ExpMoveFreeHandles函数分析和备用空闲表的关系

第一部分：ExpMoveFreeHandles和备用空闲表的关系

ULONG
ExpMoveFreeHandles (
    IN PHANDLE_TABLE HandleTable
    )
{
    ULONG OldValue, NewValue;
    ULONG Index, OldIndex, NewIndex, FreeSize;
    PHANDLE_TABLE_ENTRY Entry, FirstEntry;
    EXHANDLE Handle;
    ULONG Idx;
    BOOLEAN StrictFIFO;

    //
    // First remove all the handles from the free list so we can add them to the
    // list we use for allocates.
    //

    OldValue = InterlockedExchange ((PLONG)&HandleTable->LastFree,
                                    0);
    Index = OldValue;
    if (Index == 0) {
        //
        // There are no free handles.  Nothing to do.
        //
        return OldValue;
    }

       
    //
    // We are pushing old entries onto the free list.
    // We have the A-B-A problem here as these items may have been moved here because
    // another thread was using them in the pop code.
    //
    for (Idx = 1; Idx < HANDLE_TABLE_LOCKS; Idx++) {
        ExAcquireReleasePushLockExclusive (&HandleTable->HandleTableLock[Idx]);
    }
    StrictFIFO = HandleTable->StrictFIFO;
 
    //如果我们是严格的FIFO，那么反转列表以减少句柄重用。
    // If we are strict FIFO then reverse the list to make handle reuse rare.
    //

    if (!StrictFIFO) {
        //
        // We have a complete chain here. If there is no existing chain we
        // can just push this one without any hassles. If we can't then
        // we can just fall into the reversing code anyway as we need
        // to find the end of the chain to continue it.
        //

        //
        // This is a push so create a new sequence number
        //

        if (InterlockedCompareExchange ((PLONG)&HandleTable->FirstFree,
                                        OldValue + GetNextSeq(),
                                        0) == 0) {
            return OldValue;
        }
    }

    //
    // Loop over all the entries and reverse the chain.
    //遍历所有条目并反转链。
    FreeSize = OldIndex = 0;
    FirstEntry = NULL;
    while (1) {
        FreeSize++;
        Handle.Value = Index;
        Entry = ExpLookupHandleTableEntry (HandleTable, Handle);

        EXASSERT (Entry->Object == NULL);

        NewIndex = Entry->NextFreeTableEntry;
        Entry->NextFreeTableEntry = OldIndex;
        if (OldIndex == 0) {
            FirstEntry = Entry;
        }
        OldIndex = Index;
        if (NewIndex == 0) {
            break;
        }
        Index = NewIndex;
    }

    NewValue = ExpInterlockedExchange (&HandleTable->FirstFree,
                                       OldIndex,
                                       FirstEntry);

    //
    // If we haven't got a pool of a few handles then force
    // table expansion to keep the free handle size high
    //
    if (FreeSize < 100 && StrictFIFO) {
        OldValue = 0;
    }
    return OldValue;
}

FORCEINLINE
ULONG
ExpInterlockedExchange (
    IN OUT PULONG Index,
    IN ULONG FirstIndex,
    IN PHANDLE_TABLE_ENTRY Entry
    )
/*++

Routine Description:

    This performs the following steps:
    1. Set Entry->NextFreeTableEntry = *Index
    2. Loops until *Index == (the value of *Index when we entered the function)
       When they're equal, we set *Index = FirstIndex

Arguments:

    Index - Points to the ULONG we want to set.
    
    FirstIndex - New value to set Index to.

    Entry - TableEntry that will get the initial value of *Index before it's
            updated.

Return Value:

    New value of *Index (i.e. FirstIndex).

--*/
{
    ULONG OldIndex, NewIndex;

    EXASSERT (Entry->Object == NULL);

    //
    // Load new value and generate the sequence number on pushes
    //

    NewIndex = FirstIndex + GetNextSeq();

    while (1) {

        //
        // remember original value and
        // archive it in NextFreeTableEntry.
        //

        OldIndex = *Index;
        Entry->NextFreeTableEntry = OldIndex;

        
        //
        // Swap in the new value, and if the swap occurs
        // successfully, we're done.
        //
        if (OldIndex == (ULONG) InterlockedCompareExchange ((PLONG)Index,
                                                            NewIndex,
                                                            OldIndex)) {
            return OldIndex;
        }
    }
}

第二部分：ExpMoveFreeHandles函数的调用时机
PHANDLE_TABLE_ENTRY
ExpAllocateHandleTableEntry (
    IN PHANDLE_TABLE HandleTable,
    OUT PEXHANDLE pHandle
    )
{

......

    while (1) {

        OldValue = HandleTable->FirstFree;

        while (OldValue == 0) {
            //
            //  Lock the handle table for exclusive access as we will be
            //  allocating a new table level.
            //
            ExpLockHandleTableExclusive (HandleTable, CurrentThread);

            //
            // If we have multiple threads trying to expand the table at
            // the same time then by just acquiring the table lock we
            // force those threads to complete their allocations and
            // populate the free list. We must check the free list here
            // so we don't expand the list twice without needing to.
            //

            OldValue = HandleTable->FirstFree;
            if (OldValue != 0) {
                ExpUnlockHandleTableExclusive (HandleTable, CurrentThread);
                break;
            }

            //看看我们在备用空闲列表上是否有句柄
            // See if we have any handles on the alternate free list
            // These handles need some locking to move them over.
            //
            OldValue = ExpMoveFreeHandles (HandleTable);
            if (OldValue != 0) {
                ExpUnlockHandleTableExclusive (HandleTable, CurrentThread);
                break;
            }