Revert "Minor fixes on the MLIR ActionProfiler (NFC)"

This reverts commit 0502d83.

This introduces a race condition on the printComma variable.

Author: joker-eph

mlir/include/mlir/Debug/Observers/ActionProfiler.h

@@ -41,7 +41,7 @@ struct ActionProfiler : public ExecutionContext::Observer {
   std::chrono::time_point<std::chrono::steady_clock> startTime;
   bool printComma = false;
 
-  /// A mutex used to guard printing from multiple threads.
+  /// A mutex used to guard profiling.
   std::mutex mutex;
 };
 

mlir/lib/Debug/Observers/ActionProfiler.cpp

@@ -36,8 +36,6 @@ void ActionProfiler::print(const ActionActiveStack *action,
   // Create the event.
   std::string str;
   llvm::raw_string_ostream event(str);
-  if (printComma)
-    event << ",\n";
   event << "{";
   event << R"("name": ")" << action->getAction().getTag() << "\", ";
   event << R"("cat": "PERF", )";
@@ -54,11 +52,12 @@ void ActionProfiler::print(const ActionActiveStack *action,
     event << "\"}";
   }
   event << "}";
-  event.flush();
 
-  // Print the event, guard with a mutex to ensure the stream is correctly
-  // formed.
+  // Print the event.
   std::lock_guard<std::mutex> guard(mutex);
+  if (printComma)
+    os << ",\n";
   printComma = true;
   os << event.str();
+  os.flush();
 }