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305 lines
7.6 KiB
305 lines
7.6 KiB
/*++
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Copyright (c) 2000 Microsoft Corporation
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Module Name:
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rundown.c
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Abstract:
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This module houses routines that do fast referencing of object manager
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objects. This is just a thin layer around the fast ref package in EX.
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The EX routines are all inline so their description is here.
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The basic principle of these routines is to allow fast referencing of
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objects held in pointers protected by locks. This is done by assuming
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the pointer to an object is aligned on a 8 byte boundary and using the
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bottom 3 bits of the pointer as a fast referencing mechanism. The
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assumption of this algorithm is that the pointer changes far less
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frequently than it is referenced.
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Given the following bit definition of a
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pointer:
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+-----------+---+
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| p | n |
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+-----------+---+
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p << 3 : Object pointer. Bottom three bits are zero. p may be null in
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which case n must be zero
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n : Total number of pre-references unused
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For a non-null p the total number of references on the target object
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associated with this structure is >= 1 + 7 - n. There is an associated
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reference for the pointer itself and one for each of the possible
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extra references.
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Fast references proceed to perform one of the following transformation:
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+-----------+---+ +-----------+-----+
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| p | n | => | p | n-1 | n > 0, p != NULL
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+-----------+---+ +-----------+-----+
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+-----------+---+ +-----------+---+
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| NULL | 0 | => | NULL | 0 | NULL pointers are never fast refed
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+-----------+---+ +-----------+---+ and never have cached references
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Slow references do the following transformation:
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+-----------+---+ +-----------+-----+
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| p | 0 | => | p | 7 | An addition 8 references are
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+-----------+---+ +-----------+-----+ added to the object
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The second transformation is either done under a lock or done by the
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thread that does the transition with n = 1 => n = 0.
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The reference obtained by this fast algorithm may be released by
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dereferencing the target object directly or by attempting to return the
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reference to the pointer. Returning the reference to the pointer has
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the following transformations:
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+-----------+---+ +-----------+-----+
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| p | n | => | p | n+1 | n < 7, p != NULL
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+-----------+---+ +-----------+-----+
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+-----------+---+ +-----------+-----+
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| p | 7 | => | p | 0 | Dereference the object directly
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+-----------+---+ +-----------+-----+
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+-----------+---+ +-----------+-----+ Dereference the object directly
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| q | n | => | q | n | as the pointer p has been
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+-----------+---+ +-----------+-----+ replaced by q. q May be NULL
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Author:
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Neill Clift (NeillC) 29-Jul-2000
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Revision History:
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--*/
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#include "obp.h"
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#pragma hdrstop
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#ifdef ALLOC_PRAGMA
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#pragma alloc_text(PAGE,ObInitializeFastReference)
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#pragma alloc_text(PAGE,ObFastReferenceObject)
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#pragma alloc_text(PAGE,ObFastReferenceObjectLocked)
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#pragma alloc_text(PAGE,ObFastDereferenceObject)
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#pragma alloc_text(PAGE,ObFastReplaceObject)
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#endif
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NTKERNELAPI
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VOID
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FASTCALL
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ObInitializeFastReference (
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IN PEX_FAST_REF FastRef,
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IN PVOID Object
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)
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/*++
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Routine Description:
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Initialize a fast reference structure.
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Arguments:
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FastRef - Rundown block to be initialized
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Return Value:
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None
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--*/
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{
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//
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// If an object was given then bias the object reference by the cache size.
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//
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if (Object != NULL) {
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ObReferenceObjectEx (Object, ExFastRefGetAdditionalReferenceCount ());
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}
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ExFastRefInitialize (FastRef, Object);
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}
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NTKERNELAPI
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PVOID
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FASTCALL
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ObFastReferenceObject (
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IN PEX_FAST_REF FastRef
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)
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/*++
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Routine Description:
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This routine attempts a fast reference of an object in a fast ref
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structure.
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Arguments:
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FastRef - Rundown block to be used for the reference
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Return Value:
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PVOID - Object that was referenced or NULL if we failed
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--*/
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{
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EX_FAST_REF OldRef;
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PVOID Object;
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ULONG RefsToAdd, Unused;
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//
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// Attempt the fast reference
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//
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OldRef = ExFastReference (FastRef);
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Object = ExFastRefGetObject (OldRef);
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//
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// We fail if there wasn't an object or if it has no cached references
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// left. Both of these cases had the cached reference count zero.
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//
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Unused = ExFastRefGetUnusedReferences (OldRef);
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if (Unused <= 1) {
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if (Unused == 0) {
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return NULL;
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}
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//
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// If we took the counter to zero then attempt to make life easier for
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// the next referencer by resetting the counter to its max. Since we now
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// have a reference to the object we can do this.
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//
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RefsToAdd = ExFastRefGetAdditionalReferenceCount ();
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ObReferenceObjectEx (Object, RefsToAdd);
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//
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// Try to add the added references to the cache. If we fail then just
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// release them.
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//
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if (!ExFastRefAddAdditionalReferenceCounts (FastRef, Object, RefsToAdd)) {
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ObDereferenceObjectEx (Object, RefsToAdd);
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}
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}
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return Object;
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}
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NTKERNELAPI
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PVOID
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FASTCALL
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ObFastReferenceObjectLocked (
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IN PEX_FAST_REF FastRef
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)
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/*++
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Routine Description:
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This routine does a slow object reference. This must be called while
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holding a lock.
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Arguments:
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FastRef - Rundown block to be used to reference the object
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Return Value:
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PVOID - Object that was referenced or NULL if there was no object.
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--*/
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{
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PVOID Object;
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EX_FAST_REF OldRef;
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OldRef = *FastRef;
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Object = ExFastRefGetObject (OldRef);
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if (Object != NULL) {
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ObReferenceObject (Object);
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}
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return Object;
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}
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NTKERNELAPI
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VOID
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FASTCALL
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ObFastDereferenceObject (
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IN PEX_FAST_REF FastRef,
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IN PVOID Object
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)
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/*++
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Routine Description:
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This routine does a fast dereference if possible.
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Arguments:
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FastRef - Rundown block to be used to dereference the object
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Return Value:
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None.
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--*/
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{
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if (!ExFastRefDereference (FastRef, Object)) {
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//
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// If the object changed or there is no space left in the reference
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// cache then just deref the object.
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//
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ObDereferenceObject (Object);
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}
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}
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NTKERNELAPI
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PVOID
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FASTCALL
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ObFastReplaceObject (
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IN PEX_FAST_REF FastRef,
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IN PVOID Object
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)
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/*++
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Routine Description:
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This routine does a swap of the object. This must be called while holding
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a lock.
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Arguments:
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FastRef - Rundown block to be used to do the swap.
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Return Value:
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PVOID - Object that was in the block before the swap..
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--*/
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{
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EX_FAST_REF OldRef;
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PVOID OldObject;
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ULONG RefsToReturn;
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//
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// If we have been given an object then bias it by the correct amount.
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//
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if (Object != NULL) {
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ObReferenceObjectEx (Object, ExFastRefGetAdditionalReferenceCount ());
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}
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//
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// Do the swap
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//
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OldRef = ExFastRefSwapObject (FastRef, Object);
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OldObject = ExFastRefGetObject (OldRef);
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//
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// If there was an original object then we need to work out how many
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// cached references there were (if any) and return them.
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//
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if (OldObject != NULL) {
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RefsToReturn = ExFastRefGetUnusedReferences (OldRef);
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if (RefsToReturn > 0) {
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ObDereferenceObjectEx (OldObject, RefsToReturn);
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}
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}
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return OldObject;
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}
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