windows-nt/Source/XPSP1/NT/base/ntos/ex/callback.c

/*++

Copyright (c) 1989-1995  Microsoft Corporation

Module Name:

    callback.c

Abstract:

   This module implements the executive callbaqck object. Functions are
   provided to open, register, unregister , and notify callback objects.

Author:

    Ken Reneris  (kenr) 7-March-1995

    Neill Clift  (NeillC) 17-Feb-2001

    Added low overhead callbacks for critical components like thread/registry etc.
    These routines have a high probability of not requiring any locks for an
    individual call.

Environment:

    Kernel mode only.

Revision History:

--*/


#include "exp.h"

//
// Callback Specific Access Rights.
//

#define CALLBACK_MODIFY_STATE    0x0001

#define CALLBACK_ALL_ACCESS (STANDARD_RIGHTS_REQUIRED|SYNCHRONIZE|\
                             CALLBACK_MODIFY_STATE )



//
// Event to wait for registration to become idle
//

KEVENT ExpCallbackEvent;


//
// Lock used when fast referencing fails.
//
EX_PUSH_LOCK ExpCallBackFlush;

//
// Debug flag to force certain code paths. Let it get optimized away on free builds.
//
#if DBG

BOOLEAN ExpCallBackReturnRefs = FALSE;

#else

const
BOOLEAN ExpCallBackReturnRefs = FALSE;

#endif

//
// Address of callback object type descriptor.
//

POBJECT_TYPE ExCallbackObjectType;

//
// Structure that describes the mapping of generic access rights to object
// specific access rights for callback objects.
//

#ifdef ALLOC_DATA_PRAGMA
#pragma const_seg("INITCONST")
#endif
const GENERIC_MAPPING ExpCallbackMapping = {
    STANDARD_RIGHTS_READ ,
    STANDARD_RIGHTS_WRITE | CALLBACK_MODIFY_STATE,
    STANDARD_RIGHTS_EXECUTE | SYNCHRONIZE,
    CALLBACK_ALL_ACCESS
};
#ifdef ALLOC_DATA_PRAGMA
#pragma const_seg()
#endif

//
// Executive callback object structure definition.
//

typedef struct _CALLBACK_OBJECT {
    ULONG               Signature;
    KSPIN_LOCK          Lock;
    LIST_ENTRY          RegisteredCallbacks;
    BOOLEAN             AllowMultipleCallbacks;
    UCHAR               reserved[3];
} CALLBACK_OBJECT , *PCALLBACK_OBJECT;

//
// Executive callback registration structure definition.
//

typedef struct _CALLBACK_REGISTRATION {
    LIST_ENTRY          Link;
    PCALLBACK_OBJECT    CallbackObject;
    PCALLBACK_FUNCTION  CallbackFunction;
    PVOID               CallbackContext;
    ULONG               Busy;
    BOOLEAN             UnregisterWaiting;
} CALLBACK_REGISTRATION , *PCALLBACK_REGISTRATION;


VOID
ExpDeleteCallback (
    IN PCALLBACK_OBJECT     CallbackObject
    );

#ifdef ALLOC_PRAGMA
#pragma alloc_text(INIT, ExpInitializeCallbacks)
#pragma alloc_text(PAGE, ExCreateCallback)
#pragma alloc_text(PAGE, ExpDeleteCallback)
#pragma alloc_text(PAGE, ExInitializeCallBack)
#pragma alloc_text(PAGE, ExCompareExchangeCallBack)
#pragma alloc_text(PAGE, ExCallCallBack)
#pragma alloc_text(PAGE, ExFreeCallBack)
#pragma alloc_text(PAGE, ExAllocateCallBack)
#pragma alloc_text(PAGE, ExReferenceCallBackBlock)
#pragma alloc_text(PAGE, ExGetCallBackBlockRoutine)
#pragma alloc_text(PAGE, ExWaitForCallBacks)
#pragma alloc_text(PAGE, ExGetCallBackBlockContext)
#pragma alloc_text(PAGE, ExDereferenceCallBackBlock)
#endif

BOOLEAN
ExpInitializeCallbacks (
    )

/*++

Routine Description:

    This function creates the callback object type descriptor at system
    initialization and stores the address of the object type descriptor
    in local static storage.

