diff --git a/mirror-nixos-branch.sh b/mirror-nixos-branch.sh
index 967220c..f7b6c92 100755
--- a/mirror-nixos-branch.sh
+++ b/mirror-nixos-branch.sh
@@ -98,6 +98,10 @@ else
     trap '' EXIT
 fi
 
+# Prevent concurrent writes to the channels and the Git clone.
+exec 10>$channelsDir/.htaccess.lock
+flock 10
+
 # Copy over to nixos.org.
 cd "$releasesDir"
 rsync -avR . hydra-mirror@nixos.org:"$releasesDir" --exclude .htaccess --exclude ".tmp.*" --delete >&2
@@ -109,12 +113,12 @@ echo "Redirect /releases/nixos/channels/$channelName /releases/nixos/$branch/$re
 mv $htaccess.tmp $htaccess
 ln -sfn $releaseDir $channelsDir/$channelName # dummy symlink
 
-flock -x $channelsDir/.htaccess.lock -c "cat $channelsDir/.htaccess-nix* > $channelsDir/.htaccess"
+cat $channelsDir/.htaccess-nix* > $channelsDir/.htaccess
 
 cd "$channelsDir"
 rsync -avR . hydra-mirror@nixos.org:"$channelsDir" --delete >&2
 
-# Update the nixpkgs-channels repo. FIXME: protect against concurrent access
+# Update the nixpkgs-channels repo.
 git remote update nixpkgs >&2
 git push nixpkgs-channels "$rev:refs/heads/$channelName" >&2
 
diff --git a/mirror-nixpkgs.sh b/mirror-nixpkgs.sh
index c8fb7db..e0c12c3 100755
--- a/mirror-nixpkgs.sh
+++ b/mirror-nixpkgs.sh
@@ -42,6 +42,10 @@ else
     mv $tmpDir $releaseDir
 fi
 
+# Prevent concurrent writes to the channels and the Git clone.
+exec 10>$channelsDir/.htaccess.lock
+flock 10
+
 # Copy over to nixos.org.
 cd "$releasesDir"
 rsync -avR . hydra-mirror@nixos.org:"$releasesDir" --delete >&2
@@ -53,7 +57,7 @@ echo "Redirect /releases/nixpkgs/channels/$channelName /releases/nixpkgs/$releas
 mv $htaccess.tmp $htaccess
 ln -sfn $releaseDir $channelsDir/$channelName # dummy symlink
 
-flock -x $channelsDir/.htaccess.lock -c "cat $channelsDir/.htaccess-nix* > $channelsDir/.htaccess"
+cat $channelsDir/.htaccess-nix* > $channelsDir/.htaccess
 
 cd "$channelsDir"
 rsync -avR . hydra-mirror@nixos.org:"$channelsDir" --delete >&2