexcept {
lhs.lock_ = std::move(rhs.lock_);
lhs.parent_ = exchange(rhs.parent_, nullptr);
}
/**
* Get a reference to the std::unique_lock.
*
* This is provided so that callers can use Synchronized<T, std::mutex>
* with a std::condition_variable.
*
* While this API could be used to bypass the normal Synchronized APIs and
* manually interact with the underlying unique_lock, this is strongly
* discouraged.
*/
std::unique_lock<std::mutex>& getUniqueLock() {
return lock_;
}
/**
* Unlock the synchronized data.
*
* The LockedPtr can no longer be dereferenced after unlock() has been
* called. isValid() will return false on an unlocked LockedPtr.
*
* unlock() can only be called on a LockedPtr that is valid.
*/
void unlock() {
DCHECK(parent_ != nullptr);
lock_.unlock();
parent_ = nullptr;
}
protected:
LockedPtrBase() {}
explicit LockedPtrBase(SynchronizedType* parent)
: lock_{parent->mutex_, std::adopt_lock}, parent_{parent} {
DCHECK(parent);
if (!LockPolicy::lock(parent_->mutex_)) {
parent_ = nullptr;
lock_.release();
}
}
using UnlockerData =
std::pair<std::unique_lock<std::mutex>, SynchronizedType*>;
static SynchronizedType* getSynchronized(const UnlockerData& data) {
return data.second;
}
UnlockerData releaseLock() {
DCHECK(parent_ != nullptr);
UnlockerData data(std::move(lock_), parent_);
parent_ = nullptr;
data.first.unlock();
return data;
}
void reacquireLock(UnlockerData&& data) {
lock_ = std::move(data.first);
lock_.lock();
parent_ = data.second;
}
// The specialization for std::mutex does have to store slightly more
// state than the default implementation.
std::unique_lock<std::mutex> lock_;
SynchronizedType* parent_ = nullptr;
};
/**
* This class temporarily unlocks a LockedPtr in a scoped manner.
*/
template <class SynchronizedType, class LockPolicy>
class ScopedUnlocker {
public:
explicit ScopedUnlocker(LockedPtr<SynchronizedType, LockPolicy>* p)
: ptr_(p), data_(ptr_->releaseLock()) {}
ScopedUnlocker(const ScopedUnlocker&) = delete;
ScopedUnlocker& operator=(const ScopedUnlocker&) = delete;
ScopedUnlocker(ScopedUnlocker&& other) noexcept
: ptr_(exchange(other.ptr_, nullptr)), data_(std::move(other.data_)) {}
ScopedUnlocker& operator=(ScopedUnlocker&& other) = delete;
~ScopedUnlocker() {
if (ptr_) {
ptr_->reacquireLock(std::move(data_));
}
}
/**
* Return a pointer to the Synchronized object used by this ScopedUnlocker.
*/
SynchronizedType* getSynchronized() const {
return LockedPtr<SynchronizedType, LockPolicy>::getSynchronized(data_);
}
private:
using Data = typename LockedPtr<SynchronizedType, LockPolicy>::UnlockerData;
LockedPtr<SynchronizedType, LockPolicy>* ptr_{nullptr};
Data data_;
};
/**
* A LockedPtr keeps a Synchronized<T> object locked for the duration of
* LockedPtr's existence.
*
* It provides access the datum's members directly by using operator->() and
* operator*().
*
* The LockPolicy parameter controls whether or not the lock is acquired in
* exclusive or shared mode.
*/
template <class SynchronizedType, class LockPolicy>
class LockedPtr : public LockedPtrBase<
SynchronizedType,
typename SynchronizedType::MutexType,
LockPolicy> {
private:
using Base = LockedPtrBase<
SynchronizedType,
typename SynchronizedType::MutexType,
LockPolicy>;
using UnlockerData = typename Base::UnlockerData;
// CDataType is the DataType with the appropriate const-qualification
using CDataType = detail::SynchronizedDataType<SynchronizedType>;
// Enable only if the unlock policy of the other LockPolicy is the same as
// ours
template <typename LockPolicyOther>
using EnableIfSameUnlockPolicy = std::enable_if_t<std::is_same<
typename LockPolicy::UnlockPolicy,
typename LockPolicyOther::UnlockPolicy>::value>;
// friend other LockedPtr types
template <typename SynchronizedTypeOther, typename LockPolicyOther>
friend class LockedPtr;
public:
using DataType = typename SynchronizedType::DataType;
using MutexType = typename SynchronizedType::MutexType;
using Synchronized = typename std::remove_const<SynchronizedType>::type;
friend class ScopedUnlocker<SynchronizedType, LockPolicy>;
/**
* Creates an uninitialized LockedPtr.
