xahaud/modules/beast_basics/memory/beast_FifoFreeStoreWithoutTLS.cpp

/*============================================================================*/
/*
  VFLib: https://github.com/vinniefalco/VFLib

  Copyright (C) 2008 by Vinnie Falco <vinnie.falco@gmail.com>

  This library contains portions of other open source products covered by
  separate licenses. Please see the corresponding source files for specific
  terms.

  VFLib is provided under the terms of The MIT License (MIT):

  Permission is hereby granted, free of charge, to any person obtaining a copy
  of this software and associated documentation files (the "Software"), to deal
  in the Software without restriction, including without limitation the rights
  to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
  copies of the Software, and to permit persons to whom the Software is
  furnished to do so, subject to the following conditions:

  The above copyright notice and this permission notice shall be included in
  all copies or substantial portions of the Software.

  THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
  IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
  FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
  AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
  LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
  FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
  IN THE SOFTWARE.
*/
/*============================================================================*/

// This precedes every allocation
struct FifoFreeStoreWithoutTLS::Header
{
    union
    {
        FifoFreeStoreWithoutTLS::Block* block; // backpointer to the page

        char pad [Memory::allocAlignBytes];
    };
};

//------------------------------------------------------------------------------

class FifoFreeStoreWithoutTLS::Block : Uncopyable
{
public:
    explicit Block (const size_t bytes) : m_refs (1)
    {
        m_end = reinterpret_cast <char*> (this) + bytes;
        m_free = reinterpret_cast <char*> (
                     Memory::pointerAdjustedForAlignment (this + 1));
    }

    ~Block ()
    {
        bassert (!m_refs.isSignaled ());
    }

    inline void addref ()
    {
        m_refs.addref ();
    }

    inline bool release ()
    {
        return m_refs.release ();
    }

    enum Result
    {
        success,  // successful allocation
        ignore,   // disregard the block
        consumed  // block is consumed (1 thread sees this)
    };

    Result allocate (size_t bytes, void* pBlock)
    {
        bassert (bytes > 0);

        Result result;

        for (;;)
        {
            char* base = m_free.get ();

            if (base)
            {
                char* p = Memory::pointerAdjustedForAlignment (base);
                char* free = p + bytes;

                if (free <= m_end)
                {
                    // Try to commit the allocation
                    if (m_free.compareAndSet (free, base))
                    {
                        * (reinterpret_cast <void**> (pBlock)) = p;
                        result = success;
                        break;
                    }
                    else
                    {
                        // Someone changed m_free, retry.
                    }
                }
                else
                {
                    // Mark the block consumed.
                    if (m_free.compareAndSet (0, base))
                    {
                        // Only one caller sees this, the rest get 'ignore'
                        result = consumed;
                        break;
                    }
                    else
                    {
                        // Happens with another concurrent allocate(), retry.
                    }
                }
            }
            else
            {
                // Block is consumed, ignore it.
                result = ignore;
                break;
            }
        }

        return result;
    }

private:
    AtomicCounter m_refs;        // reference count
    AtomicPointer <char> m_free; // next free byte or 0 if inactive.
    char* m_end;                 // last free byte + 1
};

//------------------------------------------------------------------------------

inline FifoFreeStoreWithoutTLS::Block* FifoFreeStoreWithoutTLS::newBlock ()
{
    return new (m_pages->allocate ()) Block (m_pages->getPageBytes ());
}

inline void FifoFreeStoreWithoutTLS::deleteBlock (Block* b)
{
    // It is critical that we do not call the destructor,
    // because due to the lock-free implementation, a Block
    // can be accessed for a short time after it is deleted.
    /* b->~Block (); */ // DO NOT CALL!!!

    PagedFreeStoreType::deallocate (b);
}

FifoFreeStoreWithoutTLS::FifoFreeStoreWithoutTLS ()
    : m_pages (GlobalPagedFreeStore::getInstance ())
{
    if (m_pages->getPageBytes () < sizeof (Block) + 256)
        Throw (Error ().fail (__FILE__, __LINE__, TRANS ("the block size is too small")));

    m_active = newBlock ();
}

FifoFreeStoreWithoutTLS::~FifoFreeStoreWithoutTLS ()
{
    deleteBlock (m_active);
}

//------------------------------------------------------------------------------

void* FifoFreeStoreWithoutTLS::allocate (const size_t bytes)
{
    const size_t actual = sizeof (Header) + bytes;

    if (actual > m_pages->getPageBytes ())
        Throw (Error ().fail (__FILE__, __LINE__, TRANS ("the memory request was too large")));

    Header* h;

    for (;;)
    {
        // Get an active block.
        Block* b = m_active;

        while (!b)
        {
            Thread::yield ();
            b = m_active;
        }

        // (*) It is possible for the block to get a final release here
        //     In this case it will have been put in the garbage, and
        //     m_active will not match.

        // Acquire a reference.
        b->addref ();

        // Is it still active?
        if (m_active == b)
        {
            // Yes so try to allocate from it.
            const Block::Result result = b->allocate (actual, &h);

            if (result == Block::success)
            {
                // Keep the reference and return the allocation.
                h->block = b;
                break;
            }
            else if (result == Block::consumed)
            {
                // Remove block from active.
                m_active = 0;

                // Take away the reference we added
                b->release ();

                // Take away the original active reference.
                if (b->release ())
                    deleteBlock (b);

                // Install a fresh empty active block.
                m_active = newBlock ();
            }
            else
            {
                if (b->release ())
                    deleteBlock (b);
            }

            // Try again.
        }
        else
        {
            // Block became inactive, so release our reference.
            b->release ();

            // (*) It is possible for this to be a duplicate final release.
        }
    }

    return h + 1;
}

//------------------------------------------------------------------------------

void FifoFreeStoreWithoutTLS::deallocate (void* p)
{
    Block* const b = (reinterpret_cast <Header*> (p) - 1)->block;

    if (b->release ())
        deleteBlock (b);
}