folly/Synchronized.h
folly/Synchronized.h
introduces a simple abstraction for mutex-
based concurrency. It replaces convoluted, unwieldy, and just
plain wrong code with simple constructs that are easy to get
right and difficult to get wrong.
Many of our multithreaded C++ programs use shared data structures associated with locks. This follows the time-honored adage of mutex-based concurrency control "associate mutexes with data, not code". Consider the following example:
class RequestHandler {
...
RequestQueue requestQueue_;
SharedMutex requestQueueMutex_;
std::map<std::string, Endpoint> requestEndpoints_;
SharedMutex requestEndpointsMutex_;
HandlerState workState_;
SharedMutex workStateMutex_;
...
};
Whenever the code needs to read or write some of the protected data, it acquires the mutex for reading or for reading and writing. For example:
void RequestHandler::processRequest(const Request& request) {
stop_watch<> watch;
checkRequestValidity(request);
SharedMutex::WriteHolder lock(requestQueueMutex_);
requestQueue_.push_back(request);
stats_->addStatValue("requestEnqueueLatency", watch.elapsed());
LOG(INFO) << "enqueued request ID " << request.getID();
}
However, the correctness of the technique is entirely predicated on convention. Developers manipulating these data members must take care to explicitly acquire the correct lock for the data they wish to access. There is no ostensible error for code that:
const
access
to the guarded datafolly/Synchronized.h
The same code sample could be rewritten with Synchronized
as follows:
class RequestHandler {
...
Synchronized<RequestQueue> requestQueue_;
Synchronized<std::map<std::string, Endpoint>> requestEndpoints_;
Synchronized<HandlerState> workState_;
...
};
void RequestHandler::processRequest(const Request& request) {
stop_watch<> watch;
checkRequestValidity(request);
requestQueue_.wlock()->push_back(request);
stats_->addStatValue("requestEnqueueLatency", watch.elapsed());
LOG(INFO) << "enqueued request ID " << request.getID();
}
The rewrite does at maximum efficiency what needs to be done:
acquires the lock associated with the RequestQueue
object, writes to
the queue, and releases the lock immediately thereafter.
On the face of it, that's not much to write home about, and not an obvious improvement over the previous state of affairs. But the features at work invisible in the code above are as important as those that are visible:
requestQueue_
without acquiring the lock you
wouldn't be able to; it is virtually impossible to access the queue
without acquiring the correct lock.If you need to perform several operations while holding the lock,
Synchronized
provides several options for doing this.
The wlock()
method (or lock()
if you have a non-shared mutex type)
returns a LockedPtr
object that can be stored in a variable. The lock
will be held for as long as this object exists, similar to a
std::unique_lock
. This object can be used as if it were a pointer to
the underlying locked object:
{
auto lockedQueue = requestQueue_.wlock();
lockedQueue->push_back(request1);
lockedQueue->push_back(request2);
}
The rlock()
function is similar to wlock()
, but acquires a shared lock
rather than an exclusive lock.
We recommend explicitly opening a new nested scope whenever you store a
LockedPtr
object, to help visibly delineate the critical section, and
to ensure that the LockedPtr
is destroyed as soon as it is no longer
needed.
Alternatively, Synchronized
also provides mechanisms to run a function while
holding the lock. This makes it possible to use lambdas to define brief
critical sections:
void RequestHandler::processRequest(const Request& request) {
stop_watch<> watch;
checkRequestValidity(request);
requestQueue_.withWLock([&](auto& queue) {
// withWLock() automatically holds the lock for the
// duration of this lambda function
queue.push_back(request);
});
stats_->addStatValue("requestEnqueueLatency", watch.elapsed());
LOG(INFO) << "enqueued request ID " << request.getID();
}
One advantage of the withWLock()
approach is that it forces a new
scope to be used for the critical section, making the critical section
more obvious in the code, and helping to encourage code that releases
the lock as soon as possible.
Synchronized<T>
Synchronized
is a template with two parameters, the data type and a
mutex type: Synchronized<T, Mutex>
.
If not specified, the mutex type defaults to folly::SharedMutex
. However, any
mutex type supported by folly::LockTraits
can be used instead.
folly::LockTraits
can be specialized to support other custom mutex
types that it does not know about out of the box. See
folly/LockTraitsBoost.h
for an example of how to support additional mutex
types.
Synchronized
provides slightly different APIs when instantiated with a
shared mutex type or an upgrade mutex type then with a plain exclusive mutex.
