diff --git a/extensions/autodoc_placeholder/__init__.py b/extensions/autodoc_placeholder/__init__.py
index 1f4e088b2..470f53830 100644
--- a/extensions/autodoc_placeholder/__init__.py
+++ b/extensions/autodoc_placeholder/__init__.py
@@ -21,6 +21,6 @@ def setup(app):
     directives.register_directive('autoattribute', PlaceHolder)
 
     return {
-        'parallel_read_safe': False,
+        'parallel_read_safe': True,
         'parallel_write_safe': True
     }
diff --git a/extensions/embedded_video/__init__.py b/extensions/embedded_video/__init__.py
index 9119e01c9..f200deeed 100644
--- a/extensions/embedded_video/__init__.py
+++ b/extensions/embedded_video/__init__.py
@@ -66,6 +66,6 @@ def setup(app):
     directives.register_directive('vimeo', Vimeo)
 
     return {
-        'parallel_read_safe': False,
+        'parallel_read_safe': True,
         'parallel_write_safe': True
     }
diff --git a/extensions/exercise_admonition/__init__.py b/extensions/exercise_admonition/__init__.py
index fd0b0b6a5..82be4d755 100644
--- a/extensions/exercise_admonition/__init__.py
+++ b/extensions/exercise_admonition/__init__.py
@@ -21,7 +21,7 @@ def setup(app):
     ))
 
     return {
-        'parallel_read_safe': False,
+        'parallel_read_safe': True,
         'parallel_write_safe': True
     }
 
diff --git a/extensions/github_link/__init__.py b/extensions/github_link/__init__.py
index 6bde1c906..5d1752c5e 100644
--- a/extensions/github_link/__init__.py
+++ b/extensions/github_link/__init__.py
@@ -75,7 +75,7 @@ def setup(app):
     app.config.linkcode_resolve = linkcode_resolve
 
     return {
-        'parallel_read_safe': False,
+        'parallel_read_safe': True,
         'parallel_write_safe': True
     }
 
diff --git a/extensions/html_domain/__init__.py b/extensions/html_domain/__init__.py
index f3e20b465..2b2997bde 100644
--- a/extensions/html_domain/__init__.py
+++ b/extensions/html_domain/__init__.py
@@ -40,7 +40,7 @@ def setup(app):
         addnode(app, node, name)
 
     return {
-        'parallel_read_safe': False,
+        'parallel_read_safe': True,
         'parallel_write_safe': True
     }
 
@@ -183,3 +183,11 @@ class HtmlDomain(Domain):
         'var': makerole(var),
         'samp': makerole(samp),
     }
+
+    def merge_domaindata(self, docnames, otherdata) -> None:
+        """Merge in data regarding *docnames* from a different domaindata
+        inventory (coming from a subprocess in parallel builds).
+        """
+        # This extension doesn't store any data on the env
+        # and therefore doesn't need to support this method.
+        pass
diff --git a/extensions/odoo_theme/__init__.py b/extensions/odoo_theme/__init__.py
index 041f9a14e..4e61aaefb 100644
--- a/extensions/odoo_theme/__init__.py
+++ b/extensions/odoo_theme/__init__.py
@@ -16,7 +16,7 @@ def setup(app):
     app.add_js_file('js/page_toc.js')
 
     return {
-        'parallel_read_safe': False,
+        'parallel_read_safe': True,
         'parallel_write_safe': True
     }
 
diff --git a/extensions/redirects/__init__.py b/extensions/redirects/__init__.py
index f66c6b6fa..054baf733 100644
--- a/extensions/redirects/__init__.py
+++ b/extensions/redirects/__init__.py
@@ -64,6 +64,6 @@ def setup(app):
     app.connect('builder-inited', generate_redirects)
 
     return {
-        'parallel_read_safe': False,
+        'parallel_read_safe': True,
         'parallel_write_safe': True
     }
diff --git a/extensions/switcher/__init__.py b/extensions/switcher/__init__.py
index 87c36e74b..d71316786 100644
--- a/extensions/switcher/__init__.py
+++ b/extensions/switcher/__init__.py
@@ -13,7 +13,7 @@ def setup(app):
     app.connect('env-updated', add_statics)
 
     return {
-        'parallel_read_safe': False,
+        'parallel_read_safe': True,
         'parallel_write_safe': True
     }