Spin Locks

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Since IRQL is a per-CPU concept, it doesn't help you safeguard data against interference by code running on another processor in the same multiprocessor computer. A primitive object known as a spin lock serves that purpose. To acquire a spin lock, code on one CPU executes an atomic operation that tests and then sets some memory variable in such a way that no other CPU can access the variable until the operation completes. If the test indicates that the lock was previously free, the program continues. If the test indicates that the lock was previously busy, the program repeats the test-and-set in a tight loop: it "spins." Eventually the owner releases the lock by resetting the variable, whereupon one of the waiting CPUs' test-and-set operations will report the lock as free.

Two facts about spin locks are probably obvious but still worth stating. First of all, if a CPU already owns a spin lock and tries to obtain it a second time, the CPU will deadlock. No usage counter or owner identifier is associated with a spin lock; the lock is either owned by somebody or not. If you try to acquire it when it's owned, you will wait until the owner releases it. If your CPU happens to already be the owner, the code which would release the lock can never execute because you're spinning in a tight loop testing and setting the lock variable.

The second fact about spin locks is that no useful work occurs on a CPU that's waiting for a spin lock. Therefore, to avoid harming performance, you need to minimize the amount of work you do while holding a spin lock that some other CPU is likely to want.

There's another important fact about spin locks that's not obvious but still pretty important: you can only request a spin lock when you're running at or below DISPATCH_LEVEL, and the kernel will raise the IRQL to DISPATCH_LEVEL for the duration of your ownership of the lock. Internally, the kernel is able to acquire spin locks at an IRQL higher than DISPATCH_LEVEL, but you and I are unable to accomplish that feat.

Working with Spin Locks

To use a spin lock explicitly, allocate storage for a KSPIN_LOCK object in nonpaged memory. Then call KeInitializeSpinLock to initialize the object. Later, while running at or below DISPATCH_LEVEL, acquire the lock, perform the work that needs to be protected from interference, and then release the lock. For example, suppose that your device extension contains a spin lock named QLock that you use for guarding access to a special IRP queue you've set up. You'd initialize this lock in your AddDevice function:

typedef struct _DEVICE_EXTENSION { ... KSPIN_LOCK QLock; } DEVICE_EXTENSION, *PDEVICE_EXTENSION; ... NTSTATUS AddDevice(...) { ... PDEVICE_EXTENSION pdx = ...; KeInitializeSpinLock(&pdx->QLock); ... }

Elsewhere in your driver, say in the dispatch function for some type of IRP, you could claim (and quickly release) the lock around some queue manipulation that you needed to perform. Note that this function must be in nonpaged memory because it executes for some period of time at an elevated IRQL.

 1 2

NTSTATUS DispatchSomething(...) { KIRQL oldirql; PDEVICE_EXTENSION pdx = ...; KeAcquireSpinLock(&pdx->QLock, &oldirql); ... KeReleaseSpinLock(&pdx->QLock, oldirql); }

1. When KeAcquireSpinLock acquires the spin lock, it also raises IRQL to DISPATCH_LEVEL and returns the current (that is, preacquisition) level to us wherever the second argument points.
2. When KeReleaseSpinLock releases the spin lock, it also lowers IRQL back to the value specified in the second argument.

If you know you're already executing at DISPATCH_LEVEL, you can save a little time by calling two special routines. This technique is appropriate, for example, in DPC, StartIo, and other driver routines that execute at DISPATCH_LEVEL:

KeAcquireSpinLockAtDpcLevel(&pdx->QLock); ... KeReleaseSpinLockFromDpcLevel(&pdx->QLock);