Index: lldb/include/lldb/Utility/Log.h
===================================================================
--- lldb/include/lldb/Utility/Log.h
+++ lldb/include/lldb/Utility/Log.h
@@ -33,7 +33,6 @@
 class raw_ostream;
 }
 // Logging Options
-#define LLDB_LOG_OPTION_THREADSAFE (1u << 0)
 #define LLDB_LOG_OPTION_VERBOSE (1u << 1)
 #define LLDB_LOG_OPTION_PREPEND_SEQUENCE (1u << 3)
 #define LLDB_LOG_OPTION_PREPEND_TIMESTAMP (1u << 4)
@@ -50,10 +49,6 @@
 public:
   virtual ~LogHandler() = default;
   virtual void Emit(llvm::StringRef message) = 0;
-  void EmitThreadSafe(llvm::StringRef message);
-
-private:
-  std::mutex m_mutex;
 };
 
 class StreamLogHandler : public LogHandler {
@@ -62,8 +57,10 @@
   ~StreamLogHandler() override;
 
   void Emit(llvm::StringRef message) override;
+  void Flush();
 
 private:
+  std::mutex m_mutex;
   llvm::raw_fd_ostream m_stream;
 };
 
@@ -90,6 +87,7 @@
   size_t GetNumMessages() const;
   size_t GetFirstMessageIndex() const;
 
+  mutable std::mutex m_mutex;
   std::unique_ptr<std::string[]> m_messages;
   const size_t m_size = 0;
   size_t m_next_index = 0;
Index: lldb/source/Commands/CommandObjectLog.cpp
===================================================================
--- lldb/source/Commands/CommandObjectLog.cpp
+++ lldb/source/Commands/CommandObjectLog.cpp
@@ -94,9 +94,6 @@
         error =
             buffer_size.SetValueFromString(option_arg, eVarSetOperationAssign);
         break;
-      case 't':
-        log_options |= LLDB_LOG_OPTION_THREADSAFE;
-        break;
       case 'v':
         log_options |= LLDB_LOG_OPTION_VERBOSE;
         break;
Index: lldb/source/Commands/Options.td
===================================================================
--- lldb/source/Commands/Options.td
+++ lldb/source/Commands/Options.td
@@ -433,8 +433,6 @@
     Desc<"Set the destination file to log to.">;
   def log_buffer_size : Option<"buffer", "b">, Group<1>, Arg<"UnsignedInteger">,
     Desc<"Set the log to be buffered, using the specified buffer size.">;
-  def log_threadsafe : Option<"threadsafe", "t">, Group<1>,
-    Desc<"Enable thread safe logging to avoid interweaved log lines.">;
   def log_verbose : Option<"verbose", "v">, Group<1>,
     Desc<"Enable verbose logging.">;
   def log_sequence : Option<"sequence", "s">, Group<1>,
Index: lldb/source/Core/Debugger.cpp
===================================================================
--- lldb/source/Core/Debugger.cpp
+++ lldb/source/Core/Debugger.cpp
@@ -1448,8 +1448,7 @@
   assert(log_handler_sp);
 
   if (log_options == 0)
-    log_options =
-        LLDB_LOG_OPTION_PREPEND_THREAD_NAME | LLDB_LOG_OPTION_THREADSAFE;
+    log_options = LLDB_LOG_OPTION_PREPEND_THREAD_NAME;
 
   return Log::EnableLogChannel(log_handler_sp, log_options, channel, categories,
                                error_stream);
Index: lldb/source/Utility/Log.cpp
===================================================================
--- lldb/source/Utility/Log.cpp
+++ lldb/source/Utility/Log.cpp
@@ -317,12 +317,7 @@
   auto handler_sp = GetHandler();
   if (!handler_sp)
     return;
-
-  Flags options = GetOptions();
-  if (options.Test(LLDB_LOG_OPTION_THREADSAFE))
-    handler_sp->EmitThreadSafe(message);
-  else
-    handler_sp->Emit(message);
+  handler_sp->Emit(message);
 }
 
 void Log::Format(llvm::StringRef file, llvm::StringRef function,
@@ -334,11 +329,6 @@
   WriteMessage(message.str());
 }
 
-void LogHandler::EmitThreadSafe(llvm::StringRef message) {
-  std::lock_guard<std::mutex> guard(m_mutex);
-  Emit(message);
-}
-
 StreamLogHandler::StreamLogHandler(int fd, bool should_close,
                                    size_t buffer_size)
     : m_stream(fd, should_close, false) {
@@ -346,11 +336,21 @@
     m_stream.SetBufferSize(buffer_size);
 }
 
-StreamLogHandler::~StreamLogHandler() {
+StreamLogHandler::~StreamLogHandler() { Flush(); }
+
+void StreamLogHandler::Flush() {
+  std::lock_guard<std::mutex> guard(m_mutex);
   m_stream.flush();
 }
 
-void StreamLogHandler::Emit(llvm::StringRef message) { m_stream << message; }
+void StreamLogHandler::Emit(llvm::StringRef message) {
+  if (m_stream.GetBufferSize() > 0) {
+    std::lock_guard<std::mutex> guard(m_mutex);
+    m_stream << message;
+  } else {
+    m_stream << message;
+  }
+}
 
 CallbackLogHandler::CallbackLogHandler(lldb::LogOutputCallback callback,
                                        void *baton)
@@ -364,6 +364,7 @@
     : m_messages(std::make_unique<std::string[]>(size)), m_size(size) {}
 
 void RotatingLogHandler::Emit(llvm::StringRef message) {
+  std::lock_guard<std::mutex> guard(m_mutex);
   ++m_total_count;
   const size_t index = m_next_index;
   m_next_index = NormalizeIndex(index + 1);
@@ -381,6 +382,7 @@
 }
 
 void RotatingLogHandler::Dump(llvm::raw_ostream &stream) const {
+  std::lock_guard<std::mutex> guard(m_mutex);
   const size_t start_idx = GetFirstMessageIndex();
   const size_t stop_idx = start_idx + GetNumMessages();
   for (size_t i = start_idx; i < stop_idx; ++i) {