C++ mutex collections, platform-independent header-only C++11 library.
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README.md

yamc

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C++ mutex (mutual exclusion primitive for multi-threading) collections. This is header-only, cross-platform, no external dependency C++11 library.

"yamc" is an acronym for Yet Another (or Yohhoy's Ad-hoc) Mutex Collections ;)

Description

This library includes:

  • Spinlock mutex, support configurable backoff algorithm.
  • Shared mutex for readers-writer locking in C++11, support reader-prefer/writer-prefer scheduling.
  • Checked mutex for debugging, compatible with requirements in C++11/14/17 Standard.
  • Fair mutex and fair shared mutex, support FIFO scheduling to prevent from starvation.
  • shared_lock<Mutex>, scoped_lock<Mutexes...> utilities which are added in C++14/17.

Example

All mutex types in this library are compatible with corresponding mutex types in C++ Standard Library. The following toy example use spinlock mutex (yamc::spin_ttas::mutex) and scoped locking by std::lock_guard<>.

#include <mutex>  // std::lock_guard<>
#include "ttas_spin_mutex.hpp"

template <typename T>
class ValueHolder {
  // declare mutex type for this class implementation
  using MutexType = yamc::spin_ttas::mutex;

  T value_;
  mutable MutexType guard_;  // guard to value_ access

public:
  ValueHolder(const T& v = T())
    : value_(v) {}

  void set(const T& v)
  {
    std::lock_guard<MutexType> lk(guard_);  // acquire lock
    value_ = v;
  }

  T get() const
  {
    std::lock_guard<MutexType> lk(guard_);  // acquire lock
    return value_;
  }
};

Mutex characteristics

This mutex collections library provide the following types:

  • yamc::spin::mutex: TAS spinlock, non-recursive
  • yamc::spin_weak::mutex: TAS spinlock, non-recursive
  • yamc::spin_ttas::mutex: TTAS spinlock, non-recursive
  • yamc::checked::mutex: requirements debugging, non-recursive
  • yamc::checked::timed_mutex: requirements debugging, non-recursive, support timeout
  • yamc::checked::recursive_mutex: requirements debugging, recursive
  • yamc::checked::recursive_timed_mutex: requirements debugging, recursive, support timeout
  • yamc::checked::shared_mutex: requirements debugging, RW locking, non-recursive
  • yamc::checked::shared_timed_mutex: requirements debugging, RW locking, non-recursive, support timeout
  • yamc::fair::mutex: fairness, non-recursive
  • yamc::fair::recursive_mutex: fairness, recursive
  • yamc::fair::timed_mutex: fairness, non-recursive, support timeout
  • yamc::fair::recursive_timed_mutex: fairness, recursive, support timeout
  • yamc::fair::shared_mutex: fairness, RW locking, non-recursive
  • yamc::fair::shared_timed_mutex: fairness, RW locking, non-recursive, support timeout
  • yamc::alternate::recursive_mutex: recursive
  • yamc::alternate::timed_mutex: non-recursive, support timeout
  • yamc::alternate::recursive_timed_mutex: recursive, support timeout
  • yamc::alternate::shared_mutex: RW locking, non-recursive
  • yamc::alternate::shared_timed_mutex: RW locking, non-recursive, support timeout

These mutex types fulfil corresponding mutex semantics in C++ Standard. You can replace type std::mutex to yamc::*::mutex, std::recursive_mutex to yamc::*::recursive_mutex likewise, except some special case. Note: std::mutex's default constructor is constexpr, but yamc::*::mutex is not. All mutex types in C++ Standard are standard-layout class, but not all types in yamc namespace are.

C++11/14/17 Standard Library define variable mutex types:

Which mutex should I use?

Basically, you should use std::mutex or variants of C++ Standard Library in your products. Period.

