diff --git a/lib/discourse.rb b/lib/discourse.rb
index 780321a361b..eeb8d2a01b8 100644
--- a/lib/discourse.rb
+++ b/lib/discourse.rb
@@ -311,7 +311,7 @@ module Discourse
       $redis.set(key, 1)
     else
       $redis.setex(key, READONLY_MODE_KEY_TTL, 1)
-      keep_readonly_mode(key)
+      keep_readonly_mode(key) if !Rails.env.test?
     end
 
     MessageBus.publish(readonly_channel, true)
@@ -321,20 +321,24 @@ module Discourse
 
   def self.keep_readonly_mode(key)
     # extend the expiry by 1 minute every 30 seconds
-    unless Rails.env.test?
+    @mutex ||= Mutex.new
+
+    @mutex.synchronize do
       @dbs ||= Set.new
       @dbs << RailsMultisite::ConnectionManagement.current_db
       @threads ||= {}
 
       unless @threads[key]&.alive?
         @threads[key] = Thread.new do
-          while @dbs.size > 0
+          while @dbs.size > 0 do
             sleep 30
 
-            @dbs.each do |db|
-              RailsMultisite::ConnectionManagement.with_connection(db) do
-                if !$redis.expire(key, READONLY_MODE_KEY_TTL)
-                  @dbs.delete(db)
+            @mutex.synchronize do
+              @dbs.each do |db|
+                RailsMultisite::ConnectionManagement.with_connection(db) do
+                  if !$redis.expire(key, READONLY_MODE_KEY_TTL)
+                    @dbs.delete(db)
+                  end
                 end
               end
             end