.. _custom_mutex_chmap:
The customizing mutex type for ``concurrent_hash_map``
======================================================
.. note::
To enable this feature, define the ``TBB_PREVIEW_CONCURRENT_HASH_MAP_EXTENSIONS`` macro to 1.
.. contents::
:local:
:depth: 1
Description
***********
oneTBB ``concurrnent_hash_map`` class uses reader-writer mutex
to provide thread safety and avoid data races for insert, lookup, and erasure operations. This feature adds an extra template parameter
for ``concurrent_hash_map`` that allows to customize the type of the reader-writer mutex.
API
***
Header
------
.. code:: cpp
#include <oneapi/tbb/concurrent_hash_map.h>
Synopsis
--------
.. code:: cpp
namespace oneapi {
namespace tbb {
template <typename Key, typename T,
typename HashCompare = tbb_hash_compare<Key>,
typename Allocator = tbb_allocator<std::pair<const Key, T>>,
typename Mutex = spin_rw_mutex>
class concurrent_hash_map {
using mutex_type = Mutex;
};
} // namespace tbb
} // namespace oneapi
Type requirements
-----------------
The type of the mutex passed as a template argument for ``concurrent_hash_map`` should
meet the requirements of `ReaderWriterMutex <https://spec.oneapi.com/versions/latest/elements/oneTBB/source/named_requirements/mutexes/rw_mutex.html>`_.
It should also provide the following API:
.. cpp:function:: bool ReaderWriterMutex::scoped_lock::is_writer() const;
**Returns**: ``true`` if the ``scoped_lock`` object acquires the mutex as a writer, ``false`` otherwise.
The behavior is undefined if the ``scoped_lock`` object does not acquire the mutex.
``oneapi::tbb::spin_rw_mutex``, ``oneapi::tbb::speculative_spin_rw_mutex``, ``oneapi::tbb::queuing_rw_mutex``, ``oneapi::tbb::null_rw_mutex``,
and ``oneapi::tbb::rw_mutex`` meet the requirements above.
.. rubric:: Example
The example below demonstrates how to wrap ``std::shared_mutex`` (C++17) to meet the requirements
of `ReaderWriterMutex` and how to customize ``concurrent_hash_map`` to use this mutex.
.. code:: cpp
#define TBB_PREVIEW_CONCURRENT_HASH_MAP_EXTENSIONS 1
#include "oneapi/tbb/concurrent_hash_map.h"
#include <shared_mutex>
class SharedMutexWrapper {
public:
// ReaderWriterMutex requirements
static constexpr bool is_rw_mutex = true;
static constexpr bool is_recursive_mutex = false;
static constexpr bool is_fair_mutex = false;
class scoped_lock {
public:
scoped_lock() : my_mutex_ptr(nullptr), my_writer_flag(false) {}
scoped_lock(SharedMutexWrapper& mutex, bool write = true)
: my_mutex_ptr(&mutex), my_writer_flag(write)
{
if (my_writer_flag) {
my_mutex_ptr->my_mutex.lock();
} else {
my_mutex_ptr->my_mutex.lock_shared();
}
}
~scoped_lock() {
if (my_mutex_ptr) release();
}
void acquire(SharedMutexWrapper& mutex, bool write = true) {
if (my_mutex_ptr) release();
my_mutex_ptr = &mutex;
my_writer_flag = write;
if (my_writer_flag) {
my_mutex_ptr->my_mutex.lock();
} else {
my_mutex_ptr->my_mutex.lock_shared();
}
}
void release() {
if (my_writer_flag) {
my_mutex_ptr->my_mutex.unlock();
} else {
my_mutex_ptr->my_mutex.unlock_shared();
}
}
bool upgrade_to_writer() {
// std::shared_mutex does not have the upgrade/downgrade parallel_for_each_semantics
if (my_writer_flag) return true; // Already a writer
my_mutex_ptr->my_mutex.unlock_shared();
my_mutex_ptr->my_mutex.lock();
return false; // The lock was reacquired
}
bool downgrade_to_reader() {
if (!my_writer_flag) return true; // Already a reader
my_mutex_ptr->my_mutex.unlock();
my_mutex_ptr->my_mutex.lock_shared();
return false;
}
bool is_writer() const {
return my_writer_flag;
}
private:
SharedMutexWrapper* my_mutex_ptr;
bool my_writer_flag;
};
private:
std::shared_mutex my_mutex;
}; // struct SharedMutexWrapper
int main() {
using map_type = oneapi::tbb::concurrent_hash_map<int, int,
oneapi::tbb::tbb_hash_compare<int>,
oneapi::tbb::tbb_allocator<std::pair<const int, int>>,
SharedMutexWrapper>;
map_type map; // This object will use SharedMutexWrapper for thread safety of insert/find/erase operations
}
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