Arguments:

    None.

Return Value:

    A value of TRUE is returned if the timer object type descriptor is
    successfully initialized. Otherwise a value of FALSE is returned.

--*/

{
    OBJECT_TYPE_INITIALIZER ObjectTypeInitializer;
    OBJECT_ATTRIBUTES ObjectAttributes;
    NTSTATUS Status;
    UNICODE_STRING unicodeString;
    ULONG           i;
    HANDLE          handle;

    //
    // Initialize the slow referencing lock
    //
    ExInitializePushLock (&ExpCallBackFlush);

    //
    // Initialize string descriptor.
    //

    RtlInitUnicodeString(&unicodeString, L"Callback");

    //
    // Create timer object type descriptor.
    //

    RtlZeroMemory(&ObjectTypeInitializer, sizeof(ObjectTypeInitializer));
    ObjectTypeInitializer.Length = sizeof(ObjectTypeInitializer);
    ObjectTypeInitializer.InvalidAttributes = OBJ_OPENLINK;
    ObjectTypeInitializer.GenericMapping = ExpCallbackMapping;
    ObjectTypeInitializer.DeleteProcedure = ExpDeleteCallback;
    ObjectTypeInitializer.PoolType = NonPagedPool;
    ObjectTypeInitializer.ValidAccessMask = CALLBACK_ALL_ACCESS;
    Status = ObCreateObjectType(&unicodeString,
                                &ObjectTypeInitializer,
                                (PSECURITY_DESCRIPTOR)NULL,
                                &ExCallbackObjectType);

    if (!NT_SUCCESS(Status)) {
        return FALSE;
    }

    RtlInitUnicodeString( &unicodeString, ExpWstrCallback );
    InitializeObjectAttributes(
        &ObjectAttributes,
        &unicodeString,
        OBJ_CASE_INSENSITIVE | OBJ_PERMANENT,
        NULL,
        SePublicDefaultSd
        );

    Status = NtCreateDirectoryObject(
                &handle,
                DIRECTORY_ALL_ACCESS,
                &ObjectAttributes
            );

    if (!NT_SUCCESS(Status)) {
        return FALSE;
    }

    NtClose (handle);

    //
    // Initialize event to wait on for Unregisters which occur while
    // notifications are in progress
    //

    KeInitializeEvent (&ExpCallbackEvent, NotificationEvent, 0);

    //
    // Initialize NT global callbacks
    //

    for (i=0; ExpInitializeCallback[i].CallBackObject; i++) {

        //
        // Create named calledback
        //

        RtlInitUnicodeString(&unicodeString, ExpInitializeCallback[i].CallbackName);


        InitializeObjectAttributes(
            &ObjectAttributes,
            &unicodeString,
            OBJ_PERMANENT | OBJ_CASE_INSENSITIVE,
            NULL,
            NULL
        );

        Status = ExCreateCallback (
                        ExpInitializeCallback[i].CallBackObject,
                        &ObjectAttributes,
                        TRUE,
                        TRUE
                        );

        if (!NT_SUCCESS(Status)) {
            return FALSE;
        }
    }

    return TRUE;
}

NTSTATUS
ExCreateCallback (
    OUT PCALLBACK_OBJECT * CallbackObject,
    IN POBJECT_ATTRIBUTES ObjectAttributes,
    IN BOOLEAN Create,
    IN BOOLEAN AllowMultipleCallbacks
    )

/*++

Routine Description:

    This function opens a callback object with the specified callback
    object. If the callback object does not exist or it is a NULL then
    a callback object will be created if create is TRUE. If a callbackobject
    is created it will only support multiple registered callbacks if
    AllowMulitipleCallbacks is TRUE.

Arguments:

    CallbackObject - Supplies a pointer to a variable that will receive the
        Callback object.

    CallbackName  - Supplies a pointer to a object name that will receive the

    Create - Supplies a flag which indicates whether a callback object will
        be created or not .

