Mark TBufferMerger::fBuffered as atomic

io/io/inc/ROOT/TBufferMerger.hxx

@@ -117,7 +117,7 @@ private:
    bool TryMerge(TBufferMergerFile *memfile);
 
    size_t fAutoSave{0};                                          //< AutoSave only every fAutoSave bytes
-   size_t fBuffered{0};                                          //< Number of bytes currently buffered
+   std::atomic<size_t> fBuffered{0};                             //< Number of bytes currently buffered
    TFileMerger fMerger{false, false};                            //< TFileMerger used to merge all buffers
    std::mutex fMergeMutex;                                       //< Mutex used to lock fMerger
    mutable std::mutex fQueueMutex;                               //< Mutex used to lock fQueue

io/io/src/TBufferMerger.cxx

@@ -72,7 +72,6 @@ size_t TBufferMerger::GetQueueSize() const
 
 size_t TBufferMerger::GetBuffered() const
 {
-   std::lock_guard<std::mutex> lock(fQueueMutex);
    return fBuffered;
 }