lib/model: Use a single lock (phase two: cleanup) (#9276)

Cleanup after #9275.

This renames `fmut` -> `mut`, removes the deadlock detector and
associated plumbing, renames some things from `...PRLocked` to
`...RLocked` and similar, and updates comments.

Apart from the removal of the deadlock detection machinery, no
functional code changes... i.e. almost 100% diff noise, have fun
reviewing.

cmd/syncthing/main.go

@@ -88,9 +88,6 @@ above.
  STTRACE           A comma separated string of facilities to trace. The valid
                    facility strings are listed below.
 
- STDEADLOCKTIMEOUT Used for debugging internal deadlocks; sets debug
-                   sensitivity. Use only under direction of a developer.
-
  STLOCKTHRESHOLD   Used for debugging internal deadlocks; sets debug
                    sensitivity.  Use only under direction of a developer.
 
@@ -173,7 +170,6 @@ type serveOptions struct {
 	// Debug options below
 	DebugDBIndirectGCInterval time.Duration `env:"STGCINDIRECTEVERY" help:"Database indirection GC interval"`
 	DebugDBRecheckInterval    time.Duration `env:"STRECHECKDBEVERY" help:"Database metadata recalculation interval"`
-	DebugDeadlockTimeout      int           `placeholder:"SECONDS" env:"STDEADLOCKTIMEOUT" help:"Used for debugging internal deadlocks"`
 	DebugGUIAssetsDir         string        `placeholder:"PATH" help:"Directory to load GUI assets from" env:"STGUIASSETS"`
 	DebugPerfStats            bool          `env:"STPERFSTATS" help:"Write running performance statistics to perf-$pid.csv (Unix only)"`
 	DebugProfileBlock         bool          `env:"STBLOCKPROFILE" help:"Write block profiles to block-$pid-$timestamp.pprof every 20 seconds"`
@@ -623,7 +619,6 @@ func syncthingMain(options serveOptions) {
 	}
 
 	appOpts := syncthing.Options{
-		DeadlockTimeoutS:     options.DebugDeadlockTimeout,
 		NoUpgrade:            options.NoUpgrade,
 		ProfilerAddr:         options.DebugProfilerListen,
 		ResetDeltaIdxs:       options.DebugResetDeltaIdxs,
@@ -634,10 +629,6 @@ func syncthingMain(options serveOptions) {
 	if options.Audit {
 		appOpts.AuditWriter = auditWriter(options.AuditFile)
 	}
-	if t := os.Getenv("STDEADLOCKTIMEOUT"); t != "" {
-		secs, _ := strconv.Atoi(t)
-		appOpts.DeadlockTimeoutS = secs
-	}
 	if dur, err := time.ParseDuration(os.Getenv("STRECHECKDBEVERY")); err == nil {
 		appOpts.DBRecheckInterval = dur
 	}

go.mod

@@ -40,7 +40,6 @@ require (
 	github.com/prometheus/procfs v0.12.0 // indirect
 	github.com/quic-go/quic-go v0.40.0
 	github.com/rcrowley/go-metrics v0.0.0-20201227073835-cf1acfcdf475
-	github.com/sasha-s/go-deadlock v0.3.1
 	github.com/shirou/gopsutil/v3 v3.23.11
 	github.com/syncthing/notify v0.0.0-20210616190510-c6b7342338d2
 	github.com/syndtr/goleveldb v1.0.1-0.20220721030215-126854af5e6d
@@ -68,7 +67,6 @@ require (
 	github.com/matttproud/golang_protobuf_extensions/v2 v2.0.0 // indirect
 	github.com/onsi/ginkgo/v2 v2.13.2 // indirect
 	github.com/oschwald/maxminddb-golang v1.12.0 // indirect
-	github.com/petermattis/goid v0.0.0-20231126143041-f558c26febf5 // indirect
 	github.com/power-devops/perfstat v0.0.0-20221212215047-62379fc7944b // indirect
 	github.com/prometheus/client_model v0.5.0 // indirect
 	github.com/quic-go/qtls-go1-20 v0.4.1 // indirect

go.sum

@@ -139,9 +139,6 @@ github.com/oschwald/geoip2-golang v1.9.0 h1:uvD3O6fXAXs+usU+UGExshpdP13GAqp4GBrz
 github.com/oschwald/geoip2-golang v1.9.0/go.mod h1:BHK6TvDyATVQhKNbQBdrj9eAvuwOMi2zSFXizL3K81Y=
 github.com/oschwald/maxminddb-golang v1.12.0 h1:9FnTOD0YOhP7DGxGsq4glzpGy5+w7pq50AS6wALUMYs=
 github.com/oschwald/maxminddb-golang v1.12.0/go.mod h1:q0Nob5lTCqyQ8WT6FYgS1L7PXKVVbgiymefNwIjPzgY=
-github.com/petermattis/goid v0.0.0-20180202154549-b0b1615b78e5/go.mod h1:jvVRKCrJTQWu0XVbaOlby/2lO20uSCHEMzzplHXte1o=
-github.com/petermattis/goid v0.0.0-20231126143041-f558c26febf5 h1:+qIP3OMrT7SN5kLnTcVEISPOMB/97RyAKTg1UWA738E=
-github.com/petermattis/goid v0.0.0-20231126143041-f558c26febf5/go.mod h1:pxMtw7cyUw6B2bRH0ZBANSPg+AoSud1I1iyJHI69jH4=
 github.com/pierrec/lz4/v4 v4.1.18 h1:xaKrnTkyoqfh1YItXl56+6KJNVYWlEEPuAQW9xsplYQ=
 github.com/pierrec/lz4/v4 v4.1.18/go.mod h1:gZWDp/Ze/IJXGXf23ltt2EXimqmTUXEy0GFuRQyBid4=
 github.com/pkg/errors v0.8.1/go.mod h1:bwawxfHBFNV+L2hUp1rHADufV3IMtnDRdf1r5NINEl0=
@@ -168,8 +165,6 @@ github.com/rcrowley/go-metrics v0.0.0-20201227073835-cf1acfcdf475 h1:N/ElC8H3+5X
 github.com/rcrowley/go-metrics v0.0.0-20201227073835-cf1acfcdf475/go.mod h1:bCqnVzQkZxMG4s8nGwiZ5l3QUCyqpo9Y+/ZMZ9VjZe4=
 github.com/russross/blackfriday/v2 v2.1.0 h1:JIOH55/0cWyOuilr9/qlrm0BSXldqnqwMsf35Ld67mk=
 github.com/russross/blackfriday/v2 v2.1.0/go.mod h1:+Rmxgy9KzJVeS9/2gXHxylqXiyQDYRxCVz55jmeOWTM=
-github.com/sasha-s/go-deadlock v0.3.1 h1:sqv7fDNShgjcaxkO0JNcOAlr8B9+cV5Ey/OB71efZx0=
-github.com/sasha-s/go-deadlock v0.3.1/go.mod h1:F73l+cr82YSh10GxyRI6qZiCgK64VaZjwesgfQ1/iLM=
 github.com/sclevine/spec v1.4.0 h1:z/Q9idDcay5m5irkZ28M7PtQM4aOISzOpj4bUPkDee8=
 github.com/shirou/gopsutil/v3 v3.23.11 h1:i3jP9NjCPUz7FiZKxlMnODZkdSIp2gnzfrvsu9CuWEQ=
 github.com/shirou/gopsutil/v3 v3.23.11/go.mod h1:1FrWgea594Jp7qmjHUUPlJDTPgcsb9mGnXDxavtikzM=

lib/model/folder_recvonly_test.go

@@ -535,8 +535,8 @@ func setupROFolder(t *testing.T) (*testModel, *receiveOnlyFolder, context.Cancel
 	<-m.started
 	must(t, m.ScanFolder("ro"))
 
-	m.fmut.RLock()
+	m.mut.RLock()
-	defer m.fmut.RUnlock()
+	defer m.mut.RUnlock()
 	r, _ := m.folderRunners.Get("ro")
 	f := r.(*receiveOnlyFolder)
 

lib/model/mocks/model.go

@@ -531,11 +531,6 @@ type Model struct {
 	setIgnoresReturnsOnCall map[int]struct {
 		result1 error
 	}
-	StartDeadlockDetectorStub        func(time.Duration)
-	startDeadlockDetectorMutex       sync.RWMutex
-	startDeadlockDetectorArgsForCall []struct {
-		arg1 time.Duration
-	}
 	StateStub        func(string) (string, time.Time, error)
 	stateMutex       sync.RWMutex
 	stateArgsForCall []struct {
@@ -3070,38 +3065,6 @@ func (fake *Model) SetIgnoresReturnsOnCall(i int, result1 error) {
 	}{result1}
 }
 