    AllowMultipleCallbacks - Supplies a flag which indicates only support
        mulitiple registered callbacks.

Return Value:

    NTSTATUS.

--*/

{
    PCALLBACK_OBJECT cbObject;
    NTSTATUS Status;
    HANDLE Handle;

    PAGED_CODE();

    //
    // Initializing cbObject & Handle is not needed for correctness but without
    // it the compiler cannot compile this code W4 to check for use of
    // uninitialized variables.
    //

    Handle = NULL;
    cbObject = NULL;

    //
    // If named callback, open handle to it
    //

    if (ObjectAttributes->ObjectName) {
        Status = ObOpenObjectByName(ObjectAttributes,
                                    ExCallbackObjectType,
                                    KernelMode,
                                    NULL,
                                    0,   // DesiredAccess,
                                    NULL,
                                    &Handle);
    } else {
        Status = STATUS_UNSUCCESSFUL;
    }

    //
    // If not opened, check if callback should be created
    //

    if (!NT_SUCCESS(Status) && Create ) {

        Status = ObCreateObject(KernelMode,
                                ExCallbackObjectType,
                                ObjectAttributes,
                                KernelMode,
                                NULL,
                                sizeof(CALLBACK_OBJECT),
                                0,
                                0,
                                (PVOID *)&cbObject );

        if(NT_SUCCESS(Status)){

            //
            // Fill in structure signature
            //

            cbObject->Signature = 'llaC';

            //
            // It will support multiple registered callbacks if
            // AllowMultipleCallbacks is TRUE.
            //

            cbObject->AllowMultipleCallbacks = AllowMultipleCallbacks;

            //
            // Initialize CallbackObject queue.
            //

            InitializeListHead( &cbObject->RegisteredCallbacks );

            //
            // Initialize spinlock
            //

            KeInitializeSpinLock (&cbObject->Lock);


            //
            // Put the object in the root directory
            //

            Status = ObInsertObject (
                     cbObject,
                     NULL,
                     FILE_READ_DATA,
                     0,
                     NULL,
                     &Handle );

        }

    }

    if(NT_SUCCESS(Status)){

        //
        // Add one to callback object reference count.
        //

        Status = ObReferenceObjectByHandle (
                    Handle,
                    0,          // DesiredAccess
                    ExCallbackObjectType,
                    KernelMode,
                    &cbObject,
                    NULL
                    );

        ZwClose (Handle);
    }

    //
    // If success, returns a referenced pointer to the CallbackObject.
    //

    if (NT_SUCCESS(Status)) {
        *CallbackObject = cbObject;
    }

    return Status;
}

VOID
ExpDeleteCallback (
    IN PCALLBACK_OBJECT     CallbackObject
    )
{
#if !DBG
    UNREFERENCED_PARAMETER (CallbackObject);
#endif

    ASSERT (IsListEmpty(&CallbackObject->RegisteredCallbacks));
}

PVOID
ExRegisterCallback (
    IN PCALLBACK_OBJECT   CallbackObject,
    IN PCALLBACK_FUNCTION CallbackFunction,
    IN PVOID CallbackContext
    )

/*++

Routine Description:

    This routine allows a caller to register that it would like to have its
    callback Function invoked when the callback notification call occurs.

Arguments:

    CallbackObject - Supplies a pointer to a CallbackObject.

    CallbackFunction - Supplies a pointer to a function which is to
        be executed when the Callback notification occures.

    CallbackContext - Supplies a pointer to an arbitrary data structure
        that will be passed to the function specified by the CallbackFunction
        parameter.

Return Value:

    Returns handle to callback registration.

