devmapper: Implement a goroutine to cleanup deleted devices

Start a goroutine which runs every 30 seconds and if there are deferred
deleted devices, it tries to clean those up.

Also it moves the call to cleanupDeletedDevices() into goroutine and
moves the locking completely inside the function. Now function does not
assume that device lock is held at the time of entry.

Signed-off-by: Vivek Goyal <vgoyal@redhat.com>

daemon/graphdriver/devmapper/deviceset.go

@@ -111,6 +111,7 @@ type DeviceSet struct {
 	deferredDelete        bool   // use deferred deletion
 	BaseDeviceUUID        string //save UUID of base device
 	nrDeletedDevices      uint   //number of deleted devices
+	deletionWorkerTicker  *time.Ticker
 }
 
 // DiskUsage contains information about disk usage and is used when reporting Status of a device.
@@ -580,9 +581,10 @@ func (devices *DeviceSet) migrateOldMetaData() error {
 }
 
 // Cleanup deleted devices. It assumes that all the devices have been
-// loaded in the hash table. Should be called with devices.Lock() held.
-// Will drop the lock for device deletion and return with lock acquired.
+// loaded in the hash table.
 func (devices *DeviceSet) cleanupDeletedDevices() error {
+	devices.Lock()
+
 	// If there are no deleted devices, there is nothing to do.
 	if devices.nrDeletedDevices == 0 {
 		return nil
@@ -601,7 +603,6 @@ func (devices *DeviceSet) cleanupDeletedDevices() error {
 	// Delete the deleted devices. DeleteDevice() first takes the info lock
 	// and then devices.Lock(). So drop it to avoid deadlock.
 	devices.Unlock()
-	defer devices.Lock()
 
 	for _, info := range deletedDevices {
 		// This will again try deferred deletion.
@@ -622,6 +623,18 @@ func (devices *DeviceSet) countDeletedDevices() {
 	}
 }
 
+func (devices *DeviceSet) startDeviceDeletionWorker() {
+	// Deferred deletion is not enabled. Don't do anything.
+	if !devices.deferredDelete {
+		return
+	}
+
+	logrus.Debugf("devmapper: Worker to cleanup deleted devices started")
+	for range devices.deletionWorkerTicker.C {
+		devices.cleanupDeletedDevices()
+	}
+}
+
 func (devices *DeviceSet) initMetaData() error {
 	devices.Lock()
 	defer devices.Unlock()
@@ -648,10 +661,8 @@ func (devices *DeviceSet) initMetaData() error {
 		return err
 	}
 
-	if err := devices.cleanupDeletedDevices(); err != nil {
-		return err
-	}
-
+	// Start a goroutine to cleanup Deleted Devices
+	go devices.startDeviceDeletionWorker()
 	return nil
 }
 
@@ -1861,6 +1872,13 @@ func (devices *DeviceSet) Shutdown() error {
 
 	var devs []*devInfo
 
+	// Stop deletion worker. This should start delivering new events to
+	// ticker channel. That means no new instance of cleanupDeletedDevice()
+	// will run after this call. If one instance is already running at
+	// the time of the call, it must be holding devices.Lock() and
+	// we will block on this lock till cleanup function exits.
+	devices.deletionWorkerTicker.Stop()
+
 	devices.Lock()
 	// Save DeviceSet Metadata first. Docker kills all threads if they
 	// don't finish in certain time. It is possible that Shutdown()
@@ -2200,6 +2218,7 @@ func NewDeviceSet(root string, doInit bool, options []string) (*DeviceSet, error
 		doBlkDiscard:          true,
 		thinpBlockSize:        defaultThinpBlockSize,
 		deviceIDMap:           make([]byte, deviceIDMapSz),
+		deletionWorkerTicker:  time.NewTicker(time.Second * 30),
 	}
 
 	foundBlkDiscard := false