From 23288b7cb51902262dc3c8e608e3c04da5c46a03 Mon Sep 17 00:00:00 2001
From: Nikita Shulga <nikita.shulga@oculus.com>
Date: Mon, 7 Oct 2019 10:58:25 -0700
Subject: [PATCH] Use atomic operations to modify flagNestedParallelFor

This ensures uniform behavior on any C++11 compliant compiler
---
 modules/core/src/parallel.cpp | 14 +++++++++-----
 1 file changed, 9 insertions(+), 5 deletions(-)

diff --git a/modules/core/src/parallel.cpp b/modules/core/src/parallel.cpp
index 5b1c917..f4a0a75 100644
--- a/modules/core/src/parallel.cpp
+++ b/modules/core/src/parallel.cpp
@@ -134,6 +134,10 @@
 #  define CV_PARALLEL_FRAMEWORK "pthreads"
 #endif
 
+#ifdef CV_PARALLEL_FRAMEWORK
+#include <atomic>
+#endif
+
 #include "parallel_impl.hpp"
 
 #include "opencv2/core/detail/exception_ptr.hpp"  // CV__EXCEPTION_PTR = 1 if std::exception_ptr is available
@@ -487,20 +491,20 @@ void cv::parallel_for_(const cv::Range& range, const cv::ParallelLoopBody& body,
         return;
 
 #ifdef CV_PARALLEL_FRAMEWORK
-    static volatile int flagNestedParallelFor = 0;
-    bool isNotNestedRegion = flagNestedParallelFor == 0;
+    static std::atomic<bool> flagNestedParallelFor(false);
+    bool isNotNestedRegion = !flagNestedParallelFor.load();
     if (isNotNestedRegion)
-      isNotNestedRegion = CV_XADD(&flagNestedParallelFor, 1) == 0;
+      isNotNestedRegion = !flagNestedParallelFor.exchange(true);
     if (isNotNestedRegion)
     {
         try
         {
             parallel_for_impl(range, body, nstripes);
-            flagNestedParallelFor = 0;
+            flagNestedParallelFor = false;
         }
         catch (...)
         {
-            flagNestedParallelFor = 0;
+            flagNestedParallelFor = false;
             throw;
         }
     }
-- 
2.7.4