A nonblocking thread-safe memory-pool implementation

Question

I needed a simple non-blocking static-block-size memory-pool. I didn't find such on the web. So everyone, who needs such a solution. This one is free... only works on Win32.

Best regards,

Friedrich

#ifndef MEMPOOL_HPP_INCLUDED
#define MEMPOOL_HPP_INCLUDED

#include "atomic.hpp"
#include "static_assert.hpp"

#pragma warning( push )
#pragma warning( disable : 4311 ) // warning C4311: 'Typumwandlung'

/// @brief Block-free memory-pool implemenation
/// @tparam T Object-type to be saved within the memory-pool.
/// @tparam S Capacy of the memory-pool.
template <typename T, int S>
class MemoryPool
{
private:
    STATIC_ASSERT(sizeof(int) == sizeof(void*), "Well, ...");

public:
    /// @brief Object-type saved within the pool.
    typedef T TYPE;
    enum
    {
        /// @brief Capacy of the memory-pool.
        SIZE = S
    };

private:
    /// @brief Chunks, that holds the memory
    struct Chunk
    {
        /// @brief Single-linked list.
        Chunk* Next;
        /// @brief The value
        /// We do not call the default constructor this way.
        char Value[sizeof(TYPE)];
    };

    /// @brief The root object.
    Chunk* Root;

    /// @brief The pool
    Chunk Pool[SIZE];

private:
    // do not allow copying
    MemoryPool(const MemoryPool&);
    MemoryPool& operator=(const MemoryPool&);

    void free(Chunk* c)
    {
        c->Next = Root;
        while(!CompareAndSwap((int*)&Root, (int)c->Next, (int)c))
        {
            c->Next = Root;
        }
    }

public:
    /// Default constructor
    /// Creates an empty memory-pool.
    /// Invalidates all the memory.
    MemoryPool()
    :   Root(0)
    {
        for(int i = 0; i < SIZE; i++)
        {
            MemoryPool::free(&Pool[i]);
        }
    }

    /// @brief Frees a chunk of memory, that was allocated by MemoryPool::malloc
    /// @param _Chunk A chunk of memory, that was allocated by MemoryPool::malloc
    /// This function will not call the destructor.
    /// Thread-safe, non-blocking
    void free(T* _Chunk)
    {
        if(!_Chunk)
            return;

        Chunk* c = (Chunk*)((int)(_Chunk) - sizeof(Chunk*));

        if(c < &Pool[0] || c > &Pool[SIZE - 1])
            return;

        MemoryPool::free(c);
    }

    /// @brief Returns a pointer to a chunk of memory
    /// @return 0 on a memory shortage
    /// @return A pointer to a chunk of memory
    /// This function will not call the constructor.
    /// Thread-safe, non-blocking
    T* malloc()
    {
        Chunk* r = Root;
        if(!r)
            return 0;

        while(!CompareAndSwap((int*)&Root, (int)r, (int)r->Next))
        {
            r = Root;
            if(!r)
                return 0;
        }

        return &(r->Value);
    }
};

#pragma warning( pop )

#endif // MEMPOOL_HPP_INCLUDED

And the CompareAndSwap

/// @brief Atomic compare and set
/// Atomically compare the value stored at *p with cmpval and if the
/// two values are equal, update the value of *p with newval. Returns
/// zero if the compare failed, nonzero otherwise.
/// @param p Pointer to the target
/// @param cmpval Value as we excpect it
/// @param newval New value
static inline int CompareAndSwap(volatile int *_ptr, int _old, int _new)
{
    __asm {
        mov eax, [_old]                // place the value of _old to EAX
        mov ecx, [_new]                // place the value of _new to ECX
        mov edx, [_ptr]                // place the pointer of _ptr to EDX
        lock cmpxchg [edx], ecx        // cmpxchg old (EAX) and *ptr ([EDX])
    }
    return 1;
}

Ninnetta

Thanks, but SO is not for this kind of post. – Sirup

BTW: If this only works on Windows: Why not using InterlockedXYZ()? – Strath

7 questions, 0 answers, 0 votes, 0 accept. Thanks, but SO is not for this kind of user. – Poly

I went ahead and answered it because even though the original poster didn't realize it, he needed help. – Gatling

You should keep to the Question and Answer format of the site by moving the answer part of your post down to an answer. – Ethyl

Answer 1

The problem with this approach is that there is a race condition in malloc:

while(!CompareAndSwap((int*)&Root, (int)r, (int)r->Next))

Consider the following sequence of operations:

1. Initially Root = A, A->next = B, ...
2. One thread reads r = Root so r = A and (into a register) it reads ecx = r->Next = B
3. Initial thread is preempted (or, on another CPU) a series of malloc and free occur such that A is used for a while and freed last.
4. New list state is Root = A, A->next = ZZZ, ...
5. Original thread wakes up and does cmpxchg and succeeds because Root == r == A and thus sets Root = ecx = B
6. Now your list is corrupted.