--*/
{
    PCALLBACK_REGISTRATION  CallbackRegistration;
    BOOLEAN                 Inserted;
    KIRQL                   OldIrql;

    ASSERT (CallbackFunction);
    ASSERT (KeGetCurrentIrql() < DISPATCH_LEVEL);

    //
    // Add reference to object
    //

    ObReferenceObject (CallbackObject);

    //
    // Begin by attempting to allocate storage for the CallbackRegistration.
    // one cannot be allocated, return the error status.
    //

    CallbackRegistration = ExAllocatePoolWithTag(
                                NonPagedPool,
                                sizeof( CALLBACK_REGISTRATION ),
                                'eRBC'
                                );


    if( !CallbackRegistration ) {
       ObDereferenceObject (CallbackObject);
       return NULL;
    }


    //
    // Initialize the callback packet
    //

    CallbackRegistration->CallbackObject    = CallbackObject;
    CallbackRegistration->CallbackFunction  = CallbackFunction;
    CallbackRegistration->CallbackContext   = CallbackContext;
    CallbackRegistration->Busy              = 0;
    CallbackRegistration->UnregisterWaiting = FALSE;


    Inserted = FALSE;
    KeAcquireSpinLock (&CallbackObject->Lock, &OldIrql);

    if( CallbackObject->AllowMultipleCallbacks ||
        IsListEmpty( &CallbackObject->RegisteredCallbacks ) ) {

       //
       // add CallbackRegistration to tail
       //


       Inserted = TRUE;
       InsertTailList( &CallbackObject->RegisteredCallbacks,
                       &CallbackRegistration->Link );
    }

    KeReleaseSpinLock (&CallbackObject->Lock, OldIrql);

    if (!Inserted) {
       ExFreePool (CallbackRegistration);
       CallbackRegistration = NULL;
    }

    return (PVOID) CallbackRegistration;
}


VOID
ExUnregisterCallback (
    IN PVOID CbRegistration
    )

/*++

Routine Description:

    This function removes the callback registration for the callbacks
    from the list of callback object .

Arguments:

    CallbackRegistration - Pointer to device object for the file system.

Return Value:

    None.

--*/

{
    PCALLBACK_REGISTRATION  CallbackRegistration;
    PCALLBACK_OBJECT        CallbackObject;
    KIRQL                   OldIrql;

    ASSERT (KeGetCurrentIrql() < DISPATCH_LEVEL);

    CallbackRegistration = (PCALLBACK_REGISTRATION) CbRegistration;
    CallbackObject = CallbackRegistration->CallbackObject;

    KeAcquireSpinLock (&CallbackObject->Lock, &OldIrql);

    //
    // Wait for registration
    //

    while (CallbackRegistration->Busy) {

        //
        // Set waiting flag, then wait.  (not performance critical - use
        // single global event to wait for any and all unregister waits)
        //

        CallbackRegistration->UnregisterWaiting = TRUE;
        KeClearEvent (&ExpCallbackEvent);
        KeReleaseSpinLock (&CallbackObject->Lock, OldIrql);

        KeWaitForSingleObject (
            &ExpCallbackEvent,
            Executive,
            KernelMode,
            FALSE,
            NULL
        );

        //
        // Synchronize with callback object and recheck registration busy
        //

        KeAcquireSpinLock (&CallbackObject->Lock, &OldIrql);
    }

    //
    // Registration not busy, remove it from the callback object
    //

    RemoveEntryList (&CallbackRegistration->Link);
    KeReleaseSpinLock (&CallbackObject->Lock, OldIrql);

    //
    // Free memory used for CallbackRegistration
    //

    ExFreePool (CallbackRegistration);

    //
    // Remove reference count on CallbackObject
    //

    ObDereferenceObject (CallbackObject);
}

VOID
ExNotifyCallback (
    IN PCALLBACK_OBJECT     CallbackObject,
    IN PVOID                Argument1,
    IN PVOID                Argument2
    )

/*++

Routine Description:

    This function notifies all registered callbacks .

Arguments:

    CallbackObject - supplies a pointer to the callback object should be
            notified.

    SystemArgument1 - supplies a pointer will be passed to callback function.

    SystemArgument2 - supplies a pointer will be passed to callback function.

Return Value:

    None.

