diff --git a/db.go b/db.go
index 4f10fe9e8..1c56ae7d0 100644
--- a/db.go
+++ b/db.go
@@ -99,14 +99,14 @@ type DB struct {
 	metrics *dbMetrics
 	opts    *Options
 
-	// Mutex for that must be held when modifying the general
-	// block layout.
+	// Mutex for that must be held when modifying the general block layout.
+	// cmtx must be held before acquiring it.
 	mtx    sync.RWMutex
 	blocks []Block
 
 	// Mutex that must be held when modifying just the head blocks
 	// or the general layout.
-	// Must never be held when acquiring a blocks's mutex!
+	// mtx must be held before acquiring.
 	headmtx sync.RWMutex
 	heads   []headBlock
 
@@ -341,6 +341,9 @@ func (db *DB) appendableHeads() (r []headBlock) {
 }
 
 func (db *DB) completedHeads() (r []headBlock) {
+	db.mtx.RLock()
+	defer db.mtx.RUnlock()
+
 	db.headmtx.RLock()
 	defer db.headmtx.RUnlock()
 
@@ -594,12 +597,12 @@ func (db *DB) EnableCompactions() {
 
 // Snapshot writes the current data to the directory.
 func (db *DB) Snapshot(dir string) error {
-	db.mtx.Lock() // To block any appenders.
-	defer db.mtx.Unlock()
-
 	db.cmtx.Lock()
 	defer db.cmtx.Unlock()
 
+	db.mtx.Lock() // To block any appenders.
+	defer db.mtx.Unlock()
+
 	blocks := db.blocks[:]
 	for _, b := range blocks {
 		db.logger.Log("msg", "snapshotting block", "block", b)
@@ -804,6 +807,7 @@ func (a *dbAppender) Rollback() error {
 func (db *DB) Delete(mint, maxt int64, ms ...labels.Matcher) error {
 	db.cmtx.Lock()
 	defer db.cmtx.Unlock()
+
 	db.mtx.Lock()
 	defer db.mtx.Unlock()
 
diff --git a/db_test.go b/db_test.go
index e410dfceb..d4b3ca8bb 100644
--- a/db_test.go
+++ b/db_test.go
@@ -60,10 +60,10 @@ func TestDataAvailableOnlyAfterCommit(t *testing.T) {
 	require.NoError(t, err)
 
 	querier := db.Querier(0, 1)
-	defer querier.Close()
 	seriesSet, err := readSeriesSet(querier.Select(labels.NewEqualMatcher("foo", "bar")))
 	require.NoError(t, err)
 	require.Equal(t, seriesSet, map[string][]sample{})
+	require.NoError(t, querier.Close())
 
 	err = app.Commit()
 	require.NoError(t, err)
diff --git a/querier.go b/querier.go
index 523a67398..cfe53cd57 100644
--- a/querier.go
+++ b/querier.go
@@ -53,8 +53,8 @@ type querier struct {
 	blocks []Querier
 }
 
-// Querier returns a new querier over the data partition for the given
-// time range.
+// Querier returns a new querier over the data partition for the given time range.
+// A goroutine must not handle more than one open Querier.
 func (s *DB) Querier(mint, maxt int64) Querier {
 	s.mtx.RLock()