Serialize GPU context disposal

PerfectNumbers.Core/Gpu/GpuContextPool.cs

@@ -114,11 +114,14 @@ public static class GpuContextPool
 			}
 		}
 
-		else
+                else
-		{
+                {
-			ctx.Dispose();
+                        lock (CreationLock)
-		}
+                        {
-	}
+                                ctx.Dispose();
+                        }
+                }
+        }
 
     public struct GpuContextLease : IDisposable
     {