diff --git a/internal/files/concurrency2_test.go b/internal/files/concurrency2_test.go
new file mode 100644
index 00000000..d74a9026
--- /dev/null
+++ b/internal/files/concurrency2_test.go
@@ -0,0 +1,206 @@
+// Copyright (C) 2014 Jakob Borg and Contributors (see the CONTRIBUTORS file).
+//
+// This program is free software: you can redistribute it and/or modify it
+// under the terms of the GNU General Public License as published by the Free
+// Software Foundation, either version 3 of the License, or (at your option)
+// any later version.
+//
+// This program is distributed in the hope that it will be useful, but WITHOUT
+// ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+// FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for
+// more details.
+//
+// You should have received a copy of the GNU General Public License along
+// with this program. If not, see <http://www.gnu.org/licenses/>.
+
+package files_test
+
+import (
+	"fmt"
+	"log"
+	"math/rand"
+	"os"
+	"runtime"
+	"sync"
+	"testing"
+	"time"
+
+	"github.com/syncthing/syncthing/internal/files"
+	"github.com/syncthing/syncthing/internal/protocol"
+	"github.com/syndtr/goleveldb/leveldb"
+	"github.com/syndtr/goleveldb/leveldb/opt"
+)
+
+func TestLongConcurrent(t *testing.T) {
+	if testing.Short() || runtime.GOMAXPROCS(-1) < 4 {
+		return
+	}
+
+	os.RemoveAll("/tmp/test.db")
+	db, err := leveldb.OpenFile("/tmp/test.db", &opt.Options{CachedOpenFiles: 100})
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	start := make(chan struct{})
+
+	log.Println("preparing")
+	var wg sync.WaitGroup
+	for i := 0; i < runtime.GOMAXPROCS(-1); i++ {
+		i := i
+		rem0, rem1 := generateFiles()
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			longConcurrentTest(db, fmt.Sprintf("folder%d", i), rem0, rem1, start)
+		}()
+	}
+
+	log.Println("starting")
+	close(start)
+	wg.Wait()
+}
+
+func generateFiles() ([]protocol.FileInfo, []protocol.FileInfo) {
+	var rem0, rem1 fileList
+
+	for i := 0; i < 10000; i++ {
+		n := rand.Int()
+		rem0 = append(rem0, protocol.FileInfo{
+			Name:    fmt.Sprintf("path/path/path/path/path/path/path%d/path%d/path%d/file%d", n, n, n, n),
+			Version: uint64(rand.Int63()),
+			Blocks:  genBlocks(rand.Intn(25)),
+			Flags:   uint32(rand.Int31()),
+		})
+	}
+
+	for i := 0; i < 10000; i++ {
+		if i%2 == 0 {
+			// Same file as rem0, randomly newer or older
+			f := rem0[i]
+			f.Version = uint64(rand.Int63())
+			rem1 = append(rem1, f)
+		} else {
+			// Different file
+			n := rand.Int()
+			f := protocol.FileInfo{
+				Name:    fmt.Sprintf("path/path/path/path/path/path/path%d/path%d/path%d/file%d", n, n, n, n),
+				Version: uint64(rand.Int63()),
+				Blocks:  genBlocks(rand.Intn(25)),
+				Flags:   uint32(rand.Int31()),
+			}
+			rem1 = append(rem1, f)
+		}
+	}
+
+	return rem0, rem1
+}
+
+func longConcurrentTest(db *leveldb.DB, folder string, rem0, rem1 []protocol.FileInfo, start chan struct{}) {
+	s := files.NewSet(folder, db)
+
+	<-start
+
+	t0 := time.Now()
+	cont := func() bool {
+		return time.Since(t0) < 60*time.Second
+	}
+
+	log.Println(folder, "start")
+
+	var wg sync.WaitGroup
+
+	// Fast updater
+
+	wg.Add(1)
+	go func() {
+		defer wg.Done()
+		for cont() {
+			log.Println(folder, "u0")
+			for i := 0; i < 10000; i += 250 {
+				s.Update(remoteDevice0, rem0[i:i+250])
+			}
+			time.Sleep(25 * time.Millisecond)
+			s.Replace(remoteDevice0, nil)
+			time.Sleep(25 * time.Millisecond)
+		}
+	}()
+
+	// Fast updater
+
+	wg.Add(1)
+	go func() {
+		defer wg.Done()
+		for cont() {
+			log.Println(folder, "u1")
+			for i := 0; i < 10000; i += 250 {
+				s.Update(remoteDevice1, rem1[i:i+250])
+			}
+			time.Sleep(25 * time.Millisecond)
+			s.Replace(remoteDevice1, nil)
+			time.Sleep(25 * time.Millisecond)
+		}
+	}()
+
+	// Fast need list
+
+	wg.Add(1)
+	go func() {
+		defer wg.Done()
+		for cont() {
+			needList(s, protocol.LocalDeviceID)
+			time.Sleep(25 * time.Millisecond)
+		}
+	}()
+
+	// Fast global list
+
+	wg.Add(1)
+	go func() {
+		defer wg.Done()
+		for cont() {
+			globalList(s)
+			time.Sleep(25 * time.Millisecond)
+		}
+	}()
+
+	// Long running need lists
+
+	go func() {
+		for i := 0; i < 10; i++ {
+			time.Sleep(25 * time.Millisecond)
+			wg.Add(1)
+			go func() {
+				defer wg.Done()
+				for cont() {
+					s.WithNeed(protocol.LocalDeviceID, func(intf protocol.FileIntf) bool {
+						time.Sleep(50 * time.Millisecond)
+						return cont()
+					})
+				}
+			}()
+		}
+	}()
+
+	// Long running global lists
+
+	go func() {
+		for i := 0; i < 10; i++ {
+			time.Sleep(25 * time.Millisecond)
+			wg.Add(1)
+			go func() {
+				defer wg.Done()
+				for cont() {
+					s.WithGlobal(func(intf protocol.FileIntf) bool {
+						time.Sleep(50 * time.Millisecond)
+						return cont()
+					})
+				}
+			}()
+		}
+	}()
+
+	wg.Wait()
+
+	log.Println(folder, "done")
+}