If instantiated with either of the two mutex types above (either through
having a member called lock_shared() or specializing LockTraits
as in
folly/LockTraitsBoost.h
) the Synchronized
object has corresponding
wlock
, rlock
or ulock
methods to acquire different lock types. When
using a shared or upgrade mutex type, these APIs ensure that callers make an
explicit choice to acquire a shared, exclusive or upgrade lock and that
callers do not unintentionally lock the mutex in the incorrect mode. The
rlock()
APIs only provide const
access to the underlying data type,
ensuring that it cannot be modified when only holding a shared lock.
The default constructor default-initializes the data and its associated mutex.
The copy constructor locks the source for reading and copies its data into the target. (The target is not locked as an object under construction is only accessed by one thread.)
Finally, Synchronized<T>
defines an explicit constructor that
takes an object of type T
and copies it. For example:
// Default constructed
Synchronized<map<string, int>> syncMap1;
// Copy constructed
Synchronized<map<string, int>> syncMap2(syncMap1);
// Initializing from an existing map
map<string, int> init;
init["world"] = 42;
Synchronized<map<string, int>> syncMap3(init);
EXPECT_EQ(syncMap3->size(), 1);
The copy assignment operator copies the underlying source data into a temporary with the source mutex locked, and then move the temporary into the destination data with the destination mutex locked. This technique avoids the need to lock both mutexes at the same time. Mutexes are not copied or moved.
The move assignment operator assumes the source object is a true rvalue and does lock lock the source mutex. It moves the source data into the destination data with the destination mutex locked.
swap
acquires locks on both mutexes in increasing order of
object address, and then swaps the underlying data. This avoids
potential deadlock, which may otherwise happen should one thread
do a = b
while another thread does b = a
.
The data copy assignment operator copies the parameter into the destination data while the destination mutex is locked.
The data move assignment operator moves the parameter into the destination data while the destination mutex is locked.
To get a copy of the guarded data, there are two methods
available: void copy(T*)
and T copy()
. The first copies data
to a provided target and the second returns a copy by value. Both
operations are done under a read lock. Example:
Synchronized<vector<string>> syncVec1, syncVec2;
vector<string> vec;
// Assign
syncVec1 = syncVec2;
// Assign straight from vector
syncVec1 = vec;
// Swap
syncVec1.swap(syncVec2);
// Swap with vector
syncVec1.swap(vec);
// Copy to given target
syncVec1.copy(&vec);
// Get a copy by value
auto copy = syncVec1.copy();
lock()
If the mutex type used with Synchronized
is a simple exclusive mutex
type (as opposed to a shared mutex), Synchronized<T>
provides a
lock()
method that returns a LockedPtr<T>
to access the data while
holding the lock.
The LockedPtr
object returned by lock()
holds the lock for as long
as it exists. Whenever possible, prefer declaring a separate inner
scope for storing this variable, to make sure the LockedPtr
is
destroyed as soon as the lock is no longer needed:
void fun(Synchronized<vector<string>, std::mutex>& vec) {
{
auto locked = vec.lock();
locked->push_back("hello");
locked->push_back("world");
}
LOG(INFO) << "successfully added greeting";
}
wlock()
and rlock()
If the mutex type used with Synchronized
is a shared mutex type,
Synchronized<T>
provides a wlock()
method that acquires an exclusive
lock, and an rlock()
method that acquires a shared lock.
The LockedPtr
returned by rlock()
only provides const access to the
internal data, to ensure that it cannot be modified while only holding a
shared lock.
int computeSum(const Synchronized<vector<int>>& vec) {
int sum = 0;
auto locked = vec.rlock();
for (int n : *locked) {
sum += n;
}
return sum;
}
void doubleValues(Synchronized<vector<int>>& vec) {
auto locked = vec.wlock();
for (int& n : *locked) {
n *= 2;
}
}
This example brings us to a cautionary discussion. The LockedPtr
object returned by lock()
, wlock()
, or rlock()
only holds the lock
as long as it exists. This object makes it difficult to access the data
without holding the lock, but not impossible. In particular you should
never store a raw pointer or reference to the internal data for longer
than the lifetime of the LockedPtr
object.
For instance, if we had written the following code in the examples above, this would have continued accessing the vector after the lock had been released:
// No. NO. NO!
for (int& n : *vec.wlock()) {
n *= 2;
}
The vec.wlock()
return value is destroyed in this case as soon as the
internal range iterators are created. The range iterators point into
the vector's data, but lock is released immediately, before executing
the loop body.
Needless to say, this is a crime punishable by long debugging nights.
