diff --git a/mercurial/store.py b/mercurial/store.py
--- a/mercurial/store.py
+++ b/mercurial/store.py
@@ -486,7 +486,7 @@
         if self.addls:
             # if we have just new entries, let's append them to the fncache
             tr.addbackup('fncache')
-            fp = self.vfs('fncache', mode='ab', atomictemp=True)
+            fp = self.vfs('fncache', mode='ab')
             if self.addls:
                 fp.write(encodedir('\n'.join(self.addls) + '\n'))
             fp.close()