Set "allow_parallel" to false by default.

sphinx/builders/__init__.py

@@ -39,8 +39,8 @@ class Builder(object):
     format = ''
     # doctree versioning method
     versioning_method = 'none'
-    # allow parallel write_doc calls
-    allow_parallel = True
+    # allow parallel write_doc() calls
+    allow_parallel = False
 
     def __init__(self, app):
         self.env = app.env

sphinx/builders/html.py

@@ -60,6 +60,7 @@ class StandaloneHTMLBuilder(Builder):
     name = 'html'
     format = 'html'
     copysource = True
+    allow_parallel = True
     out_suffix = '.html'
     link_suffix = '.html'  # defaults to matching out_suffix
     indexer_format = js_index