-func (fake *Model) StartDeadlockDetector(arg1 time.Duration) {
-	fake.startDeadlockDetectorMutex.Lock()
-	fake.startDeadlockDetectorArgsForCall = append(fake.startDeadlockDetectorArgsForCall, struct {
-		arg1 time.Duration
-	}{arg1})
-	stub := fake.StartDeadlockDetectorStub
-	fake.recordInvocation("StartDeadlockDetector", []interface{}{arg1})
-	fake.startDeadlockDetectorMutex.Unlock()
-	if stub != nil {
-		fake.StartDeadlockDetectorStub(arg1)
-	}
-}
-
-func (fake *Model) StartDeadlockDetectorCallCount() int {
-	fake.startDeadlockDetectorMutex.RLock()
-	defer fake.startDeadlockDetectorMutex.RUnlock()
-	return len(fake.startDeadlockDetectorArgsForCall)
-}
-
-func (fake *Model) StartDeadlockDetectorCalls(stub func(time.Duration)) {
-	fake.startDeadlockDetectorMutex.Lock()
-	defer fake.startDeadlockDetectorMutex.Unlock()
-	fake.StartDeadlockDetectorStub = stub
-}
-
-func (fake *Model) StartDeadlockDetectorArgsForCall(i int) time.Duration {
-	fake.startDeadlockDetectorMutex.RLock()
-	defer fake.startDeadlockDetectorMutex.RUnlock()
-	argsForCall := fake.startDeadlockDetectorArgsForCall[i]
-	return argsForCall.arg1
-}
-
 func (fake *Model) State(arg1 string) (string, time.Time, error) {
 	fake.stateMutex.Lock()
 	ret, specificReturn := fake.stateReturnsOnCall[len(fake.stateArgsForCall)]
@@ -3351,8 +3314,6 @@ func (fake *Model) Invocations() map[string][][]interface{} {
 	defer fake.serveMutex.RUnlock()
 	fake.setIgnoresMutex.RLock()
 	defer fake.setIgnoresMutex.RUnlock()
-	fake.startDeadlockDetectorMutex.RLock()
-	defer fake.startDeadlockDetectorMutex.RUnlock()
 	fake.stateMutex.RLock()
 	defer fake.stateMutex.RUnlock()
 	fake.usageReportingStatsMutex.RLock()

lib/model/model.go

@@ -116,7 +116,6 @@ type Model interface {
 	DismissPendingDevice(device protocol.DeviceID) error
 	DismissPendingFolder(device protocol.DeviceID, folder string) error
 
-	StartDeadlockDetector(timeout time.Duration)
 	GlobalDirectoryTree(folder, prefix string, levels int, dirsOnly bool) ([]*TreeEntry, error)
 }
 
@@ -145,8 +144,8 @@ type model struct {
 	keyGen          *protocol.KeyGenerator
 	promotionTimer  *time.Timer
 
-	// fields protected by fmut
+	// fields protected by mut
-	fmut                           sync.RWMutex
+	mut                            sync.RWMutex
 	folderCfgs                     map[string]config.FolderConfiguration                  // folder -> cfg
 	folderFiles                    map[string]*db.FileSet                                 // folder -> files
 	deviceStatRefs                 map[protocol.DeviceID]*stats.DeviceStatisticsReference // deviceID -> statsRef
@@ -156,8 +155,6 @@ type model struct {
 	folderVersioners               map[string]versioner.Versioner                         // folder -> versioner (may be nil)
 	folderEncryptionPasswordTokens map[string][]byte                                      // folder -> encryption token (may be missing, and only for encryption type folders)
 	folderEncryptionFailures       map[string]map[protocol.DeviceID]error                 // folder -> device -> error regarding encryption consistency (may be missing)
-
-	// fields also protected by fmut
 	connections                    map[string]protocol.Connection                         // connection ID -> connection
 	deviceConnIDs                  map[protocol.DeviceID][]string                         // device -> connection IDs (invariant: if the key exists, the value is len >= 1, with the primary connection at the start of the slice)
 	promotedConnID                 map[protocol.DeviceID]string                           // device -> latest promoted connection ID
@@ -226,8 +223,8 @@ func NewModel(cfg config.Wrapper, id protocol.DeviceID, ldb *db.Lowlevel, protec
 		keyGen:               keyGen,
 		promotionTimer:       time.NewTimer(0),
 
-		// fields protected by fmut
+		// fields protected by mut
-		fmut:                           sync.NewRWMutex(),
+		mut:                            sync.NewRWMutex(),
 		folderCfgs:                     make(map[string]config.FolderConfiguration),
 		folderFiles:                    make(map[string]*db.FileSet),
 		deviceStatRefs:                 make(map[protocol.DeviceID]*stats.DeviceStatisticsReference),
@@ -236,8 +233,6 @@ func NewModel(cfg config.Wrapper, id protocol.DeviceID, ldb *db.Lowlevel, protec
 		folderVersioners:               make(map[string]versioner.Versioner),
 		folderEncryptionPasswordTokens: make(map[string][]byte),
 		folderEncryptionFailures:       make(map[string]map[protocol.DeviceID]error),
-
-		// ditto
 		connections:                    make(map[string]protocol.Connection),
 		deviceConnIDs:                  make(map[protocol.DeviceID][]string),
 		promotedConnID:                 make(map[protocol.DeviceID]string),
@@ -250,7 +245,7 @@ func NewModel(cfg config.Wrapper, id protocol.DeviceID, ldb *db.Lowlevel, protec
 	}
 	for devID, cfg := range cfg.Devices() {
 		m.deviceStatRefs[devID] = stats.NewDeviceStatisticsReference(m.db, devID)
-		m.setConnRequestLimitersPLocked(cfg)
+		m.setConnRequestLimitersLocked(cfg)
 	}
 	m.Add(m.folderRunners)
 	m.Add(m.progressEmitter)
@@ -310,13 +305,13 @@ func (m *model) initFolders(cfg config.Configuration) error {
 }
 
 func (m *model) closeAllConnectionsAndWait() {
-	m.fmut.RLock()
+	m.mut.RLock()
 	closed := make([]chan struct{}, 0, len(m.connections))
 	for connID, conn := range m.connections {
 		closed = append(closed, m.closed[connID])
 		go conn.Close(errStopped)
 	}
-	m.fmut.RUnlock()
+	m.mut.RUnlock()
 	for _, c := range closed {
 		<-c
 	}
@@ -329,16 +324,7 @@ func (m *model) fatal(err error) {
 	}
 }
 
-// StartDeadlockDetector starts a deadlock detector on the models locks which
+// Need to hold lock on m.mut when calling this.
-// causes panics in case the locks cannot be acquired in the given timeout
-// period.
-func (m *model) StartDeadlockDetector(timeout time.Duration) {
-	l.Infof("Starting deadlock detector with %v timeout", timeout)
-	detector := newDeadlockDetector(timeout, m.evLogger, m.fatal)
-	detector.Watch("fmut", m.fmut)
-}
-
-// Need to hold lock on m.fmut when calling this.
 func (m *model) addAndStartFolderLocked(cfg config.FolderConfiguration, fset *db.FileSet, cacheIgnoredFiles bool) {
 	ignores := ignore.New(cfg.Filesystem(nil), ignore.WithCache(cacheIgnoredFiles))
 	if cfg.Type != config.FolderTypeReceiveEncrypted {
@@ -461,14 +447,12 @@ func (m *model) warnAboutOverwritingProtectedFiles(cfg config.FolderConfiguratio
 }
 
 func (m *model) removeFolder(cfg config.FolderConfiguration) {
-	m.fmut.RLock()
+	m.mut.RLock()
 	wait := m.folderRunners.StopAndWaitChan(cfg.ID, 0)
-	m.fmut.RUnlock()
+	m.mut.RUnlock()
 	<-wait
 
-	// We need to hold both fmut and pmut and must acquire locks in the same
+	m.mut.Lock()
-	// order always. (The locks can be *released* in any order.)
-	m.fmut.Lock()
 
 	isPathUnique := true
 	for folderID, folderCfg := range m.folderCfgs {
@@ -493,13 +477,13 @@ func (m *model) removeFolder(cfg config.FolderConfiguration) {
 		return nil
 	})
 
-	m.fmut.Unlock()
+	m.mut.Unlock()
 
 	// Remove it from the database
 	db.DropFolder(m.db, cfg.ID)
 }
 
-// Need to hold lock on m.fmut when calling this.
+// Need to hold lock on m.mut when calling this.
 func (m *model) cleanupFolderLocked(cfg config.FolderConfiguration) {
 	// clear up our config maps
 	m.folderRunners.Remove(cfg.ID)
@@ -523,7 +507,7 @@ func (m *model) restartFolder(from, to config.FolderConfiguration, cacheIgnoredF
 
