rundown.c

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#include "../../includes/me.h"
#include "../../intrinsics/atomic.h"
 
#define TEARDOWN_ACTIVE (1ULL << 63)
#define REFERENCE_COUNT (0x7FFFFFFFFFFFFFFF)
 
bool 
MsAcquireRundownProtection (
    IN    PRUNDOWN_REF rundown
) 
 
/*++
 
    Routine description : Safely acquires rundown protection on a shared resource to prevent it from being deleted or "rundown" while in use.
 
    Arguments:
 
        Pointer to RUNDOWN_REF Object.
 
    Return Values:
 
        True - The rundown protection acquisition has succeeded, the object is safe from memory deletion.
        False - Teardown has started on the object, handle gracefully.
 
--*/
 
{
    uint64_t expected, desired;
    do {
        expected = __atomic_load_n(&rundown->Count, __ATOMIC_SEQ_CST);
 
        // If teardown has started we refuse.
        if (expected & TEARDOWN_ACTIVE) return false;
 
        desired = expected + 1;
    } while (!InterlockedCompareExchangeU64_bool(&rundown->Count, desired, &expected));
    return true;
}
 
void 
MsReleaseRundownProtection (
    IN    PRUNDOWN_REF rundown
) 
 
/*++
 
    Routine description : Releases rundown protection from the object.
 
    Arguments:
 
        Pointer to RUNDOWN_REF Object.
 
    Return Values:
 
        None.
 
--*/
 
{
    InterlockedDecrementU64(&rundown->Count);
}
 
// Wait for rundown (teardown)
void MsWaitForRundownProtectionRelease (
    IN    PRUNDOWN_REF rundown
) 
 
/*++
 
    Routine description : Waits until all rundown protections have been released from the object, then starts Teardown.
    Use this when you want to gurantee an object will not be used after free.
 
    Arguments:
 
        Pointer to RUNDOWN_REF Object.
 
    Return Values:
 
        None.
 
--*/
 
{
    uint64_t expected = __atomic_load_n(&rundown->Count, __ATOMIC_SEQ_CST);
    for (;;) {
        uint64_t desired = expected | TEARDOWN_ACTIVE;
 
        // try to set TEARDOWN_ACTIVE while preserving the refcount bits
        if (InterlockedCompareExchangeU64_bool(&rundown->Count, desired, &expected)) {
            // success — we hold the TEARDOWN_ACTIVE marker now
            break;
        }
 
        if (expected & TEARDOWN_ACTIVE) {
            // Another thread set teardown already
            break;
        }
        // otherwise loop and try again with updated expected
    }
 
    // Spin until no references remain 
    while ((__atomic_load_n(&rundown->Count, __ATOMIC_SEQ_CST) & REFERENCE_COUNT) != 0) {
        __pause();
    }
}