diff --git a/app/jobs/scheduled/create_missing_avatars.rb b/app/jobs/scheduled/create_missing_avatars.rb
index 42a2b01dc..8526b4115 100644
--- a/app/jobs/scheduled/create_missing_avatars.rb
+++ b/app/jobs/scheduled/create_missing_avatars.rb
@@ -1,13 +1,15 @@
 module Jobs
   class CreateMissingAvatars < Jobs::Scheduled
     every 1.hour
+
     def execute(args)
-      # backfill in batches 5000 an hour
-      UserAvatar.where(last_gravatar_download_attempt: nil).includes(:user)
-          .order("users.last_posted_at desc")
-          .limit(5000).each do |u|
+      # backfill in batches of 5000 an hour
+      UserAvatar.includes(:user)
+                .where(last_gravatar_download_attempt: nil)
+                .order("users.last_posted_at DESC")
+                .limit(5000)
+                .each do |u|
         u.user.refresh_avatar
-        u.user.save
       end
     end
   end
diff --git a/app/models/user_avatar.rb b/app/models/user_avatar.rb
index 73431a961..0a14ec745 100644
--- a/app/models/user_avatar.rb
+++ b/app/models/user_avatar.rb
@@ -10,27 +10,31 @@ class UserAvatar < ActiveRecord::Base
   end
 
   def update_gravatar!
-    # special logic for our system user, we do not want the discourse email there
-    email_hash = user.id == -1 ? User.email_hash("info@discourse.org") : user.email_hash
+    DistributedMutex.synchronize("update_gravatar_#{user.id}") do
+      begin
+        # special logic for our system user
+        email_hash = user.id == Discourse::SYSTEM_USER_ID ? User.email_hash("info@discourse.org") : user.email_hash
 
-    self.last_gravatar_download_attempt = Time.new
-    gravatar_url = "http://www.gravatar.com/avatar/#{email_hash}.png?s=500&d=404"
-    tempfile = FileHelper.download(gravatar_url, SiteSetting.max_image_size_kb.kilobytes, "gravatar")
+        self.last_gravatar_download_attempt = Time.new
 
-    upload = Upload.create_for(user.id, tempfile, 'gravatar.png', tempfile.size, { origin: gravatar_url })
+        gravatar_url = "http://www.gravatar.com/avatar/#{email_hash}.png?s=500&d=404"
+        tempfile = FileHelper.download(gravatar_url, SiteSetting.max_image_size_kb.kilobytes, "gravatar")
+        upload = Upload.create_for(user.id, tempfile, 'gravatar.png', tempfile.size, { origin: gravatar_url })
 
-    if gravatar_upload_id != upload.id
-      gravatar_upload.try(:destroy!)
-      self.gravatar_upload = upload
-      save!
+        if gravatar_upload_id != upload.id
+          gravatar_upload.try(:destroy!)
+          self.gravatar_upload = upload
+          save!
+        end
+      rescue OpenURI::HTTPError
+        save!
+      rescue SocketError
+        # skip saving, we are not connected to the net
+        Rails.logger.warn "Failed to download gravatar, socket error - user id #{user.id}"
+      ensure
+        tempfile.try(:close!)
+      end
     end
-  rescue OpenURI::HTTPError
-    save!
-  rescue SocketError
-    # skip saving, we are not connected to the net
-    Rails.logger.warn "Failed to download gravatar, socket error - user id #{user.id}"
-  ensure
-    tempfile.close! if tempfile && tempfile.respond_to?(:close!)
   end
 
 end
diff --git a/lib/discourse.rb b/lib/discourse.rb
index 2a139eb2d..845aaf482 100644
--- a/lib/discourse.rb
+++ b/lib/discourse.rb
@@ -256,7 +256,7 @@ module Discourse
     user ||= User.admins.real.order(:id).first
   end
 
-  SYSTEM_USER_ID = -1 unless defined? SYSTEM_USER_ID
+  SYSTEM_USER_ID ||= -1
 
   def self.system_user
     User.find_by(id: SYSTEM_USER_ID)