 	// This mutex protects the entirety of the restart operation, preventing
 	// there from being more than one folder restart operation in progress
-	// at any given time. The usual fmut/pmut stuff doesn't cover this,
+	// at any given time. The usual locking stuff doesn't cover this,
 	// because those locks are released while we are waiting for the folder
 	// to shut down (and must be so because the folder might need them as
 	// part of its operations before shutting down).
@@ -531,13 +515,13 @@ func (m *model) restartFolder(from, to config.FolderConfiguration, cacheIgnoredF
 	restartMut.Lock()
 	defer restartMut.Unlock()
 
-	m.fmut.RLock()
+	m.mut.RLock()
 	wait := m.folderRunners.StopAndWaitChan(from.ID, 0)
-	m.fmut.RUnlock()
+	m.mut.RUnlock()
 	<-wait
 
-	m.fmut.Lock()
+	m.mut.Lock()
-	defer m.fmut.Unlock()
+	defer m.mut.Unlock()
 
 	// Cache the (maybe) existing fset before it's removed by cleanupFolderLocked
 	fset := m.folderFiles[folder]
@@ -586,8 +570,8 @@ func (m *model) newFolder(cfg config.FolderConfiguration, cacheIgnoredFiles bool
 		return fmt.Errorf("adding %v: %w", cfg.Description(), err)
 	}
 
-	m.fmut.Lock()
+	m.mut.Lock()
-	defer m.fmut.Unlock()
+	defer m.mut.Unlock()
 
 	m.addAndStartFolderLocked(cfg, fset, cacheIgnoredFiles)
 
@@ -632,11 +616,11 @@ func (m *model) UsageReportingStats(report *contract.Report, version int, previe
 		blockStatsMut.Unlock()
 
 		// Transport stats
-		m.fmut.RLock()
+		m.mut.RLock()
 		for _, conn := range m.connections {
 			report.TransportStats[conn.Transport()]++
 		}
-		m.fmut.RUnlock()
+		m.mut.RUnlock()
 
 		// Ignore stats
 		var seenPrefix [3]bool
@@ -723,8 +707,8 @@ type ConnectionInfo struct {
 
 // ConnectionStats returns a map with connection statistics for each device.
 func (m *model) ConnectionStats() map[string]interface{} {
-	m.fmut.RLock()
+	m.mut.RLock()
-	defer m.fmut.RUnlock()
+	defer m.mut.RUnlock()
 
 	res := make(map[string]interface{})
 	devs := m.cfg.Devices()
@@ -797,8 +781,8 @@ func (m *model) ConnectionStats() map[string]interface{} {
 
 // DeviceStatistics returns statistics about each device
 func (m *model) DeviceStatistics() (map[protocol.DeviceID]stats.DeviceStatistics, error) {
-	m.fmut.RLock()
+	m.mut.RLock()
-	defer m.fmut.RUnlock()
+	defer m.mut.RUnlock()
 	res := make(map[protocol.DeviceID]stats.DeviceStatistics, len(m.deviceStatRefs))
 	for id, sr := range m.deviceStatRefs {
 		stats, err := sr.GetStatistics()
@@ -818,8 +802,8 @@ func (m *model) DeviceStatistics() (map[protocol.DeviceID]stats.DeviceStatistics
 // FolderStatistics returns statistics about each folder
 func (m *model) FolderStatistics() (map[string]stats.FolderStatistics, error) {
 	res := make(map[string]stats.FolderStatistics)
-	m.fmut.RLock()
+	m.mut.RLock()
-	defer m.fmut.RUnlock()
+	defer m.mut.RUnlock()
 	err := m.folderRunners.Each(func(id string, runner service) error {
 		stats, err := runner.GetStatistics()
 		if err != nil {
@@ -936,10 +920,10 @@ func (m *model) Completion(device protocol.DeviceID, folder string) (FolderCompl
 }
 
 func (m *model) folderCompletion(device protocol.DeviceID, folder string) (FolderCompletion, error) {
-	m.fmut.RLock()
+	m.mut.RLock()
-	err := m.checkFolderRunningLocked(folder)
+	err := m.checkFolderRunningRLocked(folder)
 	rf := m.folderFiles[folder]
-	m.fmut.RUnlock()
+	m.mut.RUnlock()
 	if err != nil {
 		return FolderCompletion{}, err
 	}
@@ -950,10 +934,10 @@ func (m *model) folderCompletion(device protocol.DeviceID, folder string) (Folde
 	}
 	defer snap.Release()
 
-	m.fmut.RLock()
+	m.mut.RLock()
 	state := m.remoteFolderStates[device][folder]
 	downloaded := m.deviceDownloads[device].BytesDownloaded(folder)
-	m.fmut.RUnlock()
+	m.mut.RUnlock()
 
 	need := snap.NeedSize(device)
 	need.Bytes -= downloaded
@@ -970,10 +954,10 @@ func (m *model) folderCompletion(device protocol.DeviceID, folder string) (Folde
 
 // DBSnapshot returns a snapshot of the database content relevant to the given folder.
 func (m *model) DBSnapshot(folder string) (*db.Snapshot, error) {
-	m.fmut.RLock()
+	m.mut.RLock()
-	err := m.checkFolderRunningLocked(folder)
+	err := m.checkFolderRunningRLocked(folder)
 	rf := m.folderFiles[folder]
-	m.fmut.RUnlock()
+	m.mut.RUnlock()
 	if err != nil {
 		return nil, err
 	}
@@ -987,11 +971,11 @@ func (m *model) FolderProgressBytesCompleted(folder string) int64 {
 // NeedFolderFiles returns paginated list of currently needed files in
 // progress, queued, and to be queued on next puller iteration.
 func (m *model) NeedFolderFiles(folder string, page, perpage int) ([]db.FileInfoTruncated, []db.FileInfoTruncated, []db.FileInfoTruncated, error) {
-	m.fmut.RLock()
+	m.mut.RLock()
 	rf, rfOk := m.folderFiles[folder]
 	runner, runnerOk := m.folderRunners.Get(folder)
 	cfg := m.folderCfgs[folder]
-	m.fmut.RUnlock()
+	m.mut.RUnlock()
 
 	if !rfOk {
 		return nil, nil, nil, ErrFolderMissing
@@ -1058,9 +1042,9 @@ func (m *model) NeedFolderFiles(folder string, page, perpage int) ([]db.FileInfo
 // RemoteNeedFolderFiles returns paginated list of currently needed files for a
 // remote device to become synced with a folder.
 func (m *model) RemoteNeedFolderFiles(folder string, device protocol.DeviceID, page, perpage int) ([]db.FileInfoTruncated, error) {
-	m.fmut.RLock()
+	m.mut.RLock()
 	rf, ok := m.folderFiles[folder]
-	m.fmut.RUnlock()
+	m.mut.RUnlock()
 
 	if !ok {
 		return nil, ErrFolderMissing
@@ -1085,9 +1069,9 @@ func (m *model) RemoteNeedFolderFiles(folder string, device protocol.DeviceID, p
 }
 
 func (m *model) LocalChangedFolderFiles(folder string, page, perpage int) ([]db.FileInfoTruncated, error) {
-	m.fmut.RLock()
+	m.mut.RLock()
 	rf, ok := m.folderFiles[folder]
-	m.fmut.RUnlock()
+	m.mut.RUnlock()
 
 	if !ok {
 		return nil, ErrFolderMissing
@@ -1176,9 +1160,9 @@ func (m *model) handleIndex(conn protocol.Connection, folder string, fs []protoc
 		return fmt.Errorf("%s: %w", folder, ErrFolderPaused)
 	}
 
-	m.fmut.RLock()
+	m.mut.RLock()
-	indexHandler, ok := m.getIndexHandlerPRLocked(conn)
+	indexHandler, ok := m.getIndexHandlerRLocked(conn)
-	m.fmut.RUnlock()
+	m.mut.RUnlock()
 	if !ok {
 		// This should be impossible, as an index handler is registered when
 		// we send a cluster config, and that is what triggers index
@@ -1255,7 +1239,7 @@ func (m *model) ClusterConfig(conn protocol.Connection, cm protocol.ClusterConfi
 		break
 	}
 