*
* Dereferencing an uninitialized LockedPtr is not allowed.
*/
LockedPtr() {}
/**
* Takes a Synchronized<T> and locks it.
*/
explicit LockedPtr(SynchronizedType* parent) : Base(parent) {}
/**
* Takes a Synchronized<T> and attempts to lock it, within the specified
* timeout.
*
* Blocks until the lock is acquired or until the specified timeout expires.
* If the timeout expired without acquiring the lock, the LockedPtr will be
* null, and LockedPtr::isNull() will return true.
*/
template <class Rep, class Period>
LockedPtr(
SynchronizedType* parent,
const std::chrono::duration<Rep, Period>& timeout)
: Base(parent, timeout) {}
/**
* Move constructor.
*/
LockedPtr(LockedPtr&& rhs) noexcept = default;
template <
typename LockPolicyType,
EnableIfSameUnlockPolicy<LockPolicyType>* = nullptr>
LockedPtr(LockedPtr<SynchronizedType, LockPolicyType>&& other) noexcept
: Base{std::move(other)} {}
/**
* Move assignment operator.
*/
LockedPtr& operator=(LockedPtr&& rhs) noexcept = default;
template <
typename LockPolicyType,
EnableIfSameUnlockPolicy<LockPolicyType>* = nullptr>
LockedPtr& operator=(
LockedPtr<SynchronizedType, LockPolicyType>&& other) noexcept {
Base::operator=(std::move(other));
return *this;
}
/*
* Copy constructor and assignment operator are deleted.
*/
LockedPtr(const LockedPtr& rhs) = delete;
LockedPtr& operator=(const LockedPtr& rhs) = delete;
/**
* Destructor releases.
*/
~LockedPtr() {}
/**
* Check if this LockedPtr is uninitialized, or points to valid locked data.
*
* This method can be used to check if a timed-acquire operation succeeded.
* If an acquire operation times out it will result in a null LockedPtr.
*
* A LockedPtr is always either null, or holds a lock to valid data.
* Methods such as scopedUnlock() reset the LockedPtr to null for the
* duration of the unlock.
*/
bool isNull() const {
return this->parent_ == nullptr;
}
/**
* Explicit boolean conversion.
*
* Returns !isNull()
*/
explicit operator bool() const {
return this->parent_ != nullptr;
}
/**
* Access the locked data.
*
* This method should only be used if the LockedPtr is valid.
*/
CDataType* operator->() const {
return &this->parent_->datum_;
}
/**
* Access the locked data.
*
* This method should only be used if the LockedPtr is valid.
*/
CDataType& operator*() const {
return this->parent_->datum_;
}
/**
* Temporarily unlock the LockedPtr, and reset it to null.
*
* Returns an helper object that will re-lock and restore the LockedPtr when
* the helper is destroyed. The LockedPtr may not be dereferenced for as
* long as this helper object exists.
*/
ScopedUnlocker<SynchronizedType, LockPolicy> scopedUnlock() {
return ScopedUnlocker<SynchronizedType, LockPolicy>(this);
}
/***************************************************************************
* Upgradable lock methods.
* These are disabled via SFINAE when the mutex is not upgradable
**************************************************************************/
/**
* Move the locked ptr from an upgrade state to an exclusive state. The
* current lock is left in a null state.
*/
template <
typename SyncType = SynchronizedType,
typename = typename std::enable_if<
LockTraits<typename SyncType::MutexType>::is_upgrade>::type>
LockedPtr<SynchronizedType, LockPolicyFromUpgradeToExclusive>
moveFromUpgradeToWrite() {
return LockedPtr<SynchronizedType, LockPolicyFromUpgradeToExclusive>(
exchange(this->parent_, nullptr));
}
/**
* Move the locked ptr from an exclusive state to an upgrade state. The
* current lock is left in a null state.