--*/

{
    PLIST_ENTRY             Link;
    PCALLBACK_REGISTRATION  CallbackRegistration;
    KIRQL                   OldIrql;

    if (CallbackObject == NULL) {
        return ;
    }

    //
    // Synchronize with callback object
    //

    KeAcquireSpinLock (&CallbackObject->Lock, &OldIrql);

    //
    // call registered callbacks at callers IRQL level
    // ( done if FIFO order of registration )
    //

    if (OldIrql == DISPATCH_LEVEL) {

        //
        // OldIrql is DISPATCH_LEVEL, just invoke all callbacks without
        // releasing the lock
        //

        for (Link = CallbackObject->RegisteredCallbacks.Flink;
             Link != &CallbackObject->RegisteredCallbacks;
             Link = Link->Flink) {

            //
            // Get current registration to notify
            //

            CallbackRegistration = CONTAINING_RECORD (Link,
                                                      CALLBACK_REGISTRATION,
                                                      Link);

            //
            // Notify reigstration
            //

            CallbackRegistration->CallbackFunction(
                       CallbackRegistration->CallbackContext,
                       Argument1,
                       Argument2
                       );

        }   // next registration

    } else {

        //
        // OldIrql is < DISPATCH_LEVEL, the code being called may be pagable
        // and the callback object spinlock needs to be released around
        // each registration callback.
        //

        for (Link = CallbackObject->RegisteredCallbacks.Flink;
             Link != &CallbackObject->RegisteredCallbacks;
             Link = Link->Flink ) {

            //
            // Get current registration to notify
            //

            CallbackRegistration = CONTAINING_RECORD (Link,
                                                      CALLBACK_REGISTRATION,
                                                      Link);

            //
            // If registration is being removed, don't bothing calling it
            //

            if (!CallbackRegistration->UnregisterWaiting) {

                //
                // Set registration busy
                //

                CallbackRegistration->Busy += 1;

                //
                // Release SpinLock and notify this callback
                //

                KeReleaseSpinLock (&CallbackObject->Lock, OldIrql);

                CallbackRegistration->CallbackFunction(
                           CallbackRegistration->CallbackContext,
                           Argument1,
                           Argument2
                           );

                //
                // Synchronize with CallbackObject
                //

                KeAcquireSpinLock (&CallbackObject->Lock, &OldIrql);

                //
                // Remove our busy count
                //

                CallbackRegistration->Busy -= 1;

                //
                // If the registriation removal is pending, kick global
                // event let unregister conitnue
                //

                if (CallbackRegistration->UnregisterWaiting  &&
                    CallbackRegistration->Busy == 0) {
                    KeSetEvent (&ExpCallbackEvent, 0, FALSE);
                }
            }
        }
    }


    //
    // Release callback
    //

    KeReleaseSpinLock (&CallbackObject->Lock, OldIrql);
}

VOID
ExInitializeCallBack (
    IN OUT PEX_CALLBACK CallBack
    )
/*++

Routine Description:

    This function initializes a low overhead callback.

Arguments:

    CallBack - Pointer to the callback structure

Return Value:

    None.

--*/
{
    ExFastRefInitialize (&CallBack->RoutineBlock, NULL);
}


PEX_CALLBACK_ROUTINE_BLOCK
ExAllocateCallBack (
    IN PEX_CALLBACK_FUNCTION Function,
    IN PVOID Context
    )
/*++

Routine Description:

    This function allocates a low overhead callback.

Arguments:

    Function - Routine to issue callbacks to
    Context  - A context value to issue

Return Value:

    PEX_CALLBACK_ROUTINE_BLOCK - Allocated block or NULL if allocation fails.

--*/
{
    PEX_CALLBACK_ROUTINE_BLOCK NewBlock;

    NewBlock = ExAllocatePoolWithTag (PagedPool,
                                      sizeof (EX_CALLBACK_ROUTINE_BLOCK),
                                      'brbC');
    if (NewBlock != NULL) {
        NewBlock->Function = Function;
        NewBlock->Context = Context;
        ExInitializeRundownProtection (&NewBlock->RundownProtect);
    }
    return NewBlock;
}

VOID
ExFreeCallBack (
    IN PEX_CALLBACK_ROUTINE_BLOCK CallBackBlock
    )
/*++

Routine Description:

    This function destroys a low overhead callback block.

Arguments:

    CallBackBlock - Call back block to destroy

Return Value:

    None.

