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# updateFence(_:)

Encodes a command that instructs the GPU to update a fence after the acceleration structure pass completes.

```
func updateFence(_ fence: any MTLFence)
```

## Parameters

`fence`

A fence the pass updates after it completes.

## Discussion

You can synchronize memory operations of an acceleration structure pass that access resources with an [`MTLFence`](/documentation/Metal/MTLFence).
This method instructs the pass to update `fence` after it runs all its memory store operations to the resources it accesses.
The fence indicates when other passes can access those resources without a race condition.

For more information about synchronization with fences, see:

- [Resource synchronization](/documentation/Metal/resource-synchronization)
- [Synchronizing passes with a fence](/documentation/Metal/synchronizing-passes-with-a-fence)

### Reuse a fence by waiting first and updating second

When encoding an acceleration structure pass that reuses a fence, wait for other passes
to update the fence before repurposing that fence to notify subsequent passes with an update:

1. Call the [`waitForFence(_:)`](/documentation/Metal/MTLAccelerationStructureCommandEncoder/waitForFence(_:)) method before encoding commands that need to wait for other passes.
2. Call the [`updateFence(_:)`](/documentation/Metal/MTLAccelerationStructureCommandEncoder/updateFence(_:)) method after encoding commands that later passes depend on.

The GPU driver evaluates the fences that apply to the pass and the commands that depend on those fences when your app commits the enclosing [`MTLCommandBuffer`](/documentation/Metal/MTLCommandBuffer).

> Warning:
> Don’t update a fence and then wait for the same fence within a pass because it can create a GPU deadlock.

---

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