Starting with Windows XP, run-down protection is available to kernel-mode drivers. Drivers can use run-down protection to safely access objects in shared system memory that are created and deleted by another kernel-mode driver.
An object is said to be run down if all outstanding accesses of the object are finished and no new requests to access the object will be granted. For example, a shared object might need to be run down so that it can be deleted and replaced with a new object.
The driver that owns the shared object can enable other drivers to acquire and release run-down protection on the object. When run-down protection is in effect, a driver other than the owner can access the object without risk that the owner will delete the object before the access completes. Before the access starts, the accessing driver requests run-down protection on the object. For a long-lived object, this request is nearly always granted. After the access finishes, the accessing driver releases its previously acquired run-down protection on the object.
Primary run-down protection routines
To start sharing an object, the driver that owns the object calls the ExInitializeRundownProtection routine to initialize run-down protection on the object. After this call, other drivers that access the object can acquire and release run-down protection on the object.
A driver that accesses the shared object calls the ExAcquireRundownProtection routine to request run-down protection on the object. After the access is finished, this driver calls the ExReleaseRundownProtection routine to release run-down protection on the object.
If the owning driver determines that the shared object must be deleted, this driver waits to delete the object until all outstanding accesses of the object are finished.
In preparation to delete the shared object, the owning driver calls the ExWaitForRundownProtectionRelease routine to wait for the object to run down. During this call, ExWaitForRundownProtectionRelease waits for all previously granted instances of run-down protection on the object to be released, but prevents new requests for run-down protection on the object from being granted. After the last protected access finishes and all instances of run-down protection are released, ExWaitForRundownProtectionRelease returns, and the owning driver can safely delete the object.
ExWaitForRundownProtectionRelease blocks the execution of the calling driver thread until all drivers that hold run-down protection on the shared object release this protection. To prevent ExWaitForRundownProtectionRelease from blocking execution for excessively long periods, drivers threads that access the shared object should avoid being suspended while they hold run-down protection on the object. For this reason, accessing drivers should call ExAcquireRundownProtection and ExReleaseRundownProtection within a critical region or guarded region, or while running at IRQL = APC_LEVEL.
Uses for run-down protection
Run-down protection is particularly useful for providing access to a shared object that is nearly always available but might occasionally need to be deleted and replaced. Drivers that access data or that call routines in this object must not try to access the object after it is deleted. Otherwise, these invalid accesses might cause unpredictable behavior, data corruption, or even system failure.
For example, an antivirus driver typically stays loaded in memory when the operating system is running. Occasionally, this driver might need to be unloaded and replaced with an updated release of the driver. Other drivers send I/O requests to the antivirus driver to access the data and routines in this driver. Before sending an I/O request, a kernel component, such as a file system filter manager, can acquire run-down protection to guard against premature unloading of the antivirus driver while it handles the I/O request. After the I/O request completes, run-down protection can be released.
Run-down protection does not serialize accesses to a shared object. If two or more accessing drivers can simultaneously hold run-down protection on an object, and accesses to the object must be serialized, some other mechanism, such as a mutual-exclusion lock, must be used to serialize the accesses.
The EX_RUNDOWN_REF structure
An EX_RUNDOWN_REF structure tracks the status of run-down protection on a shared object. This structure is opaque to drivers. The system-supplied run-down protection routines use this structure to count the number of instances of run-down protection that are currently in effect on the object. These routines also use this structure to track whether the object is run down or is in the process of being run down.
To start sharing an object, the driver that owns the object calls ExInitializeRundownProtection to initialize the EX_RUNDOWN_REF structure associated with the object. After initialization, the owning driver can make this structure available to other drivers that require access to the object. The accessing drivers pass this structure as a parameter to the ExAcquireRundownProtection and ExReleaseRundownProtection calls that acquire and release run-down protection on the object. The owning driver passes this structure as a parameter to the ExWaitForRundownProtectionRelease call that waits for the object to run down so that it can be safely deleted.
Comparison to locks
Run-down protection is one of several ways to guarantee safe access to a shared object. Another approach is to use a mutual-exclusion software lock. If a driver requires access to an object that is currently locked by another driver, the first driver must wait for the second driver to release the lock. However, acquiring and releasing locks can become a performance bottleneck, and locks can consume large amounts of memory. If used incorrectly, locks might cause drivers that compete for the same shared objects to become deadlocked. Efforts to detect and avoid deadlocks typically require the diversion of substantial computing resources.
In contrast to locks, run-down protection has relatively lightweight processing time and memory requirements. A simple reference count is associated with the object to ensure that deletion of the object is deferred until all outstanding accesses of the object are completed. With this approach, atomic, interlocked hardware instructions can be used instead of mutual-exclusion software locks to guarantee safe access to an object. Calls to acquire and release run-down protection are typically very fast. The benefits of using a lightweight mechanism, such as run-down protection, can be significant for a shared object that has a long life and is shared among many drivers.
Other run-down protection routines
Several other run-down protection routines are available, in addition to those that were mentioned previously. These additional routines might used by some drivers.
The ExRundownCompleted routine updates the EX_RUNDOWN_REF structure to indicate that the run down of the associated object has completed.
The ExAcquireRundownProtectionEx and ExReleaseRundownProtectionEx routines are similar to ExAcquireRundownProtection and ExReleaseRundownProtection. These four routines increment or decrement the count of the instances of run-down protection that are in effect on a shared object. Whereas ExAcquireRundownProtection and ExReleaseRundownProtection increment and decrement this count by one, ExAcquireRundownProtectionEx and ExReleaseRundownProtectionEx increment and decrement the count by arbitrary amounts.
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