--*/
{

    ExFreePool (CallBackBlock);
}

VOID
ExWaitForCallBacks (
    IN PEX_CALLBACK_ROUTINE_BLOCK CallBackBlock
    )
/*++

Routine Description:

    This function waits for all outcalls on the specified
    callback block to complete

Arguments:

    CallBackBlock - Call back block to wait for

Return Value:

    None.

--*/
{
    //
    // Wait for all active callbacks to be finished.
    //
    ExWaitForRundownProtectionRelease (&CallBackBlock->RundownProtect);
}


BOOLEAN
ExCompareExchangeCallBack (
    IN OUT PEX_CALLBACK CallBack,
    IN PEX_CALLBACK_ROUTINE_BLOCK NewBlock,
    IN PEX_CALLBACK_ROUTINE_BLOCK OldBlock
    )
/*++

Routine Description:

    This function assigns, removes or swaps a low overhead callback function.

Arguments:

    CallBack - Callback structure to be modified

    NewBlock - New block to be installed in the callback

    OldBlock - The old block that must be there now to be replaced

Return Value:

    BOOLEAN - TRUE: The swap occured, FALSE: The swap failed

--*/
{
    EX_FAST_REF OldRef;
    PEX_CALLBACK_ROUTINE_BLOCK ReplacedBlock;

    if (NewBlock != NULL) {
        //
        // Add the additional references to the routine block
        //
        if (!ExAcquireRundownProtectionEx (&NewBlock->RundownProtect,
                                           ExFastRefGetAdditionalReferenceCount () + 1)) {
            ASSERTMSG ("Callback block is already undergoing rundown", FALSE);
            return FALSE;
        }
    }

    //
    // Attempt to replace the existing object and balance all the reference counts
    //
    OldRef = ExFastRefCompareSwapObject (&CallBack->RoutineBlock,
                                         NewBlock,
                                         OldBlock);

    ReplacedBlock = ExFastRefGetObject (OldRef);

    //
    // See if the swap occured. If it didn't undo the original references we added.
    // If it did then release remaining references on the original
    //
    if (ReplacedBlock == OldBlock) {
        PKTHREAD CurrentThread;
        //
        // We need to flush out any slow referencers at this point. We do this by
        // acquiring and releasing a lock.
        //
        if (ReplacedBlock != NULL) {
            CurrentThread = KeGetCurrentThread ();

            KeEnterCriticalRegionThread (CurrentThread);

            ExAcquireReleasePushLockExclusive (&ExpCallBackFlush);

            KeLeaveCriticalRegionThread (CurrentThread);

            ExReleaseRundownProtectionEx (&ReplacedBlock->RundownProtect,
                                          ExFastRefGetUnusedReferences (OldRef) + 1);

        }
        return TRUE;
    } else {
        //
        // The swap failed. Remove the addition references if we had added any.
        //
        if (NewBlock != NULL) {
            ExReleaseRundownProtectionEx (&NewBlock->RundownProtect,
                                          ExFastRefGetAdditionalReferenceCount () + 1);
        }
        return FALSE;
    }
}

PEX_CALLBACK_ROUTINE_BLOCK
ExReferenceCallBackBlock (
    IN OUT PEX_CALLBACK CallBack
    )
/*++

Routine Description:

    This function takes a reference on the call back block inside the
    callback structure.

Arguments:

    CallBack - Call back to obtain the call back block from

Return Value:

    PEX_CALLBACK_ROUTINE_BLOCK - Referenced structure or NULL if these wasn't one

--*/
{
    EX_FAST_REF OldRef;
    PEX_CALLBACK_ROUTINE_BLOCK CallBackBlock;

    //
    // Get a reference to the callback block if we can.
    //
    OldRef = ExFastReference (&CallBack->RoutineBlock);

    //
    // If there is no callback then return
    //
    if (ExFastRefObjectNull (OldRef)) {
        return NULL;
    }
    //
    // If we didn't get a reference then use a lock to get one.
    //
    if (!ExFastRefCanBeReferenced (OldRef)) {
        PKTHREAD CurrentThread;
        CurrentThread = KeGetCurrentThread ();

