Fixed freeze when using multiple GPUs (#3668)

platforms/common/include/openmm/common/ComputeContext.h

@@ -9,7 +9,7 @@
  * Biological Structures at Stanford, funded under the NIH Roadmap for        *
  * Medical Research, grant U54 GM072970. See https://simtk.org.               *
  *                                                                            *
- * Portions copyright (c) 2019 Stanford University and the Authors.           *
+ * Portions copyright (c) 2019-2022 Stanford University and the Authors.      *
  * Authors: Peter Eastman                                                     *
  * Contributors:                                                              *
  *                                                                            *
@@ -555,6 +555,10 @@ public:
      * Get whether the worker thread has exited.
      */
     bool isFinished();
+    /**
+     * Get whether the thread invoking this method is the worker thread.
+     */
+    bool isCurrentThread();
     /**
      * Block until all tasks have finished executing and the worker thread is idle.
      */

platforms/common/src/CommonKernels.cpp

@@ -1812,6 +1812,7 @@ public:
     double computeForceAndEnergy(bool includeForces, bool includeEnergy, int groups) {
         if ((groups&(1<<force->getForceGroup())) == 0)
             return 0;
+        if (!cc.getWorkThread().isCurrentThread())
             cc.getWorkThread().flush();
         Vec3 a, b, c;
         cc.getPeriodicBoxVectors(a, b, c);

platforms/common/src/ComputeContext.cpp

@@ -6,7 +6,7 @@
  * Biological Structures at Stanford, funded under the NIH Roadmap for        *
  * Medical Research, grant U54 GM072970. See https://simtk.org.               *
  *                                                                            *
- * Portions copyright (c) 2019-2021 Stanford University and the Authors.      *
+ * Portions copyright (c) 2019-2022 Stanford University and the Authors.      *
  * Authors: Peter Eastman                                                     *
  * Contributors:                                                              *
  *                                                                            *
@@ -716,6 +716,10 @@ bool ComputeContext::WorkThread::isFinished() {
     return finished;
 }
 
+bool ComputeContext::WorkThread::isCurrentThread() {
+    return (pthread_self() == thread);
+}
+
 void ComputeContext::WorkThread::flush() {
     pthread_mutex_lock(&queueLock);
     while (!waiting)