From a49615e192d75a5be73c71c065b44b61bfb75493 Mon Sep 17 00:00:00 2001
From: assiduous <assiduous@diligentgraphics.com>
Date: Thu, 18 Feb 2021 16:07:52 -0800
Subject: Reworked FenceVkImpl to use atomics

---
 .../GraphicsEngineVulkan/include/FenceVkImpl.hpp   |  6 +++++-
 Graphics/GraphicsEngineVulkan/src/FenceVkImpl.cpp  | 22 ++++++++++++++--------
 2 files changed, 19 insertions(+), 9 deletions(-)

(limited to 'Graphics/GraphicsEngineVulkan')

diff --git a/Graphics/GraphicsEngineVulkan/include/FenceVkImpl.hpp b/Graphics/GraphicsEngineVulkan/include/FenceVkImpl.hpp
index ab8da006..303fe6cf 100644
--- a/Graphics/GraphicsEngineVulkan/include/FenceVkImpl.hpp
+++ b/Graphics/GraphicsEngineVulkan/include/FenceVkImpl.hpp
@@ -31,6 +31,8 @@
 /// Declaration of Diligent::FenceVkImpl class
 
 #include <deque>
+#include <atomic>
+
 #include "FenceVk.h"
 #include "FenceBase.hpp"
 #include "VulkanUtilities/VulkanFencePool.hpp"
@@ -76,9 +78,11 @@ public:
     void Wait(Uint64 Value);
 
 private:
+    inline void UpdateLastCompletedFenceValue(uint64_t NewValue);
+
     VulkanUtilities::VulkanFencePool                             m_FencePool;
     std::deque<std::pair<Uint64, VulkanUtilities::FenceWrapper>> m_PendingFences;
-    volatile Uint64                                              m_LastCompletedFenceValue = 0;
+    std::atomic_uint64_t                                         m_LastCompletedFenceValue{0};
 };
 
 } // namespace Diligent
diff --git a/Graphics/GraphicsEngineVulkan/src/FenceVkImpl.cpp b/Graphics/GraphicsEngineVulkan/src/FenceVkImpl.cpp
index 0d109b38..fa4c4536 100644
--- a/Graphics/GraphicsEngineVulkan/src/FenceVkImpl.cpp
+++ b/Graphics/GraphicsEngineVulkan/src/FenceVkImpl.cpp
@@ -64,6 +64,15 @@ FenceVkImpl::~FenceVkImpl()
     }
 }
 
+void FenceVkImpl::UpdateLastCompletedFenceValue(uint64_t NewValue)
+{
+    auto LastCompletedValue = m_LastCompletedFenceValue.load();
+    while (!m_LastCompletedFenceValue.compare_exchange_strong(LastCompletedValue, std::max(LastCompletedValue, NewValue)))
+    {
+        // If exchange fails, LastCompletedValue will hold the actual value of m_LastCompletedFenceValue.
+    }
+}
+
 Uint64 FenceVkImpl::GetCompletedValue()
 {
     const auto& LogicalDevice = m_pDevice->GetLogicalDevice();
@@ -74,8 +83,7 @@ Uint64 FenceVkImpl::GetCompletedValue()
         auto status = LogicalDevice.GetFenceStatus(Value_Fence.second);
         if (status == VK_SUCCESS)
         {
-            if (Value_Fence.first > m_LastCompletedFenceValue)
-                m_LastCompletedFenceValue = Value_Fence.first;
+            UpdateLastCompletedFenceValue(Value_Fence.first);
             m_FencePool.DisposeFence(std::move(Value_Fence.second));
             m_PendingFences.pop_front();
         }
@@ -85,14 +93,13 @@ Uint64 FenceVkImpl::GetCompletedValue()
         }
     }
 
-    return m_LastCompletedFenceValue;
+    return m_LastCompletedFenceValue.load();
 }
 
 void FenceVkImpl::Reset(Uint64 Value)
 {
-    DEV_CHECK_ERR(Value >= m_LastCompletedFenceValue, "Resetting fence '", m_Desc.Name, "' to the value (", Value, ") that is smaller than the last completed value (", m_LastCompletedFenceValue, ")");
-    if (Value > m_LastCompletedFenceValue)
-        m_LastCompletedFenceValue = Value;
+    DEV_CHECK_ERR(Value >= m_LastCompletedFenceValue.load(), "Resetting fence '", m_Desc.Name, "' to the value (", Value, ") that is smaller than the last completed value (", m_LastCompletedFenceValue, ")");
+    UpdateLastCompletedFenceValue(Value);
 }
 
 
@@ -115,8 +122,7 @@ void FenceVkImpl::Wait(Uint64 Value)
 
         DEV_CHECK_ERR(status == VK_SUCCESS, "All pending fences must now be complete!");
         (void)status;
-        if (val_fence.first > m_LastCompletedFenceValue)
-            m_LastCompletedFenceValue = val_fence.first;
+        UpdateLastCompletedFenceValue(val_fence.first);
         m_FencePool.DisposeFence(std::move(val_fence.second));
 
         m_PendingFences.pop_front();
-- 
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