-	// Needs to happen outside of the fmut, as can cause CommitConfiguration
+	// Needs to happen outside of the mut, as can cause CommitConfiguration
 	if deviceCfg.AutoAcceptFolders {
 		w, _ := m.cfg.Modify(func(cfg *config.Configuration) {
 			changedFcfg := make(map[string]config.FolderConfiguration)
@@ -1291,9 +1275,9 @@ func (m *model) ClusterConfig(conn protocol.Connection, cm protocol.ClusterConfi
 		return err
 	}
 
-	m.fmut.Lock()
+	m.mut.Lock()
 	m.remoteFolderStates[deviceID] = states
-	m.fmut.Unlock()
+	m.mut.Unlock()
 
 	m.evLogger.Log(events.ClusterConfigReceived, ClusterConfigReceivedEventData{
 		Device: deviceID,
@@ -1302,11 +1286,11 @@ func (m *model) ClusterConfig(conn protocol.Connection, cm protocol.ClusterConfi
 	if len(tempIndexFolders) > 0 {
 		var connOK bool
 		var conn protocol.Connection
-		m.fmut.RLock()
+		m.mut.RLock()
 		if connIDs, connIDOK := m.deviceConnIDs[deviceID]; connIDOK {
 			conn, connOK = m.connections[connIDs[0]]
 		}
-		m.fmut.RUnlock()
+		m.mut.RUnlock()
 		// In case we've got ClusterConfig, and the connection disappeared
 		// from infront of our nose.
 		if connOK {
@@ -1339,8 +1323,8 @@ func (m *model) ensureIndexHandler(conn protocol.Connection) *indexHandlerRegist
 	deviceID := conn.DeviceID()
 	connID := conn.ConnectionID()
 
-	m.fmut.Lock()
+	m.mut.Lock()
-	defer m.fmut.Unlock()
+	defer m.mut.Unlock()
 
 	indexHandlerRegistry, ok := m.indexHandlers.Get(deviceID)
 	if ok && indexHandlerRegistry.conn.ConnectionID() == connID {
@@ -1370,8 +1354,8 @@ func (m *model) ensureIndexHandler(conn protocol.Connection) *indexHandlerRegist
 	return indexHandlerRegistry
 }
 
-func (m *model) getIndexHandlerPRLocked(conn protocol.Connection) (*indexHandlerRegistry, bool) {
+func (m *model) getIndexHandlerRLocked(conn protocol.Connection) (*indexHandlerRegistry, bool) {
-	// Reads from index handlers, which requires pmut to be read locked
+	// Reads from index handlers, which requires the mutex to be read locked
 
 	deviceID := conn.DeviceID()
 	connID := conn.ConnectionID()
@@ -1450,14 +1434,14 @@ func (m *model) ccHandleFolders(folders []protocol.Folder, deviceCfg config.Devi
 
 		if err := m.ccCheckEncryption(cfg, folderDevice, ccDeviceInfos[folder.ID], deviceCfg.Untrusted); err != nil {
 			sameError := false
-			m.fmut.Lock()
+			m.mut.Lock()
 			if devs, ok := m.folderEncryptionFailures[folder.ID]; ok {
 				sameError = devs[deviceID] == err
 			} else {
 				m.folderEncryptionFailures[folder.ID] = make(map[protocol.DeviceID]error)
 			}
 			m.folderEncryptionFailures[folder.ID][deviceID] = err
-			m.fmut.Unlock()
+			m.mut.Unlock()
 			msg := fmt.Sprintf("Failure checking encryption consistency with device %v for folder %v: %v", deviceID, cfg.Description(), err)
 			if sameError {
 				l.Debugln(msg)
@@ -1470,7 +1454,7 @@ func (m *model) ccHandleFolders(folders []protocol.Folder, deviceCfg config.Devi
 			}
 			return tempIndexFolders, seenFolders, err
 		}
-		m.fmut.Lock()
+		m.mut.Lock()
 		if devErrs, ok := m.folderEncryptionFailures[folder.ID]; ok {
 			if len(devErrs) == 1 {
 				delete(m.folderEncryptionFailures, folder.ID)
@@ -1478,7 +1462,7 @@ func (m *model) ccHandleFolders(folders []protocol.Folder, deviceCfg config.Devi
 				delete(m.folderEncryptionFailures[folder.ID], deviceID)
 			}
 		}
-		m.fmut.Unlock()
+		m.mut.Unlock()
 
 		// Handle indexes
 
@@ -1582,9 +1566,9 @@ func (m *model) ccCheckEncryption(fcfg config.FolderConfiguration, folderDevice
 		// hasTokenRemote == true
 		ccToken = ccDeviceInfos.remote.EncryptionPasswordToken
 	}
-	m.fmut.RLock()
+	m.mut.RLock()
 	token, ok := m.folderEncryptionPasswordTokens[fcfg.ID]
-	m.fmut.RUnlock()
+	m.mut.RUnlock()
 	if !ok {
 		var err error
 		token, err = readEncryptionToken(fcfg)
@@ -1598,9 +1582,9 @@ func (m *model) ccCheckEncryption(fcfg config.FolderConfiguration, folderDevice
 			}
 		}
 		if err == nil {
-			m.fmut.Lock()
+			m.mut.Lock()
 			m.folderEncryptionPasswordTokens[fcfg.ID] = token
-			m.fmut.Unlock()
+			m.mut.Unlock()
 		} else {
 			if err := writeEncryptionToken(ccToken, fcfg); err != nil {
 				if rerr, ok := redactPathError(err); ok {
@@ -1612,9 +1596,9 @@ func (m *model) ccCheckEncryption(fcfg config.FolderConfiguration, folderDevice
 					}
 				}
 			}
-			m.fmut.Lock()
+			m.mut.Lock()
 			m.folderEncryptionPasswordTokens[fcfg.ID] = ccToken
-			m.fmut.Unlock()
+			m.mut.Unlock()
 			// We can only announce ourselves once we have the token,
 			// thus we need to resend CCs now that we have it.
 			m.sendClusterConfig(fcfg.DeviceIDs())
@@ -1632,14 +1616,14 @@ func (m *model) sendClusterConfig(ids []protocol.DeviceID) {
 		return
 	}
 	ccConns := make([]protocol.Connection, 0, len(ids))
-	m.fmut.RLock()
+	m.mut.RLock()
 	for _, id := range ids {
 		if connIDs, ok := m.deviceConnIDs[id]; ok {
 			ccConns = append(ccConns, m.connections[connIDs[0]])
 		}
 	}
-	m.fmut.RUnlock()
+	m.mut.RUnlock()
-	// Generating cluster-configs acquires fmut -> must happen outside of pmut.
+	// Generating cluster-configs acquires the mutex.
 	for _, conn := range ccConns {
 		cm, passwords := m.generateClusterConfig(conn.DeviceID())
 		conn.SetFolderPasswords(passwords)
@@ -1875,10 +1859,10 @@ func (m *model) Closed(conn protocol.Connection, err error) {
 	connID := conn.ConnectionID()
 	deviceID := conn.DeviceID()
 
-	m.fmut.Lock()
+	m.mut.Lock()
 	conn, ok := m.connections[connID]
 	if !ok {
-		m.fmut.Unlock()
+		m.mut.Unlock()
 		return
 	}
 
@@ -1909,14 +1893,14 @@ func (m *model) Closed(conn protocol.Connection, err error) {
 		m.deviceConnIDs[deviceID] = remainingConns
 	}
 
-	m.fmut.Unlock()
+	m.mut.Unlock()
 	if wait != nil {
 		<-wait
 	}
 
-	m.fmut.RLock()
+	m.mut.RLock()
-	m.deviceDidCloseFRLocked(deviceID, time.Since(conn.EstablishedAt()))
+	m.deviceDidCloseRLocked(deviceID, time.Since(conn.EstablishedAt()))
-	m.fmut.RUnlock()
+	m.mut.RUnlock()
 
 	k := map[bool]string{false: "secondary", true: "primary"}[removedIsPrimary]
 	l.Infof("Lost %s connection to %s at %s: %v (%d remain)", k, deviceID.Short(), conn, err, len(remainingConns))
@@ -1969,10 +1953,10 @@ func (m *model) Request(conn protocol.Connection, folder, name string, _, size i
 
 	deviceID := conn.DeviceID()
 
-	m.fmut.RLock()
+	m.mut.RLock()
 	folderCfg, ok := m.folderCfgs[folder]
 	folderIgnores := m.folderIgnores[folder]
-	m.fmut.RUnlock()
+	m.mut.RUnlock()
 	if !ok {
 		// The folder might be already unpaused in the config, but not yet
 		// in the model.
@@ -2011,9 +1995,9 @@ func (m *model) Request(conn protocol.Connection, folder, name string, _, size i
 
