Set writer_thread_id earlier, when new exclusive owener is waiting for existing readers to finish

src/Common/SharedMutex.cpp

@@ -31,11 +31,13 @@ void SharedMutex::lock()
             break;
     }
 
+    /// The first step of acquiring the exclusive ownership is finished.
+    /// Now we just wait until all readers release the shared ownership.
+    writer_thread_id.store(getThreadId());
+
     value |= writers;
     while (value & readers)
         futexWaitLowerFetch(state, value);
-
-    writer_thread_id.store(getThreadId());
 }
 
 bool SharedMutex::try_lock()

src/Common/SharedMutex.h

@@ -38,7 +38,7 @@ private:
 
     alignas(64) std::atomic<UInt64> state;
     std::atomic<UInt32> waiters;
-    /// Is set while the lock is held in exclusive mode only to facilitate debugging
+    /// Is set while the lock is held (or is in the process of being acquired) in exclusive mode only to facilitate debugging
     std::atomic<UInt64> writer_thread_id;
 };