diff --git a/executor/executor.go b/executor/executor.go
index 706348f80ecb5..e03f6fe7e5697 100644
--- a/executor/executor.go
+++ b/executor/executor.go
@@ -28,6 +28,7 @@ import (
 	"github.com/cznic/mathutil"
 	"github.com/opentracing/opentracing-go"
 	"github.com/pingcap/errors"
+	"github.com/pingcap/failpoint"
 	"github.com/pingcap/parser/ast"
 	"github.com/pingcap/parser/auth"
 	"github.com/pingcap/parser/model"
@@ -1439,6 +1440,12 @@ type UnionExec struct {
 	results     []*chunk.Chunk
 	wg          sync.WaitGroup
 	initialized bool
+	mu          struct {
+		*sync.Mutex
+		maxOpenedChildID int
+	}
+
+	childInFlightForTest int32
 }
 
 // unionWorkerResult stores the result for a union worker.
@@ -1458,12 +1465,11 @@ func (e *UnionExec) waitAllFinished() {
 
 // Open implements the Executor Open interface.
 func (e *UnionExec) Open(ctx context.Context) error {
-	if err := e.baseExecutor.Open(ctx); err != nil {
-		return err
-	}
 	e.stopFetchData.Store(false)
 	e.initialized = false
 	e.finished = make(chan struct{})
+	e.mu.Mutex = &sync.Mutex{}
+	e.mu.maxOpenedChildID = -1
 	return nil
 }
 
@@ -1509,6 +1515,19 @@ func (e *UnionExec) resultPuller(ctx context.Context, workerID int) {
 		e.wg.Done()
 	}()
 	for childID := range e.childIDChan {
+		e.mu.Lock()
+		if childID > e.mu.maxOpenedChildID {
+			e.mu.maxOpenedChildID = childID
+		}
+		e.mu.Unlock()
+		if err := e.children[childID].Open(ctx); err != nil {
+			result.err = err
+			e.stopFetchData.Store(true)
+			e.resultPool <- result
+		}
+		failpoint.Inject("issue21441", func() {
+			atomic.AddInt32(&e.childInFlightForTest, 1)
+		})
 		for {
 			if e.stopFetchData.Load().(bool) {
 				return
@@ -1523,12 +1542,20 @@ func (e *UnionExec) resultPuller(ctx context.Context, workerID int) {
 				e.resourcePools[workerID] <- result.chk
 				break
 			}
+			failpoint.Inject("issue21441", func() {
+				if int(atomic.LoadInt32(&e.childInFlightForTest)) > e.concurrency {
+					panic("the count of child in flight is larger than e.concurrency unexpectedly")
+				}
+			})
 			e.resultPool <- result
 			if result.err != nil {
 				e.stopFetchData.Store(true)
 				return
 			}
 		}
+		failpoint.Inject("issue21441", func() {
+			atomic.AddInt32(&e.childInFlightForTest, -1)
+		})
 	}
 }
 
@@ -1567,7 +1594,15 @@ func (e *UnionExec) Close() error {
 		for range e.childIDChan {
 		}
 	}
-	return e.baseExecutor.Close()
+	// We do not need to acquire the e.mu.Lock since all the resultPuller can be
+	// promised to exit when reaching here (e.childIDChan been closed).
+	var firstErr error
+	for i := 0; i <= e.mu.maxOpenedChildID; i++ {
+		if err := e.children[i].Close(); err != nil && firstErr == nil {
+			firstErr = err
+		}
+	}
+	return firstErr
 }
 
 // ResetContextOfStmt resets the StmtContext and session variables.
diff --git a/executor/executor_test.go b/executor/executor_test.go
index 2affecc1728c7..7f5055a1334ba 100644
--- a/executor/executor_test.go
+++ b/executor/executor_test.go
@@ -7067,6 +7067,36 @@ func (s *testSuite) TestOOMActionPriority(c *C) {
 	c.Assert(action.GetPriority(), Equals, int64(memory.DefLogPriority))
 }
 
+func (s *testSerialSuite) TestIssue21441(c *C) {
+	failpoint.Enable("github.com/pingcap/tidb/executor/issue21441", `return`)
+	defer failpoint.Disable("github.com/pingcap/tidb/executor/issue21441")
+
+	tk := testkit.NewTestKit(c, s.store)
+	tk.MustExec("use test")
+	tk.MustExec("drop table if exists t")
+	tk.MustExec("create table t(a int)")
+	tk.MustExec(`insert into t values(1),(2),(3)`)
+	tk.Se.GetSessionVars().InitChunkSize = 1
+	tk.Se.GetSessionVars().MaxChunkSize = 1
+	sql := `
+select a from t union all
+select a from t union all
+select a from t union all
+select a from t union all
+select a from t union all
+select a from t union all
+select a from t union all
+select a from t`
+	tk.MustQuery(sql).Sort().Check(testkit.Rows(
+		"1", "1", "1", "1", "1", "1", "1", "1",
+		"2", "2", "2", "2", "2", "2", "2", "2",
+		"3", "3", "3", "3", "3", "3", "3", "3",
+	))
+
+	tk.MustQuery("select a from (" + sql + ") t order by a limit 4").Check(testkit.Rows("1", "1", "1", "1"))
+	tk.MustQuery("select a from (" + sql + ") t order by a limit 7, 4").Check(testkit.Rows("1", "2", "2", "2"))
+}
+
 func (s *testSuite) Test17780(c *C) {
 	tk := testkit.NewTestKit(c, s.store)
 	tk.MustExec("use test")
diff --git a/store/tikv/snapshot.go b/store/tikv/snapshot.go
index 05ddf49c033c9..c99a771ec64c5 100644
--- a/store/tikv/snapshot.go
+++ b/store/tikv/snapshot.go
@@ -502,9 +502,13 @@ func (s *tikvSnapshot) SetOption(opt kv.Option, val interface{}) {
 	case kv.SnapshotTS:
 		s.setSnapshotTS(val.(uint64))
 	case kv.ReplicaRead:
+		s.mu.Lock()
 		s.replicaRead = val.(kv.ReplicaReadType)
+		s.mu.Unlock()
 	case kv.TaskID:
+		s.mu.Lock()
 		s.taskID = val.(uint64)
+		s.mu.Unlock()
 	case kv.CollectRuntimeStats:
 		s.mu.Lock()
 		s.mu.stats = val.(*SnapshotRuntimeStats)