 	// Restrict parallel requests by connection/device
 
-	m.fmut.RLock()
+	m.mut.RLock()
 	limiter := m.connRequestLimiters[deviceID]
-	m.fmut.RUnlock()
+	m.mut.RUnlock()
 
 	// The requestResponse releases the bytes to the buffer pool and the
 	// limiters when its Close method is called.
@@ -2142,9 +2126,9 @@ func (m *model) recheckFile(deviceID protocol.DeviceID, folder, name string, off
 	// to what we have in the database, yet the content we've read off the filesystem doesn't
 	// Something is fishy, invalidate the file and rescan it.
 	// The file will temporarily become invalid, which is ok as the content is messed up.
-	m.fmut.RLock()
+	m.mut.RLock()
 	runner, ok := m.folderRunners.Get(folder)
-	m.fmut.RUnlock()
+	m.mut.RUnlock()
 	if !ok {
 		l.Debugf("%v recheckFile: %s: %q / %q: Folder stopped before rescan could be scheduled", m, deviceID, folder, name)
 		return
@@ -2156,9 +2140,9 @@ func (m *model) recheckFile(deviceID protocol.DeviceID, folder, name string, off
 }
 
 func (m *model) CurrentFolderFile(folder string, file string) (protocol.FileInfo, bool, error) {
-	m.fmut.RLock()
+	m.mut.RLock()
 	fs, ok := m.folderFiles[folder]
-	m.fmut.RUnlock()
+	m.mut.RUnlock()
 	if !ok {
 		return protocol.FileInfo{}, false, ErrFolderMissing
 	}
@@ -2172,9 +2156,9 @@ func (m *model) CurrentFolderFile(folder string, file string) (protocol.FileInfo
 }
 
 func (m *model) CurrentGlobalFile(folder string, file string) (protocol.FileInfo, bool, error) {
-	m.fmut.RLock()
+	m.mut.RLock()
 	ffs, ok := m.folderFiles[folder]
-	m.fmut.RUnlock()
+	m.mut.RUnlock()
 	if !ok {
 		return protocol.FileInfo{}, false, ErrFolderMissing
 	}
@@ -2188,10 +2172,10 @@ func (m *model) CurrentGlobalFile(folder string, file string) (protocol.FileInfo
 }
 
 func (m *model) GetMtimeMapping(folder string, file string) (fs.MtimeMapping, error) {
-	m.fmut.RLock()
+	m.mut.RLock()
 	ffs, ok := m.folderFiles[folder]
 	fcfg := m.folderCfgs[folder]
-	m.fmut.RUnlock()
+	m.mut.RUnlock()
 	if !ok {
 		return fs.MtimeMapping{}, ErrFolderMissing
 	}
@@ -2200,19 +2184,19 @@ func (m *model) GetMtimeMapping(folder string, file string) (fs.MtimeMapping, er
 
 // Connection returns if we are connected to the given device.
 func (m *model) ConnectedTo(deviceID protocol.DeviceID) bool {
-	m.fmut.RLock()
+	m.mut.RLock()
 	_, ok := m.deviceConnIDs[deviceID]
-	m.fmut.RUnlock()
+	m.mut.RUnlock()
 	return ok
 }
 
 // LoadIgnores loads or refreshes the ignore patterns from disk, if the
 // folder is healthy, and returns the refreshed lines and patterns.
 func (m *model) LoadIgnores(folder string) ([]string, []string, error) {
-	m.fmut.RLock()
+	m.mut.RLock()
 	cfg, cfgOk := m.folderCfgs[folder]
 	ignores, ignoresOk := m.folderIgnores[folder]
-	m.fmut.RUnlock()
+	m.mut.RUnlock()
 
 	if !cfgOk {
 		cfg, cfgOk = m.cfg.Folder(folder)
@@ -2244,10 +2228,10 @@ func (m *model) LoadIgnores(folder string) ([]string, []string, error) {
 // whichever it may be. No attempt is made to load or refresh ignore
 // patterns from disk.
 func (m *model) CurrentIgnores(folder string) ([]string, []string, error) {
-	m.fmut.RLock()
+	m.mut.RLock()
 	_, cfgOk := m.folderCfgs[folder]
 	ignores, ignoresOk := m.folderIgnores[folder]
-	m.fmut.RUnlock()
+	m.mut.RUnlock()
 
 	if !cfgOk {
 		return nil, nil, fmt.Errorf("folder %s does not exist", folder)
@@ -2286,9 +2270,9 @@ func (m *model) setIgnores(cfg config.FolderConfiguration, content []string) err
 		return err
 	}
 
-	m.fmut.RLock()
+	m.mut.RLock()
 	runner, ok := m.folderRunners.Get(cfg.ID)
-	m.fmut.RUnlock()
+	m.mut.RUnlock()
 	if ok {
 		runner.ScheduleScan()
 	}
@@ -2335,7 +2319,7 @@ func (m *model) AddConnection(conn protocol.Connection, hello protocol.Hello) {
 	connID := conn.ConnectionID()
 	closed := make(chan struct{})
 
-	m.fmut.Lock()
+	m.mut.Lock()
 
 	m.connections[connID] = conn
 	m.closed[connID] = closed
@@ -2366,7 +2350,7 @@ func (m *model) AddConnection(conn protocol.Connection, hello protocol.Hello) {
 		l.Infof(`Additional connection (+%d) for device %s at %s`, len(m.deviceConnIDs[deviceID])-1, deviceID.Short(), conn)
 	}
 
-	m.fmut.Unlock()
+	m.mut.Unlock()
 
 	if (deviceCfg.Name == "" || m.cfg.Options().OverwriteRemoteDevNames) && hello.DeviceName != "" {
 		m.cfg.Modify(func(cfg *config.Configuration) {
@@ -2397,11 +2381,11 @@ func (m *model) scheduleConnectionPromotion() {
 // be called after adding new connections, and after closing a primary
 // device connection.
 func (m *model) promoteConnections() {
-	m.fmut.Lock()
+	m.mut.Lock()
-	defer m.fmut.Unlock()
+	defer m.mut.Unlock()
 
 	for deviceID, connIDs := range m.deviceConnIDs {
-		cm, passwords := m.generateClusterConfigFRLocked(deviceID)
+		cm, passwords := m.generateClusterConfigRLocked(deviceID)
 		if m.promotedConnID[deviceID] != connIDs[0] {
 			// The previously promoted connection is not the current
 			// primary; we should promote the primary connection to be the
@@ -2435,17 +2419,17 @@ func (m *model) promoteConnections() {
 func (m *model) DownloadProgress(conn protocol.Connection, folder string, updates []protocol.FileDownloadProgressUpdate) error {
 	deviceID := conn.DeviceID()
 
-	m.fmut.RLock()
+	m.mut.RLock()
 	cfg, ok := m.folderCfgs[folder]
-	m.fmut.RUnlock()
+	m.mut.RUnlock()
 
 	if !ok || cfg.DisableTempIndexes || !cfg.SharedWith(deviceID) {
 		return nil
 	}
 
-	m.fmut.RLock()
+	m.mut.RLock()
 	downloads := m.deviceDownloads[deviceID]
-	m.fmut.RUnlock()
+	m.mut.RUnlock()
 	downloads.Update(folder, updates)
 	state := downloads.GetBlockCounts(folder)
 
@@ -2459,15 +2443,15 @@ func (m *model) DownloadProgress(conn protocol.Connection, folder string, update
 }
 
 func (m *model) deviceWasSeen(deviceID protocol.DeviceID) {
-	m.fmut.RLock()
+	m.mut.RLock()
 	sr, ok := m.deviceStatRefs[deviceID]
-	m.fmut.RUnlock()
+	m.mut.RUnlock()
 	if ok {
 		_ = sr.WasSeen()
 	}
 }
 