Range-based for loops are slightly subtle about the lifetime of objects
used in the initializer statement. Most other problematic use cases are
a bit easier to spot than this, since the lifetime of the LockedPtr
is
more explicitly visible.
withLock()
As an alternative to the lock()
API, Synchronized
also provides a
withLock()
method that executes a function or lambda expression while
holding the lock. The function receives a reference to the data as its
only argument.
This has a few benefits compared to lock()
:
lock()
if they choose to, but this is not
required. withLock()
ensures that a new scope must always be
defined.withLock()
also helps encourage
users to release the lock as soon as possible. Because the critical
section scope is easily visible in the code, it is harder to
accidentally put extraneous code inside the critical section without
realizing it.For example, withLock()
makes the range-based for loop mistake from
above much harder to accidentally run into:
vec.withLock([](auto& locked) {
for (int& n : locked) {
n *= 2;
}
});
This code does not have the same problem as the counter-example with
wlock()
above, since the lock is held for the duration of the loop.
When using Synchronized
with a shared mutex type, it provides separate
withWLock()
and withRLock()
methods instead of withLock()
.
ulock()
and withULockPtr()
Synchronized
also supports upgrading and downgrading mutex lock levels as
long as the mutex type used to instantiate the Synchronized
type has the
same interface as the mutex types in the C++ standard library, or if
LockTraits
is specialized for the mutex type and the specialization is
visible. See below for an intro to upgrade mutexes.
An upgrade lock can be acquired as usual either with the ulock()
method or
the withULockPtr()
method as so
{
// only const access allowed to the underlying object when an upgrade lock
// is acquired
auto ulock = vec.ulock();
auto newSize = ulock->size();
}
auto newSize = vec.withULockPtr([](auto ulock) {
// only const access allowed to the underlying object when an upgrade lock
// is acquired
return ulock->size();
});
An upgrade lock acquired via ulock()
or withULockPtr()
can be upgraded or
downgraded by calling any of the following methods on the LockedPtr
proxy
moveFromUpgradeToWrite()
moveFromWriteToUpgrade()
moveFromWriteToRead()
moveFromUpgradeToRead()
Calling these leaves the LockedPtr
object on which the method was called in
an invalid null
state and returns another LockedPtr proxy holding the
specified lock. The upgrade or downgrade is done atomically - the
Synchronized
object is never in an unlocked state during the lock state
transition. For example
auto ulock = obj.ulock();
if (ulock->needsUpdate()) {
auto wlock = ulock.moveFromUpgradeToWrite();
// ulock is now null
wlock->updateObj();
}
This "move" can also occur in the context of a withULockPtr()
(withWLockPtr()
or withRLockPtr()
work as well!) function as so
auto newSize = obj.withULockPtr([](auto ulock) {
if (ulock->needsUpdate()) {
// release upgrade lock get write lock atomically
auto wlock = ulock.moveFromUpgradeToWrite();
// ulock is now null
wlock->updateObj();
// release write lock and acquire read lock atomically
auto rlock = wlock.moveFromWriteToRead();
// wlock is now null
return rlock->newSize();
} else {
// release upgrade lock and acquire read lock atomically
auto rlock = ulock.moveFromUpgradeToRead();
// ulock is now null
return rlock->newSize();
}
});
An upgrade mutex is a shared mutex with an extra state called upgrade
and an
atomic state transition from upgrade
to unique
. The upgrade
state is more
powerful than the shared
state but less powerful than the unique
state.
An upgrade lock permits only const access to shared state for doing reads. It does not permit mutable access to shared state for doing writes. Only a unique lock permits mutable access for doing writes.
An upgrade lock may be held concurrently with any number of shared locks on the same mutex. An upgrade lock is exclusive with other upgrade locks and unique locks on the same mutex - only one upgrade lock or unique lock may be held at a time.
The upgrade mutex solves the problem of doing a read of shared state and then optionally doing a write to shared state efficiently under contention. Consider this scenario with a shared mutex:
struct MyObect {
bool isUpdateRequired() const;
void doUpdate();
};
struct MyContainingObject {
folly::Synchronized<MyObject> sync;
void mightHappenConcurrently() {
// first check
if (!sync.rlock()->isUpdateRequired()) {
return;
}
sync.withWLock([&](auto& state) {
// second check
if (!state.isUpdateRequired()) {
return;
}
state.doUpdate();
});
}
};
Here, the second isUpdateRequired
check happens under a unique lock. This
means that the second check cannot be done concurrently with other threads doing
first isUpdateRequired
checks under the shared lock, even though the second
check, like the first check, is read-only and requires only const access to the
shared state.
This may even introduce unnecessary blocking under contention. Since the default
mutex type, folly::SharedMutex
, has write priority, the unique lock protecting
the second check may introduce unnecessary blocking to all the other threads
that are attempting to acquire a shared lock to protect the first check. This
problem is called reader starvation.