You can solve this problem if you have a double-word cmpxchg, such as cmpxchg8b. You just include a serial number next to the list head so that if the compare fails if you are interrupted as in (3) above. The free side can use the narrow version as long as each malloc both exchanges the pointer and increments the serial number.

Gatling

AKA the ABA problem: en.wikipedia.org/wiki/ABA_problem :) But I think you knew that. – Conservatoire

That looks like an implementation of what we're talking about. Your own version isn't so far from working, though, as I describe in my last paragraph. – Gatling

I got blindsided by this problem as well: linkedin.com/feed/update/… – Culicid

Answer 2

Thank you for any comments. This one may be used with WinXP and newer. The implementation mentioned before may still be used with a PowerPC architecture (if you have a proper implementation of CompareAndSwap, see "http://publib.boulder.ibm.com/infocenter/aix/v6r1/topic/com.ibm.aix.aixassem/doc/alangref/stwcx.htm").

Best regards,

Friedrich

/// @brief Lock-free memory-pool implementation
/// @tparam T Type stored within the memory-pool
/// @tparam S Number of elements stored in the memory-pool.
template <typename T, int S>
class MemoryPool
{
public:
    /// @brief Type stored within the memory-pool.
    typedef T TYPE;
    enum
    {
        /// @brief Number of enrties in the memory-pool.
        SIZE = S
    };

private:

// we need to align the memory-pool-chunks.
#pragma pack(push, MEMORY_ALLOCATION_ALIGNMENT)

    /// @brief The memory-chunk used by the memory-pool.
    template <typename TYPE>
    struct MemoryChunk
    {
        /// @brief Next entry in the single-linked list.
        SLIST_ENTRY Next;
        /// @brief The value stored within the memory-pool.
        /// Do not call the constructor
        char Value[sizeof(TYPE)];
    };
    typedef MemoryChunk<TYPE> CHUNK_TYPE;

#pragma pack(pop, MEMORY_ALLOCATION_ALIGNMENT)

    /// @brief Head of the single-linked list.
    SLIST_HEADER Head;

    /// @brief The pool itself
    CHUNK_TYPE Pool[SIZE];

    // no copying is supported
    MemoryPool& operator=(const MemoryPool&);
    MemoryPool(const MemoryPool&);

public:
    /// @brief Constructs the memory-pool.
    MemoryPool()
    {
        InitializeSListHead(&Head);
        for(int i = 0; i < SIZE; i++)
        {
            InterlockedPushEntrySList(&Head, &Pool[i].Next);
        }
    }

    /// @brief Free the memory-pool.
    ~MemoryPool()
    {
        InterlockedFlushSList(&Head);
    }

    /// @brief Allocates a memory chunk
    /// @return 0 if none is free
    /// @return Pointer to a free memory chunk (the constructor is not called!)
    TYPE* Allocate()
    {
        CHUNK_TYPE* c = reinterpret_cast<CHUNK_TYPE*>(InterlockedPopEntrySList(&Head));
        if(c)
            return reinterpret_cast<TYPE*>(&c->Value[0]);
        else
            return 0;
    }

    /// @brief Deallocates a memory chunk (the destructor is not called)
    /// @param c Point to the memory-chunk allocated by us.
    void Deallocate(void* c)
    {
        if(c < static_cast<void*>(&Pool[0]) || c > static_cast<void*>(&Pool[SIZE]))
            return; // was not allocated by us
        char* p = static_cast<char*>(c);
        p -= sizeof(SLIST_ENTRY);
        CHUNK_TYPE* t = reinterpret_cast<CHUNK_TYPE*>(p);
        InterlockedPushEntrySList(&Head, &t->Next);
    }
};

Ninnetta