-func (m *model) deviceDidCloseFRLocked(deviceID protocol.DeviceID, duration time.Duration) {
+func (m *model) deviceDidCloseRLocked(deviceID protocol.DeviceID, duration time.Duration) {
 	if sr, ok := m.deviceStatRefs[deviceID]; ok {
 		_ = sr.LastConnectionDuration(duration)
 		_ = sr.WasSeen()
@@ -2490,8 +2474,8 @@ func (m *model) requestGlobal(ctx context.Context, deviceID protocol.DeviceID, f
 // ("primary") connection, which is dedicated to index data, and pick a
 // random one of the others.
 func (m *model) requestConnectionForDevice(deviceID protocol.DeviceID) (protocol.Connection, bool) {
-	m.fmut.RLock()
+	m.mut.RLock()
-	defer m.fmut.RUnlock()
+	defer m.mut.RUnlock()
 
 	connIDs, ok := m.deviceConnIDs[deviceID]
 	if !ok {
@@ -2512,12 +2496,12 @@ func (m *model) requestConnectionForDevice(deviceID protocol.DeviceID) (protocol
 }
 
 func (m *model) ScanFolders() map[string]error {
-	m.fmut.RLock()
+	m.mut.RLock()
 	folders := make([]string, 0, len(m.folderCfgs))
 	for folder := range m.folderCfgs {
 		folders = append(folders, folder)
 	}
-	m.fmut.RUnlock()
+	m.mut.RUnlock()
 
 	errors := make(map[string]error, len(m.folderCfgs))
 	errorsMut := sync.NewMutex()
@@ -2545,10 +2529,10 @@ func (m *model) ScanFolder(folder string) error {
 }
 
 func (m *model) ScanFolderSubdirs(folder string, subs []string) error {
-	m.fmut.RLock()
+	m.mut.RLock()
-	err := m.checkFolderRunningLocked(folder)
+	err := m.checkFolderRunningRLocked(folder)
 	runner, _ := m.folderRunners.Get(folder)
-	m.fmut.RUnlock()
+	m.mut.RUnlock()
 
 	if err != nil {
 		return err
@@ -2558,9 +2542,9 @@ func (m *model) ScanFolderSubdirs(folder string, subs []string) error {
 }
 
 func (m *model) DelayScan(folder string, next time.Duration) {
-	m.fmut.RLock()
+	m.mut.RLock()
 	runner, ok := m.folderRunners.Get(folder)
-	m.fmut.RUnlock()
+	m.mut.RUnlock()
 	if !ok {
 		return
 	}
@@ -2570,10 +2554,10 @@ func (m *model) DelayScan(folder string, next time.Duration) {
 // numHashers returns the number of hasher routines to use for a given folder,
 // taking into account configuration and available CPU cores.
 func (m *model) numHashers(folder string) int {
-	m.fmut.RLock()
+	m.mut.RLock()
 	folderCfg := m.folderCfgs[folder]
 	numFolders := len(m.folderCfgs)
-	m.fmut.RUnlock()
+	m.mut.RUnlock()
 
 	if folderCfg.Hashers > 0 {
 		// Specific value set in the config, use that.
@@ -2599,12 +2583,12 @@ func (m *model) numHashers(folder string) int {
 // generateClusterConfig returns a ClusterConfigMessage that is correct and the
 // set of folder passwords for the given peer device
 func (m *model) generateClusterConfig(device protocol.DeviceID) (protocol.ClusterConfig, map[string]string) {
-	m.fmut.RLock()
+	m.mut.RLock()
-	defer m.fmut.RUnlock()
+	defer m.mut.RUnlock()
-	return m.generateClusterConfigFRLocked(device)
+	return m.generateClusterConfigRLocked(device)
 }
 
-func (m *model) generateClusterConfigFRLocked(device protocol.DeviceID) (protocol.ClusterConfig, map[string]string) {
+func (m *model) generateClusterConfigRLocked(device protocol.DeviceID) (protocol.ClusterConfig, map[string]string) {
 	var message protocol.ClusterConfig
 	folders := m.cfg.FolderList()
 	passwords := make(map[string]string, len(folders))
@@ -2678,9 +2662,9 @@ func (m *model) generateClusterConfigFRLocked(device protocol.DeviceID) (protoco
 }
 
 func (m *model) State(folder string) (string, time.Time, error) {
-	m.fmut.RLock()
+	m.mut.RLock()
 	runner, ok := m.folderRunners.Get(folder)
-	m.fmut.RUnlock()
+	m.mut.RUnlock()
 	if !ok {
 		// The returned error should be an actual folder error, so returning
 		// errors.New("does not exist") or similar here would be
@@ -2692,10 +2676,10 @@ func (m *model) State(folder string) (string, time.Time, error) {
 }
 
 func (m *model) FolderErrors(folder string) ([]FileError, error) {
-	m.fmut.RLock()
+	m.mut.RLock()
-	err := m.checkFolderRunningLocked(folder)
+	err := m.checkFolderRunningRLocked(folder)
 	runner, _ := m.folderRunners.Get(folder)
-	m.fmut.RUnlock()
+	m.mut.RUnlock()
 	if err != nil {
 		return nil, err
 	}
@@ -2703,10 +2687,10 @@ func (m *model) FolderErrors(folder string) ([]FileError, error) {
 }
 
 func (m *model) WatchError(folder string) error {
-	m.fmut.RLock()
+	m.mut.RLock()
-	err := m.checkFolderRunningLocked(folder)
+	err := m.checkFolderRunningRLocked(folder)
 	runner, _ := m.folderRunners.Get(folder)
-	m.fmut.RUnlock()
+	m.mut.RUnlock()
 	if err != nil {
 		return nil // If the folder isn't running, there's no error to report.
 	}
@@ -2716,9 +2700,9 @@ func (m *model) WatchError(folder string) error {
 func (m *model) Override(folder string) {
 	// Grab the runner and the file set.
 
-	m.fmut.RLock()
+	m.mut.RLock()
 	runner, ok := m.folderRunners.Get(folder)
-	m.fmut.RUnlock()
+	m.mut.RUnlock()
 	if !ok {
 		return
 	}
@@ -2731,9 +2715,9 @@ func (m *model) Override(folder string) {
 func (m *model) Revert(folder string) {
 	// Grab the runner and the file set.
 
-	m.fmut.RLock()
+	m.mut.RLock()
 	runner, ok := m.folderRunners.Get(folder)
-	m.fmut.RUnlock()
+	m.mut.RUnlock()
 	if !ok {
 		return
 	}
@@ -2761,9 +2745,9 @@ func findByName(slice []*TreeEntry, name string) *TreeEntry {
 }
 
 func (m *model) GlobalDirectoryTree(folder, prefix string, levels int, dirsOnly bool) ([]*TreeEntry, error) {
-	m.fmut.RLock()
+	m.mut.RLock()
 	files, ok := m.folderFiles[folder]
-	m.fmut.RUnlock()
+	m.mut.RUnlock()
 	if !ok {
 		return nil, ErrFolderMissing
 	}
@@ -2833,10 +2817,10 @@ func (m *model) GlobalDirectoryTree(folder, prefix string, levels int, dirsOnly
 }
 
 func (m *model) GetFolderVersions(folder string) (map[string][]versioner.FileVersion, error) {
-	m.fmut.RLock()
+	m.mut.RLock()
-	err := m.checkFolderRunningLocked(folder)
+	err := m.checkFolderRunningRLocked(folder)
 	ver := m.folderVersioners[folder]
-	m.fmut.RUnlock()
+	m.mut.RUnlock()
 	if err != nil {
 		return nil, err
 	}
@@ -2848,11 +2832,11 @@ func (m *model) GetFolderVersions(folder string) (map[string][]versioner.FileVer
 }
 
 func (m *model) RestoreFolderVersions(folder string, versions map[string]time.Time) (map[string]error, error) {
-	m.fmut.RLock()
+	m.mut.RLock()
-	err := m.checkFolderRunningLocked(folder)
+	err := m.checkFolderRunningRLocked(folder)
 	fcfg := m.folderCfgs[folder]
 	ver := m.folderVersioners[folder]
-	m.fmut.RUnlock()
+	m.mut.RUnlock()
 	if err != nil {
 		return nil, err
 	}
@@ -2877,12 +2861,8 @@ func (m *model) RestoreFolderVersions(folder string, versions map[string]time.Ti
 }
 
 func (m *model) Availability(folder string, file protocol.FileInfo, block protocol.BlockInfo) ([]Availability, error) {
-	// The slightly unusual locking sequence here is because we need to hold
+	m.mut.RLock()
-	// pmut for the duration (as the value returned from foldersFiles can
+	defer m.mut.RUnlock()
-	// get heavily modified on Close()), but also must acquire fmut before
-	// pmut. (The locks can be *released* in any order.)
-	m.fmut.RLock()
-	defer m.fmut.RUnlock()
 
 	fs, ok := m.folderFiles[folder]
 	cfg := m.folderCfgs[folder]
@@ -2897,16 +2877,16 @@ func (m *model) Availability(folder string, file protocol.FileInfo, block protoc
 	}
 	defer snap.Release()
 
-	return m.availabilityInSnapshotPRlocked(cfg, snap, file, block), nil
+	return m.availabilityInSnapshotRLocked(cfg, snap, file, block), nil
 }
 
 func (m *model) availabilityInSnapshot(cfg config.FolderConfiguration, snap *db.Snapshot, file protocol.FileInfo, block protocol.BlockInfo) []Availability {
-	m.fmut.RLock()
+	m.mut.RLock()
-	defer m.fmut.RUnlock()
+	defer m.mut.RUnlock()
-	return m.availabilityInSnapshotPRlocked(cfg, snap, file, block)
+	return m.availabilityInSnapshotRLocked(cfg, snap, file, block)
 }
 