One solution is to use a shared mutex type with read priority, such as
folly::SharedMutexReadPriority
. That can introduce less blocking under
contention to the other threads attemping to acquire a shared lock to do the
first check. However, that may backfire and cause threads which are attempting
to acquire a unique lock (for the second check) to stall, waiting for a moment
in time when there are no shared locks held on the mutex, a moment in time that
may never even happen. This problem is called writer starvation.
Starvation is a tricky problem to solve in general. But we can partially side- step it in our case.
An alternative solution is to use an upgrade lock for the second check. Threads attempting to acquire an upgrade lock for the second check do not introduce unnecessary blocking to all other threads that are attempting to acquire a shared lock for the first check. Only after the second check passes, and the upgrade lock transitions atomically from an upgrade lock to a unique lock, does the unique lock introduce necessary blocking to the other threads attempting to acquire a shared lock. With this solution, unlike the solution without the upgrade lock, the second check may be done concurrently with all other first checks rather than blocking or being blocked by them.
The example would then look like:
struct MyObect {
bool isUpdateRequired() const;
void doUpdate();
};
struct MyContainingObject {
folly::Synchronized<MyObject> sync;
void mightHappenConcurrently() {
// first check
if (!sync.rlock()->isUpdateRequired()) {
return;
}
sync.withULockPtr([&](auto ulock) {
// second check
if (!ulock->isUpdateRequired()) {
return;
}
auto wlock = ulock.moveFromUpgradeToWrite();
wlock->doUpdate();
});
}
};
Note: Some shared mutex implementations offer an atomic state transition from
shared
to unique
and some upgrade mutex implementations offer an atomic
state transition from shared
to upgrade
. These atomic state transitions are
dangerous, however, and can deadlock when done concurrently on the same mutex.
For example, if threads A and B both hold shared locks on a mutex and are both
attempting to transition atomically from shared to upgrade locks, the threads
are deadlocked. Likewise if they are both attempting to transition atomically
from shared to unique locks, or one is attempting to transition atomically from
shared to upgrade while the other is attempting to transition atomically from
shared to unique. Therefore, LockTraits
does not expose either of these
dangerous atomic state transitions even when the underlying mutex type supports
them. Likewise, Synchronized
's LockedPtr
proxies do not expose these
dangerous atomic state transitions either.
When Synchronized
is used with a mutex type that supports timed lock
acquisition, lock()
, wlock()
, and rlock()
can all take an optional
std::chrono::duration
argument. This argument specifies a timeout to
use for acquiring the lock. If the lock is not acquired before the
timeout expires, a null LockedPtr
object will be returned. Callers
must explicitly check the return value before using it:
void fun(Synchronized<vector<string>>& vec) {
{
auto locked = vec.lock(10ms);
if (!locked) {
throw std::runtime_error("failed to acquire lock");
}
locked->push_back("hello");
locked->push_back("world");
}
LOG(INFO) << "successfully added greeting";
}
unlock()
and scopedUnlock()
Synchronized
is a good mechanism for enforcing scoped
synchronization, but it has the inherent limitation that it
requires the critical section to be, well, scoped. Sometimes the
code structure requires a fleeting "escape" from the iron fist of
synchronization, while still inside the critical section scope.
One common pattern is releasing the lock early on error code paths,
prior to logging an error message. The LockedPtr
class provides an
unlock()
method that makes this possible:
Synchronized<map<int, string>> dic;
...
{
auto locked = dic.rlock();
auto iter = locked->find(0);
if (iter == locked.end()) {
locked.unlock(); // don't hold the lock while logging
LOG(ERROR) << "key 0 not found";
return false;
}
processValue(*iter);
}
LOG(INFO) << "succeeded";
For more complex nested control flow scenarios, scopedUnlock()
returns
an object that will release the lock for as long as it exists, and will
reacquire the lock when it goes out of scope.
Synchronized<map<int, string>> dic;
...
{
auto locked = dic.wlock();
auto iter = locked->find(0);
if (iter == locked->end()) {
{
auto unlocker = locked.scopedUnlock();
LOG(INFO) << "Key 0 not found, inserting it."
}
locked->emplace(0, "zero");
} else {
*iter = "zero";
}
}
Clearly scopedUnlock()
comes with specific caveats and
liabilities. You must assume that during the scopedUnlock()
section, other threads might have changed the protected structure
in arbitrary ways. In the example above, you cannot use the
iterator iter
and you cannot assume that the key 0
is not in the
map; another thread might have inserted it while you were
bragging on LOG(INFO)
.