  • When you debug misuse of mutex object, checked mutex in yamc::checked::* will help you.
  • When you really need spinlock mutex, I suppose yamc::spin_ttas::mutex may be best choice.
  • When you actually need fairness of locking order, try to use fair mutex in yamc::fair::*.
  • Mutex in yamc::alternate::* has the same semantics of C++ Standard mutex, no additional features.
  • When your compiler doesn't support C++14/17 Standard Library, shared mutex in yamc::alternate::* and yamc::shared_lock<Mutex> which emulate C++14 std::shared_lock<Mutex> are useful.

Requirements

A C++ compiler and standard library support C++11. No need for external library.

NOTE: The implementation of this library depends on C++11 Standard threading primitives only std::mutex, std::condition_variable and std::atomic<T>. This means that you can use shared mutex variants (shared_mutex, shared_timed_mutex) with C++11 compiler which doesn't not support C++14/17 yet.

CI building and unit-testing on the following environments:

  • Linux/G++ 5.4
  • Linux/Clang 3.7
  • OSX/Clang (Xcode 8.3)
  • Windows/MSVC 14.0 (Visual Studio 2015)

Tweaks

Busy waiting in spinlock mutex

The spinlock mutexes use an exponential backoff algorithm in busy waiting to acquire lock as default. These backoff algorithm of spinlock mutex-es are implemented with policy-based template class basic_mutex<BackoffPolicy>. You can tweak the algorithm by specifying BackoffPolicy when you instantiate spinlock mutex type, or define the following macros to change default behavior of all spinlock mutex types.

Customizable macros:

  • YAMC_BACKOFF_SPIN_DEFAULT: BackoffPolicy of spinlock mutex types. Default policy is yamc::backoff::exponential<>.
  • YAMC_BACKOFF_EXPONENTIAL_INITCOUNT: An initial count of yamc::backoff::exponential<N> policy class. Default value is 4000.

Pre-defined BackoffPolicy classes:

  • yamc::backoff::exponential<N>: An exponential backoff waiting algorithm, N denotes initial count. Yield the thread at an exponential decaying intervals in busy waiting loop.
  • yamc::backoff::yield: Always yield the thread by calling std::this_thread::yield().
  • yamc::backoff::busy: Do nothing. Real busy-loop may waste CPU time and increase power consumption.

Sample code:

// change default BackoffPolicy
#define YAMC_BACKOFF_SPIN_DEFAULT yamc::backoff::yield
#include "naive_spin_mutex.hpp"

// define spinlock mutex type with exponential backoff (initconut=1000)
using MyMutex = yamc::spin::basic_mutex<yamc::backoff::exponential<1000>>;

Readers-Writer lock by shared mutex

The shared mutex types provide "Readers-Writer lock" (a.k.a. "Shared-Exclusive lock") semantics. They implement data sharing mechanism between multiple-readers and single-writer threads. Multiple threads can acquire shared lock to concurrently read shared data, or single thread can acquire exclusive lock to modify shared data. When readers and writers threads try to acquire lock simultaneously, there are some scheduling algorithm that determinate which thread can acquire next lock.

These scheduling algorithm of shared mutex types are implemented with policy-based template class basic_shared_(timed_)mutex<RwLockPolicy>, except yamc::fair::shared_(timed_)mutex which implement fairness locking between readers and writers. You can tweak the algorithm by specifying RwLockPolicy when you instantiate shared mutex type, or define the following macro to change default behavior of these shared mutex types.

Customizable macro:

  • YAMC_RWLOCK_SCHED_DEFAULT: RwLockPolicy of shared mutex types. Default policy is yamc::rwlock::ReaderPrefer.

Pre-defined RwLockPolicy classes:

  • yamc::rwlock::ReaderPrefer: Reader prefer locking. While any reader thread owns shared lock, subsequent other reader threads can immediately acquire shared lock, but subsequent writer threads will be blocked until all reader threads release shared lock. This policy might introduce "Writer Starvation" if reader threads continuously hold shared lock.
  • yamc::rwlock::WriterPrefer: Writer prefer locking. While any reader thread owns shared lock and there are a waiting writer thread, subsequent other reader threads which try to acquire shared lock are blocked until writer thread's work is done. This policy might introduce "Reader Starvation" if writer threads continuously try to acquire exclusive lock.