-func (m *model) availabilityInSnapshotPRlocked(cfg config.FolderConfiguration, snap *db.Snapshot, file protocol.FileInfo, block protocol.BlockInfo) []Availability {
+func (m *model) availabilityInSnapshotRLocked(cfg config.FolderConfiguration, snap *db.Snapshot, file protocol.FileInfo, block protocol.BlockInfo) []Availability {
 	var availabilities []Availability
 	for _, device := range snap.Availability(file.Name) {
 		if _, ok := m.remoteFolderStates[device]; !ok {
@@ -2932,9 +2912,9 @@ func (m *model) availabilityInSnapshotPRlocked(cfg config.FolderConfiguration, s
 
 // BringToFront bumps the given files priority in the job queue.
 func (m *model) BringToFront(folder, file string) {
-	m.fmut.RLock()
+	m.mut.RLock()
 	runner, ok := m.folderRunners.Get(folder)
-	m.fmut.RUnlock()
+	m.mut.RUnlock()
 
 	if ok {
 		runner.BringToFront(file)
@@ -2942,8 +2922,8 @@ func (m *model) BringToFront(folder, file string) {
 }
 
 func (m *model) ResetFolder(folder string) error {
-	m.fmut.RLock()
+	m.mut.RLock()
-	defer m.fmut.RUnlock()
+	defer m.mut.RUnlock()
 	_, ok := m.folderRunners.Get(folder)
 	if ok {
 		return errors.New("folder must be paused when resetting")
@@ -3037,9 +3017,9 @@ func (m *model) CommitConfiguration(from, to config.Configuration) bool {
 				// If we don't have the encryption token yet, we need to drop
 				// the connection to make the remote re-send the cluster-config
 				// and with it the token.
-				m.fmut.RLock()
+				m.mut.RLock()
 				_, ok := m.folderEncryptionPasswordTokens[toCfg.ID]
-				m.fmut.RUnlock()
+				m.mut.RUnlock()
 				if !ok {
 					closeDevices = append(closeDevices, toCfg.DeviceIDs()...)
 				} else {
@@ -3065,9 +3045,9 @@ func (m *model) CommitConfiguration(from, to config.Configuration) bool {
 		fromCfg, ok := fromDevices[deviceID]
 		if !ok {
 			sr := stats.NewDeviceStatisticsReference(m.db, deviceID)
-			m.fmut.Lock()
+			m.mut.Lock()
 			m.deviceStatRefs[deviceID] = sr
-			m.fmut.Unlock()
+			m.mut.Unlock()
 			continue
 		}
 		delete(fromDevices, deviceID)
@@ -3090,23 +3070,23 @@ func (m *model) CommitConfiguration(from, to config.Configuration) bool {
 		}
 
 		if toCfg.MaxRequestKiB != fromCfg.MaxRequestKiB {
-			m.fmut.Lock()
+			m.mut.Lock()
-			m.setConnRequestLimitersPLocked(toCfg)
+			m.setConnRequestLimitersLocked(toCfg)
-			m.fmut.Unlock()
+			m.mut.Unlock()
 		}
 	}
 
 	// Clean up after removed devices
 	removedDevices := make([]protocol.DeviceID, 0, len(fromDevices))
-	m.fmut.Lock()
+	m.mut.Lock()
 	for deviceID := range fromDevices {
 		delete(m.deviceStatRefs, deviceID)
 		removedDevices = append(removedDevices, deviceID)
 		delete(clusterConfigDevices, deviceID)
 	}
-	m.fmut.Unlock()
+	m.mut.Unlock()
 
-	m.fmut.RLock()
+	m.mut.RLock()
 	for _, id := range closeDevices {
 		delete(clusterConfigDevices, id)
 		if conns, ok := m.deviceConnIDs[id]; ok {
@@ -3123,8 +3103,8 @@ func (m *model) CommitConfiguration(from, to config.Configuration) bool {
 			}
 		}
 	}
-	m.fmut.RUnlock()
+	m.mut.RUnlock()
-	// Generating cluster-configs acquires fmut -> must happen outside of pmut.
+	// Generating cluster-configs acquires the mutex.
 	m.sendClusterConfig(clusterConfigDevices.AsSlice())
 
 	ignoredDevices := observedDeviceSet(to.IgnoredDevices)
@@ -3144,8 +3124,8 @@ func (m *model) CommitConfiguration(from, to config.Configuration) bool {
 	return true
 }
 
-func (m *model) setConnRequestLimitersPLocked(cfg config.DeviceConfiguration) {
+func (m *model) setConnRequestLimitersLocked(cfg config.DeviceConfiguration) {
-	// Touches connRequestLimiters which is protected by pmut.
+	// Touches connRequestLimiters which is protected by the mutex.
 	// 0: default, <0: no limiting
 	switch {
 	case cfg.MaxRequestKiB > 0:
@@ -3251,10 +3231,10 @@ func (m *model) cleanPending(existingDevices map[protocol.DeviceID]config.Device
 	}
 }
 
-// checkFolderRunningLocked returns nil if the folder is up and running and a
+// checkFolderRunningRLocked returns nil if the folder is up and running and a
 // descriptive error if not.
-// Need to hold (read) lock on m.fmut when calling this.
+// Need to hold (read) lock on m.mut when calling this.
-func (m *model) checkFolderRunningLocked(folder string) error {
+func (m *model) checkFolderRunningRLocked(folder string) error {
 	_, ok := m.folderRunners.Get(folder)
 	if ok {
 		return nil

lib/model/model_test.go

@@ -902,13 +902,13 @@ func TestIssue5063(t *testing.T) {
 	defer cleanupModel(m)
 	defer cancel()
 
-	m.fmut.Lock()
+	m.mut.Lock()
 	for _, c := range m.connections {
 		conn := c.(*fakeConnection)
 		conn.CloseCalls(func(_ error) {})
 		defer m.Closed(c, errStopped) // to unblock deferred m.Stop()
 	}
-	m.fmut.Unlock()
+	m.mut.Unlock()
 
 	wg := sync.WaitGroup{}
 
@@ -1524,10 +1524,10 @@ func TestIgnores(t *testing.T) {
 		FilesystemType: fs.FilesystemTypeFake,
 	}
 	ignores := ignore.New(fcfg.Filesystem(nil), ignore.WithCache(m.cfg.Options().CacheIgnoredFiles))
-	m.fmut.Lock()
+	m.mut.Lock()
 	m.folderCfgs[fcfg.ID] = fcfg
 	m.folderIgnores[fcfg.ID] = ignores
-	m.fmut.Unlock()
+	m.mut.Unlock()
 
 	_, _, err = m.LoadIgnores("fresh")
 	if err != nil {
@@ -2973,7 +2973,7 @@ func TestConnCloseOnRestart(t *testing.T) {
 	ci := &protocolmocks.ConnectionInfo{}
 	ci.ConnectionIDReturns(srand.String(16))
 	m.AddConnection(protocol.NewConnection(device1, br, nw, testutil.NoopCloser{}, m, ci, protocol.CompressionNever, nil, m.keyGen), protocol.Hello{})
-	m.fmut.RLock()
+	m.mut.RLock()
 	if len(m.closed) != 1 {
 		t.Fatalf("Expected just one conn (len(m.closed) == %v)", len(m.closed))
 	}
@@ -2981,7 +2981,7 @@ func TestConnCloseOnRestart(t *testing.T) {
 	for _, c := range m.closed {
 		closed = c
 	}
-	m.fmut.RUnlock()
+	m.mut.RUnlock()
 
 	waiter, err := w.RemoveDevice(device1)
 	if err != nil {
@@ -3074,12 +3074,12 @@ func TestDevicePause(t *testing.T) {
 	sub := m.evLogger.Subscribe(events.DevicePaused)
 	defer sub.Unsubscribe()
 
-	m.fmut.RLock()
+	m.mut.RLock()
 	var closed chan struct{}
 	for _, c := range m.closed {
 		closed = c
 	}
-	m.fmut.RUnlock()
+	m.mut.RUnlock()
 
 	pauseDevice(t, m.cfg, device1, true)
 
