Carla/source/modules/water/threads/SpinLock.h

/*
  ==============================================================================

   This file is part of the Water library.
   Copyright (c) 2016 ROLI Ltd.
   Copyright (C) 2017 Filipe Coelho <falktx@falktx.com>

   Permission is granted to use this software under the terms of the ISC license
   http://www.isc.org/downloads/software-support-policy/isc-license/

   Permission to use, copy, modify, and/or distribute this software for any
   purpose with or without fee is hereby granted, provided that the above
   copyright notice and this permission notice appear in all copies.

   THE SOFTWARE IS PROVIDED "AS IS" AND ISC DISCLAIMS ALL WARRANTIES WITH REGARD
   TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY AND
   FITNESS. IN NO EVENT SHALL ISC BE LIABLE FOR ANY SPECIAL, DIRECT, INDIRECT,
   OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF
   USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER
   TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR PERFORMANCE
   OF THIS SOFTWARE.

  ==============================================================================
*/

#ifndef WATER_SPINLOCK_H_INCLUDED
#define WATER_SPINLOCK_H_INCLUDED

#include "ScopedLock.h"
#include "../memory/Atomic.h"

namespace water {

//==============================================================================
/**
    A simple spin-lock class that can be used as a simple, low-overhead mutex for
    uncontended situations.

    Note that unlike a CriticalSection, this type of lock is not re-entrant, and may
    be less efficient when used it a highly contended situation, but it's very small and
    requires almost no initialisation.
    It's most appropriate for simple situations where you're only going to hold the
    lock for a very brief time.

    @see CriticalSection
*/
class SpinLock
{
public:
    inline SpinLock() noexcept {}
    inline ~SpinLock() noexcept {}

    /** Acquires the lock.
        This will block until the lock has been successfully acquired by this thread.
        Note that a SpinLock is NOT re-entrant, and is not smart enough to know whether the
        caller thread already has the lock - so if a thread tries to acquire a lock that it
        already holds, this method will never return!

        It's strongly recommended that you never call this method directly - instead use the
        ScopedLockType class to manage the locking using an RAII pattern instead.
    */
    void enter() const noexcept
    {
        if (! tryEnter())
        {
            for (int i = 20; --i >= 0;)
                if (tryEnter())
                    return;

            while (! tryEnter())
            {
#ifdef CARLA_OS_WIN
                Sleep (0);
#else
                sched_yield();
#endif
            }
        }
    }

    /** Attempts to acquire the lock, returning true if this was successful. */
    inline bool tryEnter() const noexcept
    {
        return lock.compareAndSetBool (1, 0);
    }

    /** Releases the lock. */
    inline void exit() const noexcept
    {
        CARLA_SAFE_ASSERT_RETURN(lock.value == 1,);
        lock = 0;
    }

    //==============================================================================
    /** Provides the type of scoped lock to use for locking a SpinLock. */
    typedef GenericScopedLock <SpinLock>       ScopedLockType;

    /** Provides the type of scoped unlocker to use with a SpinLock. */
    typedef GenericScopedUnlock <SpinLock>     ScopedUnlockType;

private:
    //==============================================================================
    mutable Atomic<int> lock;

    CARLA_DECLARE_NON_COPY_CLASS (SpinLock)
};

}

#endif // WATER_SPINLOCK_H_INCLUDED