*/
template <
typename SyncType = SynchronizedType,
typename = typename std::enable_if<
LockTraits<typename SyncType::MutexType>::is_upgrade>::type>
LockedPtr<SynchronizedType, LockPolicyFromExclusiveToUpgrade>
moveFromWriteToUpgrade() {
return LockedPtr<SynchronizedType, LockPolicyFromExclusiveToUpgrade>(
exchange(this->parent_, nullptr));
}
/**
* Move the locked ptr from an upgrade state to a shared state. The
* current lock is left in a null state.
*/
template <
typename SyncType = SynchronizedType,
typename = typename std::enable_if<
LockTraits<typename SyncType::MutexType>::is_upgrade>::type>
LockedPtr<SynchronizedType, LockPolicyFromUpgradeToShared>
moveFromUpgradeToRead() {
return LockedPtr<SynchronizedType, LockPolicyFromUpgradeToShared>(
exchange(this->parent_, nullptr));
}
/**
* Move the locked ptr from an exclusive state to a shared state. The
* current lock is left in a null state.
*/
template <
typename SyncType = SynchronizedType,
typename = typename std::enable_if<
LockTraits<typename SyncType::MutexType>::is_upgrade>::type>
LockedPtr<SynchronizedType, LockPolicyFromExclusiveToShared>
moveFromWriteToRead() {
return LockedPtr<SynchronizedType, LockPolicyFromExclusiveToShared>(
exchange(this->parent_, nullptr));
}
};
/**
* Helper functions that should be passed to either a lock() or synchronized()
* invocation, these return implementation defined structs that will be used
* to lock the synchronized instance appropriately.
*
* lock(wlock(one), rlock(two), wlock(three));
* synchronized([](auto one, two) { ... }, wlock(one), rlock(two));
*
* For example in the above rlock() produces an implementation defined read
* locking helper instance and wlock() a write locking helper
*
* Subsequent arguments passed to these locking helpers, after the first, will
* be passed by const-ref to the corresponding function on the synchronized
* instance. This means that if the function accepts these parameters by
* value, they will be copied. Note that it is not necessary that the primary
* locking function will be invoked at all (for eg. the implementation might
* just invoke the try*Lock() method)
*
* // Try to acquire the lock for one second
* synchronized([](auto) { ... }, wlock(one, 1s));
*
* // The timed lock acquire might never actually be called, if it is not
* // needed by the underlying deadlock avoiding algorithm
* synchronized([](auto, auto) { ... }, rlock(one), wlock(two, 1s));
*
* Note that the arguments passed to to *lock() calls will be passed by
* const-ref to the function invocation, as the implementation might use them
* many times
*/
template <typename D, typename M, typename... Args>
auto wlock(Synchronized<D, M>& synchronized, Args&&... args) {
return detail::wlock(synchronized, std::forward<Args>(args)...);
}
template <typename Data, typename Mutex, typename... Args>
auto rlock(const Synchronized<Data, Mutex>& synchronized, Args&&... args) {
return detail::rlock(synchronized, std::forward<Args>(args)...);
}
template <typename D, typename M, typename... Args>
auto ulock(Synchronized<D, M>& synchronized, Args&&... args) {
return detail::ulock(synchronized, std::forward<Args>(args)...);
}
template <typename D, typename M, typename... Args>
auto lock(Synchronized<D, M>& synchronized, Args&&... args) {
return detail::lock(synchronized, std::forward<Args>(args)...);
}
template <typename D, typename M, typename... Args>
auto lock(const Synchronized<D, M>& synchronized, Args&&... args) {
return detail::lock(synchronized, std::forward<Args>(args)...);
}
/**
* Acquire locks for multiple Synchronized<> objects, in a deadlock-safe
* manner.
*
* Wrap the synchronized instances with the appropriate locking strategy by
* using one of the four strategies - folly::lock (exclusive acquire for
* exclusive only mutexes), folly::rlock (shared acquire for shareable
* mutexes), folly::wlock (exclusive acquire for shareable mutexes) or
* folly::ulock (upgrade acquire for upgrade mutexes) (see above)
*
* The locks will be acquired and the passed callable will be invoked with the
* LockedPtr instances in the order that they were passed to the function
*/
template <typename Func, typename... SynchronizedLockers>
decltype(auto) synchronized(Func&& func, SynchronizedLockers&&... lockers) {
return apply(
std::forward<Func>(func),
lock(std::forward<SynchronizedLockers>(lockers)...));
}
/**
* Acquire locks on many lockables or synchronized instances in such a way
* that the sequence of calls within the function does not cause deadlocks.