Sample code:

// change default RwLockPolicy
#define YAMC_RWLOCK_SCHED_DEFAULT yamc::rwlock::WriterPrefer
#include "alternate_shared_mutex.hpp"

// define shared mutex type with ReaderPrefer policy
using MySharedMutex = yamc::alternate::basic_shared_mutex<yamc::rwlock::ReaderPrefer>;

Readers-Writer lock fairness

The shared mutex types in yamc::fair namespace implement "Task/Phase-fair Readers-Writer lock", which ensure not to cause writer starvation nor reader starvation. That fair shared mutex has FIFO(First-In-First-Out) queue of threads wait for lock acquisition, and switch the turn that threads acquire exclusive lock or shared locks. For example, 4 threads try to acquire lock in W1 -> R2 -> R3 -> W4 order (W=exclusive lock / R=shared lock), each threads will acquire the lock in that order. In this case, 2 reader threads can concurrently acquire R2 and R3.

These scheduling algorithm of fair shared mutex types are implemented with policy-based template class yamc::fair::basic_shared_(timed_)mutex<RwLockFairness>. You can tweak the algorithm by specifying RwLockFairness when you instantiate fair shared mutex type, or define the following macro to change default behavior of these fair shared mutex types.

Customizable macro:

  • YAMC_RWLOCK_FAIRNESS_DEFAULT: RwLockFairness of fair shared mutex types. Default policy is yamc::rwlock::PhaseFairness.

Pre-defined RwLockFairness classes:

  • yamc::rwlock::TaskFairness: Task-fairness RW locking schedule, which provides simple FIFO lock ordering. When lock request order is W1 -> R2 -> W3 -> R4, each waiting threads will acquire RW lock in the request order.
  • yamc::rwlock::PhaseFairness: Phase-fairness RW locking schedule, which provides "phasing" FIFO lock ordering. When lock request order is W1 -> R2 -> W3 -> R4, the acquisition order will be W1 -> R2,R4 -> W3. Because releasing exclusive lock (W1) switches the RW phase, so all waiting reader threads acquire shared locks (R2,R4) concurrently.

Check requirements of mutex operation

Some operation of mutex type has pre-condition statement, for instance, the thread which call m.unlock() shall own its lock of mutex m. C++ Standard say that the behavior is undefined when your program violate any requirements. This means incorrect usage of mutex might cause deadlock, data corruption, or anything wrong.

Checked mutex types which are defined in yamc::checked::* validate the following requirements on run-time:

  • A thread that call unlock() SHALL own its lock. (Unpaired Lock/Unlock)
  • For mutex and timed_mutex, a thread that call lock() or try_lock family SHALL NOT own its lock. (Non-recursive Semantics)
  • When a thread destruct mutex object, all threads (include this thread) SHALL NOT own its lock. (Abandoned Lock)

Checked mutexes are designed for debugging purpose, so the operation on checked mutex have some overhead. The default behavior is throwing std::system_error exception when checked mutex detect any violation. If you #define YAMC_CHECKED_CALL_ABORT 1 before #include "checked_(shared_)mutex.hpp", checked mutex will call std::abort() instead of throwing exception and the program immediately terminate.

Deadlock detection

Checked mutex types (yamc::checked::*) also provide "Deadlock detection" by default. The runtime deadlock detector tracks all lock ownership and waiting thread on checked mutexes, lock() and lock_shared() which cause deadlock methods will throw exception or abort the program (described in above section).

Such tracking increase additional runtime overhead, and will affect whole program progress. To disable deadlock detection, #define YAMC_CHECKED_DETECT_DEADLOCK 0 before #include "checked_(shared_)mutex.hpp".

CAVEAT: This feature depends on the intrinsic mechanism of yamc::checked::* mutexes. The deadlock detector CAN NOT detect any deadlock come from other mutex types. If you need to detect general deadlock, consider Valgrind/Helgrind and Clang/ThreadSanitizer, etc.

Licence

MIT License