From bfcfa4f6276935ed28b93642d9789e04294a78cf Mon Sep 17 00:00:00 2001
From: Nico de Poel <ndepoel@gmail.com>
Date: Tue, 18 Mar 2025 19:58:05 +0100
Subject: [PATCH] Avoid locking the mutex and blocking the main/render thread
 on destroy at the same time, which could cause deadlocks under some
 circumstances.

---
 FSR3UnityPlugin.cpp | 40 ++++++++++++++++++++++++++++++----------
 1 file changed, 30 insertions(+), 10 deletions(-)

diff --git a/FSR3UnityPlugin.cpp b/FSR3UnityPlugin.cpp
index 77c5c83..17a44cd 100644
--- a/FSR3UnityPlugin.cpp
+++ b/FSR3UnityPlugin.cpp
@@ -292,18 +292,27 @@ extern "C" bool UNITY_INTERFACE_EXPORT UNITY_INTERFACE_API AMDUP_InitApi()
 
 static void DestroyFeature(uint32_t featureSlot)
 {
-    auto& feature = s_Features[featureSlot];
-    if (feature.upscalingContext == nullptr && !feature.isValid)
-        return;
+    uint64_t dispatchFrameValue = 0;
+    {
+        // We need to lock the features list while we're accessing it, but we also can't hold the lock while blocking the main/render thread at the same time.
+        // Otherwise we're very likely to create a deadlock. So instead we only lock for as long as is necessary to grab the data we need from the feature.
+        std::lock_guard<std::mutex> lock(s_FeatureMutex);
+
+        auto& feature = s_Features[featureSlot];
+        if (feature.upscalingContext == nullptr && !feature.isValid)
+            return;
+
+        dispatchFrameValue = feature.dispatchFrameValue;
+    }
 
     if (s_GraphicsD3D12 != nullptr)
     {
         // If there's still an upscale dispatch executing on the current frame, wait until rendering is finished before destroying its context.
         // Thanks to https://alextardif.com/D3D11To12P1.html for explaining how fences work and how to make the CPU wait for a command queue to complete.
         ID3D12Fence* frameFence = s_GraphicsD3D12->GetFrameFence();
-        if (feature.dispatchFrameValue > frameFence->GetCompletedValue())
+        if (dispatchFrameValue > frameFence->GetCompletedValue())
         {
-            frameFence->SetEventOnCompletion(feature.dispatchFrameValue, s_FrameFenceEventHandle);
+            frameFence->SetEventOnCompletion(dispatchFrameValue, s_FrameFenceEventHandle);
             WaitForSingleObjectEx(s_FrameFenceEventHandle, INFINITE, false);
         }
     }
@@ -311,13 +320,16 @@ static void DestroyFeature(uint32_t featureSlot)
     {
         UnityVulkanRecordingState state;
         s_GraphicsVulkan->CommandRecordingState(&state, kUnityVulkanGraphicsQueueAccess_DontCare);
-        if (feature.dispatchFrameValue > state.safeFrameNumber)
+        if (dispatchFrameValue > state.safeFrameNumber)
         {
             UnityVulkanInstance instance = s_GraphicsVulkan->Instance();
             vkQueueWaitIdle(instance.graphicsQueue);
         }
     }
 
+    std::lock_guard<std::mutex> lock(s_FeatureMutex);
+
+    auto& feature = s_Features[featureSlot];
     if (feature.upscalingContext != nullptr)
     {
         s_ffxFunctions.DestroyContext(&feature.upscalingContext, nullptr);
@@ -332,13 +344,19 @@ static void DestroyFeature(uint32_t featureSlot)
 
 extern "C" void UNITY_INTERFACE_EXPORT UNITY_INTERFACE_API AMDUP_ShutdownApi()
 {
-    std::lock_guard<std::mutex> lock(s_FeatureMutex);
+    size_t numFeatures = 0;
+    {
+        std::lock_guard<std::mutex> lock(s_FeatureMutex);
+        numFeatures = s_Features.size();
+    }
 
-    for (uint32_t slot = 0; slot < s_Features.size(); ++slot)
+    for (uint32_t slot = 0; slot < numFeatures; ++slot)
     {
         DestroyFeature(slot);
     }
 
+    std::lock_guard<std::mutex> lock(s_FeatureMutex);
+
     if (s_ffxModule != nullptr)
     {
         FreeLibrary(s_ffxModule);
@@ -442,8 +460,6 @@ static FfxApiResource GetVulkanTextureResource(UnityVulkanInstance& instance, FS
 // Plugin function to handle a specific rendering event
 static void UNITY_INTERFACE_API OnRenderEventAndData(int eventID, void* data)
 {
-    std::lock_guard<std::mutex> lock(s_FeatureMutex);
-
     if (s_DX11Device == nullptr && s_DX12BackendDesc.device == nullptr && s_VulkanBackendDesc.vkDevice == nullptr)
         return;
 
@@ -461,6 +477,8 @@ static void UNITY_INTERFACE_API OnRenderEventAndData(int eventID, void* data)
     }
     case BaseEventId + FSR3PluginEvent::eExecute:
     {
+        std::lock_guard<std::mutex> lock(s_FeatureMutex);
+
         auto* params = (FSR3CommandExecutionData*)data;
         if (params->featureSlot < 0 || params->featureSlot >= s_Features.size())
             return;
@@ -598,6 +616,8 @@ static void UNITY_INTERFACE_API OnRenderEventAndData(int eventID, void* data)
     }
     case BaseEventId + FSR3PluginEvent::eInit:
     {
+        std::lock_guard<std::mutex> lock(s_FeatureMutex);
+
         auto* params = (FSR3CommandInitializationData*)data;
         if (params->featureSlot < 0 || params->featureSlot >= s_Features.size())
             return;