*
* This can often result in a performance boost as compared to simply
* acquiring your locks in an ordered manner. Even for very simple cases.
* The algorithm tried to adjust to contention by blocking on the mutex it
* thinks is the best fit, leaving all other mutexes open to be locked by
* other threads. See the benchmarks in folly/test/SynchronizedBenchmark.cpp
* for more
*
* This works differently as compared to the locking algorithm in libstdc++
* and is the recommended way to acquire mutexes in a generic order safe
* manner. Performance benchmarks show that this does better than the one in
* libstdc++ even for the simple cases
*
* Usage is the same as std::lock() for arbitrary lockables
*
* folly::lock(one, two, three);
*
* To make it work with folly::Synchronized you have to specify how you want
* the locks to be acquired, use the folly::wlock(), folly::rlock(),
* folly::ulock() and folly::lock() helpers defined below
*
* auto [one, two] = lock(folly::wlock(a), folly::rlock(b));
*
* Note that you can/must avoid the folly:: namespace prefix on the lock()
* function if you use the helpers, ADL lookup is done to find the lock function
*
* This will execute the deadlock avoidance algorithm and acquire a write lock
* for a and a read lock for b
*/
template <typename LockableOne, typename LockableTwo, typename... Lockables>
void lock(LockableOne& one, LockableTwo& two, Lockables&... lockables) {
auto locker = [](auto& lockable) {
using Lockable = std::remove_reference_t<decltype(lockable)>;
return detail::makeSynchronizedLocker(
lockable,
[](auto& l) { return std::unique_lock<Lockable>{l}; },
[](auto& l) {
auto lock = std::unique_lock<Lockable>{l, std::defer_lock};
lock.try_lock();
return lock;
});
};
auto locks = lock(locker(one), locker(two), locker(lockables)...);
// release ownership of the locks from the RAII lock wrapper returned by the
// function above
for_each(locks, [&](auto& lock) { lock.release(); });
}
/**
* Acquire locks for multiple Synchronized<T> objects, in a deadlock-safe
* manner.
*
* The locks are acquired in order from lowest address to highest address.
* (Note that this is not necessarily the same algorithm used by std::lock().)
* For parameters that are const and support shared locks, a read lock is
* acquired. Otherwise an exclusive lock is acquired.
*
* use lock() with folly::wlock(), folly::rlock() and folly::ulock() for
* arbitrary locking without causing a deadlock (as much as possible), with the
* same effects as std::lock()
*/
template <class Sync1, class Sync2>
std::tuple<detail::LockedPtrType<Sync1>, detail::LockedPtrType<Sync2>>
acquireLocked(Sync1& l1, Sync2& l2) {
if (static_cast<const void*>(&l1) < static_cast<const void*>(&l2)) {
auto p1 = l1.contextualLock();
auto p2 = l2.contextualLock();
return std::make_tuple(std::move(p1), std::move(p2));
} else {
auto p2 = l2.contextualLock();
auto p1 = l1.contextualLock();
return std::make_tuple(std::move(p1), std::move(p2));
}
}
/**
* A version of acquireLocked() that returns a std::pair rather than a
* std::tuple, which is easier to use in many places.
*/
template <class Sync1, class Sync2>
std::pair<detail::LockedPtrType<Sync1>, detail::LockedPtrType<Sync2>>
acquireLockedPair(Sync1& l1, Sync2& l2) {
auto lockedPtrs = acquireLocked(l1, l2);
return {std::move(std::get<0>(lockedPtrs)),
std::move(std::get<1>(lockedPtrs))};
}
/************************************************************************
* NOTE: All APIs below this line will be deprecated in upcoming diffs.
************************************************************************/
// Non-member swap primitive
template <class T, class M>
void swap(Synchronized<T, M>& lhs, Synchronized<T, M>& rhs) {
lhs.swap(rhs);
}
/**
* Disambiguate the name var by concatenating the line number of the original
* point of expansion. This avoids shadowing warnings for nested
* SYNCHRONIZEDs. The name is consistent if used multiple times within
* another macro.
* Only for internal use.
*/
#define SYNCHRONIZED_VAR(var) FB_CONCATENATE(SYNCHRONIZED_##var##_, __LINE__)
/**
* SYNCHRONIZED is the main facility that makes Synchronized<T>
* helpful. It is a pseudo-statement that introduces a scope where the
* object is locked. Inside that scope you get to access the unadorned
* datum.
