diff --git a/Annex/RepoSize/LiveUpdate.hs b/Annex/RepoSize/LiveUpdate.hs
index 6f27617369..17dae1dec7 100644
--- a/Annex/RepoSize/LiveUpdate.hs
+++ b/Annex/RepoSize/LiveUpdate.hs
@@ -15,8 +15,6 @@ import qualified Database.RepoSize as Db
 import Annex.UUID
 
 import Control.Concurrent
-import qualified Data.Map.Strict as M
-import qualified Data.Set as S
 import System.Process
 
 {- Called when a location log change is journalled, so the LiveUpdate
@@ -93,15 +91,21 @@ needLiveUpdate lu = liftIO $ void $ tryPutMVar (liveUpdateNeeded lu) ()
 --
 -- This can be called more than once on the same LiveUpdate. It will
 -- only start it once.
+--
+-- This serializes calls to the action, so that if the action
+-- queries getLiveRepoSizes it will not race with another such action
+-- that may also be starting a live update.
 checkLiveUpdate :: LiveUpdate -> Annex Bool -> Annex Bool
 checkLiveUpdate NoLiveUpdate a = a
-checkLiveUpdate lu a = do
-	r <- a
-	needed <- liftIO $ isJust <$> tryTakeMVar (liveUpdateNeeded lu)
-	when (r && needed) $ do
-		liftIO $ void $ tryPutMVar (liveUpdateStart lu) ()
-		liftIO $ void $ readMVar (liveUpdateReady lu)
-	return r
+checkLiveUpdate lu a = Db.lockDbWhile (const go) go
+  where
+	go = do
+		r <- a
+		needed <- liftIO $ isJust <$> tryTakeMVar (liveUpdateNeeded lu)
+		when (r && needed) $ do
+			liftIO $ void $ tryPutMVar (liveUpdateStart lu) ()
+			liftIO $ void $ readMVar (liveUpdateReady lu)
+		return r
 
 finishedLiveUpdate :: LiveUpdate -> UUID -> Key -> SizeChange -> IO ()
 finishedLiveUpdate NoLiveUpdate _ _ _ = noop
diff --git a/Database/RepoSize.hs b/Database/RepoSize.hs
index 0b3e15fe98..d25ca5374b 100644
--- a/Database/RepoSize.hs
+++ b/Database/RepoSize.hs
@@ -23,6 +23,7 @@ module Database.RepoSize (
 	getRepoSizeHandle,
 	openDb,
 	closeDb,
+	lockDbWhile,
 	getRepoSizes,
 	setRepoSizes,
 	startingLiveSizeChange,
@@ -48,6 +49,7 @@ import Database.Persist.TH
 import qualified System.FilePath.ByteString as P
 import qualified Data.Map.Strict as M
 import qualified Data.Set as S
+import Control.Exception
 
 share [mkPersist sqlSettings, mkMigrate "migrateRepoSizes"] [persistLowerCase|
 -- Corresponds to location log information from the git-annex branch.
@@ -101,16 +103,14 @@ getRepoSizeHandle = Annex.getState Annex.reposizehandle >>= \case
  - can create it undisturbed.
  -}
 openDb :: Annex RepoSizeHandle
-openDb = do
-	lck <- calcRepo' gitAnnexRepoSizeDbLock
-	catchPermissionDenied permerr $ withExclusiveLock lck $ do
-		dbdir <- calcRepo' gitAnnexRepoSizeDbDir
-		let db = dbdir P.</> "db"
-		unlessM (liftIO $ R.doesPathExist db) $ do
-			initDb db $ void $
-				runMigrationSilent migrateRepoSizes
-		h <- liftIO $ H.openDb db "repo_sizes"
-		return $ RepoSizeHandle (Just h)
+openDb = lockDbWhile permerr $ do
+	dbdir <- calcRepo' gitAnnexRepoSizeDbDir
+	let db = dbdir P.</> "db"
+	unlessM (liftIO $ R.doesPathExist db) $ do
+		initDb db $ void $
+			runMigrationSilent migrateRepoSizes
+	h <- liftIO $ H.openDb db "repo_sizes"
+	return $ RepoSizeHandle (Just h)
   where
 	-- If permissions don't allow opening the database,
 	-- just don't use it. Since this database is just a cache
@@ -123,6 +123,13 @@ closeDb :: RepoSizeHandle -> Annex ()
 closeDb (RepoSizeHandle (Just h)) = liftIO $ H.closeDb h
 closeDb (RepoSizeHandle Nothing) = noop
 
+-- This does not prevent another process that has already 
+-- opened the db from changing it at the same time.
+lockDbWhile :: (IOException -> Annex a) -> Annex a -> Annex a
+lockDbWhile permerr a = do
+	lck <- calcRepo' gitAnnexRepoSizeDbLock
+	catchPermissionDenied permerr $ withExclusiveLock lck a
+
 {- Gets the sizes of repositories as of a commit to the git-annex
  - branch. -}
 getRepoSizes :: RepoSizeHandle -> IO (M.Map UUID RepoSize, Maybe Sha)
diff --git a/doc/todo/git-annex_proxies.mdwn b/doc/todo/git-annex_proxies.mdwn
index bace31f5e7..05cbf74c44 100644
--- a/doc/todo/git-annex_proxies.mdwn
+++ b/doc/todo/git-annex_proxies.mdwn
@@ -35,9 +35,11 @@ Planned schedule of work:
 
   May not be a bug, needs reproducing and analysis.
 
-* Make sure that two threads don't check balanced preferred content at the
-  same time, so each thread always sees a consistent picture of what is
-  happening. Use locking as necessary.
+* Test that live repo size data is correct and really works.
+
+* Avoid using checkLiveUpdate except when checking a preferred content
+  expression that does use balanced preferred content. No reason to pay
+  its time penalty otherwise.
 
 * When loading the live update table, check if PIDs in it are still
   running (and are still git-annex), and if not, remove stale entries