metrics/cgroups: fix deadlock issue in Add during Collect

The Collector.Collect will be the field ns'Collect's callback, which be
invoked periodically with internal lock. And Collector.Add also runs
with ns.Lock in Collector.Lock, which is easy to cause deadlock.

Goroutine X:

	ns.Collect
	  ns.Lock
	    Collector.Collect
	      Collector.RLock

Goroutine Y:

	Collector.Add
	  Collector.Lock
	    ns.Lock

We should use ns.Lock without Collector.Lock in Add.

Fix: #6772

Signed-off-by: Wei Fu <fuweid89@gmail.com>

metrics/cgroups/v2/metrics.go

@@ -25,21 +25,14 @@ import (
 	"sync"
 
 	"github.com/containerd/containerd/log"
+	"github.com/containerd/containerd/metrics/cgroups/common"
 	v2 "github.com/containerd/containerd/metrics/types/v2"
 	"github.com/containerd/containerd/namespaces"
 	"github.com/containerd/typeurl"
 	"github.com/docker/go-metrics"
-	"github.com/gogo/protobuf/types"
 	"github.com/prometheus/client_golang/prometheus"
 )
 
-// Statable type that returns cgroup metrics
-type Statable interface {
-	ID() string
-	Namespace() string
-	Stats(context.Context) (*types.Any, error)
-}
-
 // NewCollector registers the collector with the provided namespace and returns it so
 // that cgroups can be added for collection
 func NewCollector(ns *metrics.Namespace) *Collector {
@@ -64,7 +57,7 @@ func taskID(id, namespace string) string {
 }
 
 type entry struct {
-	task Statable
+	task common.Statable
 	// ns is an optional child namespace that contains additional to parent labels.
 	// This can be used to append task specific labels to be able to differentiate the different containerd metrics.
 	ns *metrics.Namespace
@@ -73,12 +66,34 @@ type entry struct {
 // Collector provides the ability to collect container stats and export
 // them in the prometheus format
 type Collector struct {
-	mu sync.RWMutex
-
-	tasks         map[string]entry
 	ns            *metrics.Namespace
-	metrics       []*metric
 	storedMetrics chan prometheus.Metric
+
+	// TODO(fuweid):
+	//
+	// The Collector.Collect will be the field ns'Collect's callback,
+	// which be invoked periodically with internal lock. And Collector.Add
+	// might also invoke ns.Lock if the labels is not nil, which is easy to
+	// cause dead-lock.
+	//
+	// Goroutine X:
+	//
+	//	ns.Collect
+	//   	  ns.Lock
+	//          Collector.Collect
+	//            Collector.RLock
+	//
+	//
+	// Goroutine Y:
+	//
+	//	Collector.Add
+	//        ...(RLock/Lock)
+	//	    ns.Lock
+	//
+	// I think we should seek the way to decouple ns from Collector.
+	mu      sync.RWMutex
+	tasks   map[string]entry
+	metrics []*metric
 }
 
 // Describe prometheus metrics
@@ -141,26 +156,29 @@ func (c *Collector) collect(entry entry, ch chan<- prometheus.Metric, block bool
 }
 
 // Add adds the provided cgroup and id so that metrics are collected and exported
-func (c *Collector) Add(t Statable, labels map[string]string) error {
+func (c *Collector) Add(t common.Statable, labels map[string]string) error {
 	if c.ns == nil {
 		return nil
 	}
-	c.mu.Lock()
-	defer c.mu.Unlock()
+	c.mu.RLock()
 	id := taskID(t.ID(), t.Namespace())
-	if _, ok := c.tasks[id]; ok {
+	_, ok := c.tasks[id]
+	c.mu.RUnlock()
+	if ok {
 		return nil // requests to collect metrics should be idempotent
 	}
 	entry := entry{task: t}
 	if labels != nil {
 		entry.ns = c.ns.WithConstLabels(labels)
 	}
+	c.mu.Lock()
 	c.tasks[id] = entry
+	c.mu.Unlock()
 	return nil
 }
 
 // Remove removes the provided cgroup by id from the collector
-func (c *Collector) Remove(t Statable) {
+func (c *Collector) Remove(t common.Statable) {
 	if c.ns == nil {
 		return
 	}