*
* Example:
*
* Synchronized<vector<int>> svector;
* ...
* SYNCHRONIZED (svector) { ... use svector as a vector<int> ... }
* or
* SYNCHRONIZED (v, svector) { ... use v as a vector<int> ... }
*
* Refer to folly/docs/Synchronized.md for a detailed explanation and more
* examples.
*/
#define SYNCHRONIZED(...) \
FOLLY_PUSH_WARNING \
FOLLY_GNU_DISABLE_WARNING("-Wshadow") \
FOLLY_MSVC_DISABLE_WARNING(4189) /* initialized but unreferenced */ \
FOLLY_MSVC_DISABLE_WARNING(4456) /* declaration hides local */ \
FOLLY_MSVC_DISABLE_WARNING(4457) /* declaration hides parameter */ \
FOLLY_MSVC_DISABLE_WARNING(4458) /* declaration hides member */ \
FOLLY_MSVC_DISABLE_WARNING(4459) /* declaration hides global */ \
FOLLY_GCC_DISABLE_NEW_SHADOW_WARNINGS \
if (bool SYNCHRONIZED_VAR(state) = false) { \
} else \
for (auto SYNCHRONIZED_VAR(lockedPtr) = \
(FB_VA_GLUE(FB_ARG_2_OR_1, (__VA_ARGS__))).operator->(); \
!SYNCHRONIZED_VAR(state); \
SYNCHRONIZED_VAR(state) = true) \
for (auto& FB_VA_GLUE(FB_ARG_1, (__VA_ARGS__)) = \
*SYNCHRONIZED_VAR(lockedPtr).operator->(); \
!SYNCHRONIZED_VAR(state); \
SYNCHRONIZED_VAR(state) = true) \
FOLLY_POP_WARNING
#define TIMED_SYNCHRONIZED(timeout, ...) \
if (bool SYNCHRONIZED_VAR(state) = false) { \
} else \
for (auto SYNCHRONIZED_VAR(lockedPtr) = \
(FB_VA_GLUE(FB_ARG_2_OR_1, (__VA_ARGS__))).timedAcquire(timeout); \
!SYNCHRONIZED_VAR(state); \
SYNCHRONIZED_VAR(state) = true) \
for (auto FB_VA_GLUE(FB_ARG_1, (__VA_ARGS__)) = \
(!SYNCHRONIZED_VAR(lockedPtr) \
? nullptr \
: SYNCHRONIZED_VAR(lockedPtr).operator->()); \
!SYNCHRONIZED_VAR(state); \
SYNCHRONIZED_VAR(state) = true)
/**
* Similar to SYNCHRONIZED, but only uses a read lock.
*/
#define SYNCHRONIZED_CONST(...) \
SYNCHRONIZED( \
FB_VA_GLUE(FB_ARG_1, (__VA_ARGS__)), \
as_const(FB_VA_GLUE(FB_ARG_2_OR_1, (__VA_ARGS__))))
/**
* Similar to TIMED_SYNCHRONIZED, but only uses a read lock.
*/
#define TIMED_SYNCHRONIZED_CONST(timeout, ...) \
TIMED_SYNCHRONIZED( \
timeout, \
FB_VA_GLUE(FB_ARG_1, (__VA_ARGS__)), \
as_const(FB_VA_GLUE(FB_ARG_2_OR_1, (__VA_ARGS__))))
/**
* Synchronizes two Synchronized objects (they may encapsulate
* different data). Synchronization is done in increasing address of
* object order, so there is no deadlock risk.
*/
#define SYNCHRONIZED_DUAL(n1, e1, n2, e2) \
if (bool SYNCHRONIZED_VAR(state) = false) { \
} else \
for (auto SYNCHRONIZED_VAR(ptrs) = acquireLockedPair(e1, e2); \
!SYNCHRONIZED_VAR(state); \
SYNCHRONIZED_VAR(state) = true) \
for (auto& n1 = *SYNCHRONIZED_VAR(ptrs).first; !SYNCHRONIZED_VAR(state); \
SYNCHRONIZED_VAR(state) = true) \
for (auto& n2 = *SYNCHRONIZED_VAR(ptrs).second; \
!SYNCHRONIZED_VAR(state); \
SYNCHRONIZED_VAR(state) = true)
} /* namespace folly */