@@ -3754,9 +3754,9 @@ func TestCompletionEmptyGlobal(t *testing.T) {
 	defer wcfgCancel()
 	defer cleanupModelAndRemoveDir(m, fcfg.Filesystem(nil).URI())
 	files := []protocol.FileInfo{{Name: "foo", Version: protocol.Vector{}.Update(myID.Short()), Sequence: 1}}
-	m.fmut.Lock()
+	m.mut.Lock()
 	m.folderFiles[fcfg.ID].Update(protocol.LocalDeviceID, files)
-	m.fmut.Unlock()
+	m.mut.Unlock()
 	files[0].Deleted = true
 	files[0].Version = files[0].Version.Update(device1.Short())
 	must(t, m.IndexUpdate(conn, fcfg.ID, files))

lib/model/requests_test.go

@@ -1287,9 +1287,9 @@ func TestRequestReceiveEncrypted(t *testing.T) {
 
 	files := genFiles(2)
 	files[1].LocalFlags = protocol.FlagLocalReceiveOnly
-	m.fmut.RLock()
+	m.mut.RLock()
 	fset := m.folderFiles[fcfg.ID]
-	m.fmut.RUnlock()
+	m.mut.RUnlock()
 	fset.Update(protocol.LocalDeviceID, files)
 
 	indexChan := make(chan []protocol.FileInfo, 10)

lib/model/testutils_test.go

@@ -295,9 +295,9 @@ func folderIgnoresAlwaysReload(t testing.TB, m *testModel, fcfg config.FolderCon
 	m.removeFolder(fcfg)
 	fset := newFileSet(t, fcfg.ID, m.db)
 	ignores := ignore.New(fcfg.Filesystem(nil), ignore.WithCache(true), ignore.WithChangeDetector(newAlwaysChanged()))
-	m.fmut.Lock()
+	m.mut.Lock()
 	m.addAndStartFolderLockedWithIgnores(fcfg, fset, ignores)
-	m.fmut.Unlock()
+	m.mut.Unlock()
 }
 
 func basicClusterConfig(local, remote protocol.DeviceID, folders ...string) protocol.ClusterConfig {
@@ -319,9 +319,9 @@ func basicClusterConfig(local, remote protocol.DeviceID, folders ...string) prot
 }
 
 func localIndexUpdate(m *testModel, folder string, fs []protocol.FileInfo) {
-	m.fmut.RLock()
+	m.mut.RLock()
 	fset := m.folderFiles[folder]
-	m.fmut.RUnlock()
+	m.mut.RUnlock()
 
 	fset.Update(protocol.LocalDeviceID, fs)
 	seq := fset.Sequence(protocol.LocalDeviceID)

lib/model/util.go

@@ -11,100 +11,12 @@ import (
 	"errors"
 	"fmt"
 	"path/filepath"
-	"strings"
-	"sync"
 	"time"
 
 	"github.com/prometheus/client_golang/prometheus"
-	"github.com/syncthing/syncthing/lib/events"
 	"github.com/syncthing/syncthing/lib/fs"
-	"github.com/syncthing/syncthing/lib/ur"
 )
 
-type Holdable interface {
-	Holders() string
-}
-
-func newDeadlockDetector(timeout time.Duration, evLogger events.Logger, fatal func(error)) *deadlockDetector {
-	return &deadlockDetector{
-		warnTimeout:  timeout,
-		fatalTimeout: 10 * timeout,
-		lockers:      make(map[string]sync.Locker),
-		evLogger:     evLogger,
-		fatal:        fatal,
-	}
-}
-
-type deadlockDetector struct {
-	warnTimeout, fatalTimeout time.Duration
-	lockers                   map[string]sync.Locker
-	evLogger                  events.Logger
-	fatal                     func(error)
-}
-
-func (d *deadlockDetector) Watch(name string, mut sync.Locker) {
-	d.lockers[name] = mut
-	go func() {
-		for {
-			time.Sleep(d.warnTimeout / 4)
-			done := make(chan struct{}, 1)
-
-			go func() {
-				mut.Lock()
-				_ = 1 // empty critical section
-				mut.Unlock()
-				done <- struct{}{}
-			}()
-
-			d.watchInner(name, done)
-		}
-	}()
-}
-
-func (d *deadlockDetector) watchInner(name string, done chan struct{}) {
-	warn := time.NewTimer(d.warnTimeout)
-	fatal := time.NewTimer(d.fatalTimeout)
-	defer func() {
-		warn.Stop()
-		fatal.Stop()
-	}()
-
-	select {
-	case <-warn.C:
-		failure := ur.FailureDataWithGoroutines(fmt.Sprintf("potential deadlock detected at %s (short timeout)", name))
-		failure.Extra["timeout"] = d.warnTimeout.String()
-		d.evLogger.Log(events.Failure, failure)
-	case <-done:
-		return
-	}
-
-	select {
-	case <-fatal.C:
-		err := fmt.Errorf("potential deadlock detected at %s (long timeout)", name)
-		failure := ur.FailureDataWithGoroutines(err.Error())
-		failure.Extra["timeout"] = d.fatalTimeout.String()
-		others := d.otherHolders()
-		failure.Extra["other-holders"] = others
-		d.evLogger.Log(events.Failure, failure)
-		d.fatal(err)
-		// Give it a minute to shut down gracefully, maybe shutting down
-		// can get out of the deadlock (or it's not really a deadlock).
-		time.Sleep(time.Minute)
-		panic(fmt.Sprintf("%v:\n%v", err, others))
-	case <-done:
-	}
-}
-
-func (d *deadlockDetector) otherHolders() string {
-	var b strings.Builder
-	for otherName, otherMut := range d.lockers {
-		if otherHolder, ok := otherMut.(Holdable); ok {
-			b.WriteString("===" + otherName + "===\n" + otherHolder.Holders() + "\n")
-		}
-	}
-	return b.String()
-}
-
 // inWritableDir calls fn(path), while making sure that the directory
 // containing `path` is writable for the duration of the call.
 func inWritableDir(fn func(string) error, targetFs fs.Filesystem, path string, ignorePerms bool) error {

lib/sync/debug.go

@@ -11,7 +11,6 @@ import (
 	"strconv"
 	"time"
 
-	deadlock "github.com/sasha-s/go-deadlock"
 	"github.com/syncthing/syncthing/lib/logger"
 )
 
@@ -23,7 +22,6 @@ var (
 	// }" variable, as it may be rather performance critical and does
 	// nonstandard things (from a debug logging PoV).
 	debug = logger.DefaultLogger.ShouldDebug("sync")
-	useDeadlock = false
 )
 
 func init() {
@@ -31,10 +29,4 @@ func init() {
 		threshold = time.Duration(n) * time.Millisecond
 	}
 	l.Debugf("Enabling lock logging at %v threshold", threshold)
-
-	if n, _ := strconv.Atoi(os.Getenv("STDEADLOCKTIMEOUT")); n > 0 {
-		deadlock.Opts.DeadlockTimeout = time.Duration(n) * time.Second
-		l.Debugf("Enabling lock deadlocking at %v", deadlock.Opts.DeadlockTimeout)
-		useDeadlock = true
-	}
 }

lib/sync/sync.go

@@ -15,8 +15,6 @@ import (
 	"sync"
 	"sync/atomic"
 	"time"
-
-	"github.com/sasha-s/go-deadlock"
 )
 
 var timeNow = time.Now
@@ -39,9 +37,6 @@ type WaitGroup interface {
 }
 
 func NewMutex() Mutex {
-	if useDeadlock {
-		return &deadlock.Mutex{}
-	}
 	if debug {
 		mutex := &loggedMutex{}
 		mutex.holder.Store(holder{})
@@ -51,9 +46,6 @@ func NewMutex() Mutex {
 }
 
 func NewRWMutex() RWMutex {
-	if useDeadlock {
-		return &deadlock.RWMutex{}
-	}
 	if debug {
 		mutex := &loggedRWMutex{
 			readHolders: make(map[int][]holder),

lib/syncthing/syncthing.go

@@ -55,7 +55,6 @@ const (
 
 type Options struct {
 	AuditWriter    io.Writer
-	DeadlockTimeoutS int
 	NoUpgrade      bool
 	ProfilerAddr   string
 	ResetDeltaIdxs bool
@@ -251,12 +250,6 @@ func (a *App) startup() error {
 	keyGen := protocol.NewKeyGenerator()
 	m := model.NewModel(a.cfg, a.myID, a.ll, protectedFiles, a.evLogger, keyGen)
 
-	if a.opts.DeadlockTimeoutS > 0 {
-		m.StartDeadlockDetector(time.Duration(a.opts.DeadlockTimeoutS) * time.Second)
-	} else if !build.IsRelease || build.IsBeta {
-		m.StartDeadlockDetector(20 * time.Minute)
-	}
-
 	a.mainService.Add(m)
 
 	// The TLS configuration is used for both the listening socket and outgoing