diff --git a/db/db_impl/db_impl.cc b/db/db_impl/db_impl.cc
index 4089c0acb9a..4c0798531e7 100644
--- a/db/db_impl/db_impl.cc
+++ b/db/db_impl/db_impl.cc
@@ -690,11 +690,14 @@ Status DBImpl::CloseHelper() {
     delete txn_entry.second;
   }
 
+  mutex_.Unlock();
   // We can only access cf_based_write_buffer_manager_ before versions_.reset(),
   // after which all cf write buffer managers will be freed.
   for (auto m : cf_based_write_buffer_manager_) {
     m->UnregisterDB(this);
   }
+  mutex_.Lock();
+
   // versions need to be destroyed before table_cache since it can hold
   // references to table_cache.
   versions_.reset();
diff --git a/include/rocksdb/write_buffer_manager.h b/include/rocksdb/write_buffer_manager.h
index f2b93d8aa55..444e383080d 100644
--- a/include/rocksdb/write_buffer_manager.h
+++ b/include/rocksdb/write_buffer_manager.h
@@ -141,6 +141,8 @@ class WriteBufferManager final {
     }
   }
 
+  // Must ensure that the mutex of all dbs except this_db are not held. If
+  // this_db is not nullptr, the mutex of it must be held.
   void MaybeFlushLocked(DB* this_db = nullptr);
 
   // Returns true if total memory usage exceeded buffer_size.