        KeEnterCriticalRegionThread (CurrentThread);

        ExAcquirePushLockExclusive (&ExpCallBackFlush);

        CallBackBlock = ExFastRefGetObject (CallBack->RoutineBlock);
        if (CallBackBlock && !ExAcquireRundownProtection (&CallBackBlock->RundownProtect)) {
            CallBackBlock = NULL;
        }

        ExReleasePushLockExclusive (&ExpCallBackFlush);

        KeLeaveCriticalRegionThread (CurrentThread);

        if (CallBackBlock == NULL) {
            return NULL;
        }

    } else {
        CallBackBlock = ExFastRefGetObject (OldRef);

        //
        // If we just removed the last reference then attempt fix it up.
        //
        if (ExFastRefIsLastReference (OldRef) && !ExpCallBackReturnRefs) {
            ULONG RefsToAdd;

            RefsToAdd = ExFastRefGetAdditionalReferenceCount ();

            //
            // If we can't add the references then just give up
            //
            if (ExAcquireRundownProtectionEx (&CallBackBlock->RundownProtect,
                                              RefsToAdd)) {
                //
                // Repopulate the cached refs. If this fails we just give them back.
                //
                if (!ExFastRefAddAdditionalReferenceCounts (&CallBack->RoutineBlock,
                                                            CallBackBlock,
                                                            RefsToAdd)) {
                    ExReleaseRundownProtectionEx (&CallBackBlock->RundownProtect,
                                                  RefsToAdd);
                }
            }
        }
    }

    return CallBackBlock;
}

PEX_CALLBACK_FUNCTION
ExGetCallBackBlockRoutine (
    IN PEX_CALLBACK_ROUTINE_BLOCK CallBackBlock
    )
/*++

Routine Description:

    This function gets the routine associated with a call back block

Arguments:

    CallBackBlock - Call back block to obtain routine for

Return Value:

    PEX_CALLBACK_FUNCTION - The function pointer associated with this block

--*/
{
    return CallBackBlock->Function;
}

PVOID
ExGetCallBackBlockContext (
    IN PEX_CALLBACK_ROUTINE_BLOCK CallBackBlock
    )
/*++

Routine Description:

    This function gets the context associated with a call back block

Arguments:

    CallBackBlock - Call back block to obtain context for

Return Value:

    PVOID - The context associated with this block

--*/
{
    return CallBackBlock->Context;
}


VOID
ExDereferenceCallBackBlock (
    IN OUT PEX_CALLBACK CallBack,
    IN PEX_CALLBACK_ROUTINE_BLOCK CallBackBlock
    )
/*++

Routine Description:

    This returns a reference previous obtained on a call back block

Arguments:

    CallBackBlock - Call back block to return reference to

Return Value:

    None

--*/
{
    if (ExpCallBackReturnRefs || !ExFastRefDereference (&CallBack->RoutineBlock, CallBackBlock)) {
        ExReleaseRundownProtection (&CallBackBlock->RundownProtect);
    }
}


NTSTATUS
ExCallCallBack (
    IN OUT PEX_CALLBACK CallBack,
    IN PVOID Argument1,
    IN PVOID Argument2
    )
/*++

Routine Description:

    This function calls the callback thats inside a callback structure

Arguments:

    CallBack - Call back that needs to be called through

    Argument1 - Caller provided argument to pass on

    Argument2 - Caller provided argument to pass on

Return Value:

    NTSTATUS - Status returned by callback or STATUS_SUCCESS if theres wasn't one

--*/
{
    PEX_CALLBACK_ROUTINE_BLOCK CallBackBlock;
    NTSTATUS Status;

    CallBackBlock = ExReferenceCallBackBlock (CallBack);
    if (CallBackBlock) {
        //
        // Call the function
        //
        Status = CallBackBlock->Function (CallBackBlock->Context, Argument1, Argument2);

        ExDereferenceCallBackBlock (CallBack, CallBackBlock);
    } else {
        Status = STATUS_SUCCESS;
    }
    return Status;
}