Whenever a LockedPtr
object has been unlocked, whether with unlock()
or scopedUnlock()
, it will behave as if it is null. isNull()
will
return true. Dereferencing an unlocked LockedPtr
is not allowed and
will result in undefined behavior.
Synchronized
and std::condition_variable
When used with a std::mutex
, Synchronized
supports using a
std::condition_variable
with its internal mutex. This allows a
condition_variable
to be used to wait for a particular change to occur
in the internal data.
The LockedPtr
returned by Synchronized<T, std::mutex>::lock()
has a
getUniqueLock()
method that returns a reference to a
std::unique_lock<std::mutex>
, which can be given to the
std::condition_variable
:
Synchronized<vector<string>, std::mutex> vec;
std::condition_variable emptySignal;
// Assuming some other thread will put data on vec and signal
// emptySignal, we can then wait on it as follows:
auto locked = vec.lock();
emptySignal.wait(locked.getUniqueLock(),
[&] { return !locked->empty(); });
acquireLocked()
Sometimes locking just one object won't be able to cut the mustard. Consider a
function that needs to lock two Synchronized
objects at the
same time - for example, to copy some data from one to the other.
At first sight, it looks like sequential wlock()
calls will work just
fine:
void fun(Synchronized<vector<int>>& a, Synchronized<vector<int>>& b) {
auto lockedA = a.wlock();
auto lockedB = b.wlock();
... use lockedA and lockedB ...
}
This code compiles and may even run most of the time, but embeds
a deadly peril: if one threads call fun(x, y)
and another
thread calls fun(y, x)
, then the two threads are liable to
deadlocking as each thread will be waiting for a lock the other
is holding. This issue is a classic that applies regardless of
the fact the objects involved have the same type.
This classic problem has a classic solution: all threads must
acquire locks in the same order. The actual order is not
important, just the fact that the order is the same in all
threads. Many libraries simply acquire mutexes in increasing
order of their address, which is what we'll do, too. The
acquireLocked()
function takes care of all details of proper
locking of two objects and offering their innards. It returns a
std::tuple
of LockedPtr
s:
void fun(Synchronized<vector<int>>& a, Synchronized<vector<int>>& b) {
auto ret = folly::acquireLocked(a, b);
auto& lockedA = std::get<0>(ret);
auto& lockedB = std::get<1>(ret);
... use lockedA and lockedB ...
}
Note that C++ 17 introduces (structured binding syntax)[(http://wg21.link/P0144r2)] which will make the returned tuple more convenient to use:
void fun(Synchronized<vector<int>>& a, Synchronized<vector<int>>& b) {
auto [lockedA, lockedB] = folly::acquireLocked(a, b);
... use lockedA and lockedB ...
}
An acquireLockedPair()
function is also available, which returns a
std::pair
instead of a std::tuple
. This is more convenient to use
in many situations, until compiler support for structured bindings is
more widely available.
The library is geared at protecting one object of a given type
with a mutex. However, sometimes we'd like to protect two or more
members with the same mutex. Consider for example a bidirectional
map, i.e. a map that holds an int
to string
mapping and also
the converse string
to int
mapping. The two maps would need
to be manipulated simultaneously. There are at least two designs
that come to mind.
struct
You can easily pack the needed data items in a little struct. For example:
class Server {
struct BiMap {
map<int, string> direct;
map<string, int> inverse;
};
Synchronized<BiMap> bimap_;
...
};
...
bimap_.withLock([](auto& locked) {
locked.direct[0] = "zero";
locked.inverse["zero"] = 0;
});
With this code in tow you get to use bimap_
just like any other
Synchronized
object, without much effort.
std::tuple
If you won't stop short of using a spaceship-era approach,
std::tuple
is there for you. The example above could be
rewritten for the same functionality like this:
class Server {
Synchronized<tuple<map<int, string>, map<string, int>>> bimap_;
...
};
...
bimap_.withLock([](auto& locked) {
get<0>(locked)[0] = "zero";
get<1>(locked)["zero"] = 0;
});
The code uses std::get
with compile-time integers to access the
fields in the tuple. The relative advantages and disadvantages of
using a local struct vs. std::tuple
are quite obvious - in the
first case you need to invest in the definition, in the second
case you need to put up with slightly more verbose and less clear
access syntax.
Synchronized
and its supporting tools offer you a simple,
robust paradigm for mutual exclusion-based concurrency. Instead
of manually pairing data with the mutexes that protect it and
relying on convention to use them appropriately, you can benefit
of encapsulation and typechecking to offload a large part of that
task and to provide good guarantees.