Revert "Using atomic instead of mutex and delete scratch slice" (#1846)

* Remove a redundant field and clarify the comments.

* Revert "Using atomic instead of mutex and delete scratch slice (#1833)"

This reverts commit 19c50cd.

workerpool.go

@@ -6,7 +6,6 @@ import (
 	"runtime"
 	"strings"
 	"sync"
-	"sync/atomic"
 	"time"
 )
 
@@ -22,57 +21,29 @@ type workerPool struct {
 
 	// Function for serving server connections.
 	// It must leave c unclosed.
-	ready      workerChanStack
 	WorkerFunc ServeHandler
 
 	stopCh chan struct{}
 
 	connState func(net.Conn, ConnState)
 
+	ready []*workerChan
+
 	MaxWorkersCount int
 
 	MaxIdleWorkerDuration time.Duration
 
-	workersCount int32
+	workersCount int
 
-	mustStop atomic.Bool
+	lock sync.Mutex
 
 	LogAllErrors bool
+	mustStop     bool
 }
 
 type workerChan struct {
-	next *workerChan
-
-	ch chan net.Conn
-
-	lastUseTime int64
-}
-
-type workerChanStack struct {
-	head atomic.Pointer[workerChan]
-}
-
-func (s *workerChanStack) push(ch *workerChan) {
-	for {
-		oldHead := s.head.Load()
-		ch.next = oldHead
-		if s.head.CompareAndSwap(oldHead, ch) {
-			break
-		}
-	}
-}
-
-func (s *workerChanStack) pop() *workerChan {
-	for {
-		oldHead := s.head.Load()
-		if oldHead == nil {
-			return nil
-		}
-
-		if s.head.CompareAndSwap(oldHead, oldHead.next) {
-			return oldHead
-		}
-	}
+	lastUseTime time.Time
+	ch          chan net.Conn
 }
 
 func (wp *workerPool) Start() {
@@ -87,8 +58,9 @@ func (wp *workerPool) Start() {
 		}
 	}
 	go func() {
+		var scratch []*workerChan
 		for {
-			wp.clean()
+			wp.clean(&scratch)
 			select {
 			case <-stopCh:
 				return
@@ -109,15 +81,15 @@ func (wp *workerPool) Stop() {
 	// Stop all the workers waiting for incoming connections.
 	// Do not wait for busy workers - they will stop after
 	// serving the connection and noticing wp.mustStop = true.
-
-	for {
-		ch := wp.ready.pop()
-		if ch == nil {
-			break
-		}
-		ch.ch <- nil
+	wp.lock.Lock()
+	ready := wp.ready
+	for i := range ready {
+		ready[i].ch <- nil
+		ready[i] = nil
 	}
-	wp.mustStop.Store(true)
+	wp.ready = ready[:0]
+	wp.mustStop = true
+	wp.lock.Unlock()
 }
 
 func (wp *workerPool) getMaxIdleWorkerDuration() time.Duration {
@@ -127,22 +99,50 @@ func (wp *workerPool) getMaxIdleWorkerDuration() time.Duration {
 	return wp.MaxIdleWorkerDuration
 }
 
-func (wp *workerPool) clean() {
+func (wp *workerPool) clean(scratch *[]*workerChan) {
 	maxIdleWorkerDuration := wp.getMaxIdleWorkerDuration()
-	criticalTime := time.Now().Add(-maxIdleWorkerDuration).UnixNano()
 
-	for {
-		current := wp.ready.head.Load()
-		if current == nil || atomic.LoadInt64(&current.lastUseTime) >= criticalTime {
-			break
-		}
+	// Clean least recently used workers if they didn't serve connections
+	// for more than maxIdleWorkerDuration.
+	criticalTime := time.Now().Add(-maxIdleWorkerDuration)
+
+	wp.lock.Lock()
+	ready := wp.ready
+	n := len(ready)
 
-		next := current.next
-		if wp.ready.head.CompareAndSwap(current, next) {
-			current.ch <- nil
-			wp.workerChanPool.Put(current)
+	// Use binary-search algorithm to find out the index of the least recently worker which can be cleaned up.
+	l, r := 0, n-1
+	for l <= r {
+		mid := (l + r) / 2
+		if criticalTime.After(wp.ready[mid].lastUseTime) {
+			l = mid + 1
+		} else {
+			r = mid - 1
 		}
 	}
+	i := r
+	if i == -1 {
+		wp.lock.Unlock()
+		return
+	}
+
+	*scratch = append((*scratch)[:0], ready[:i+1]...)
+	m := copy(ready, ready[i+1:])
+	for i = m; i < n; i++ {
+		ready[i] = nil
+	}
+	wp.ready = ready[:m]
+	wp.lock.Unlock()
+
+	// Notify obsolete workers to stop.
+	// This notification must be outside the wp.lock, since ch.ch
+	// may be blocking and may consume a lot of time if many workers
+	// are located on non-local CPUs.
+	tmp := *scratch
+	for i := range tmp {
+		tmp[i].ch <- nil
+		tmp[i] = nil
+	}
 }
 
 func (wp *workerPool) Serve(c net.Conn) bool {
@@ -169,32 +169,47 @@ var workerChanCap = func() int {
 }()
 
 func (wp *workerPool) getCh() *workerChan {
-	for {
-		ch := wp.ready.pop()
-		if ch != nil {
-			return ch
+	var ch *workerChan
+	createWorker := false
+
+	wp.lock.Lock()
+	ready := wp.ready
+	n := len(ready) - 1
+	if n < 0 {
+		if wp.workersCount < wp.MaxWorkersCount {
+			createWorker = true
+			wp.workersCount++
 		}
+	} else {
+		ch = ready[n]
+		ready[n] = nil
+		wp.ready = ready[:n]
+	}
+	wp.lock.Unlock()
 
-		currentWorkers := atomic.LoadInt32(&wp.workersCount)
-		if int(currentWorkers) < wp.MaxWorkersCount {
-			if atomic.CompareAndSwapInt32(&wp.workersCount, currentWorkers, currentWorkers+1) {
-				ch = wp.workerChanPool.Get().(*workerChan)
-				go wp.workerFunc(ch)
-				return ch
-			}
-		} else {
-			break
+	if ch == nil {
+		if !createWorker {
+			return nil
 		}
+		vch := wp.workerChanPool.Get()
+		ch = vch.(*workerChan)
+		go func() {
+			wp.workerFunc(ch)
+			wp.workerChanPool.Put(vch)
+		}()
 	}
-	return nil
+	return ch
 }
 
 func (wp *workerPool) release(ch *workerChan) bool {
-	atomic.StoreInt64(&ch.lastUseTime, time.Now().UnixNano())
-	if wp.mustStop.Load() {
+	ch.lastUseTime = time.Now()
+	wp.lock.Lock()
+	if wp.mustStop {
+		wp.lock.Unlock()
 		return false
 	}
-	wp.ready.push(ch)
+	wp.ready = append(wp.ready, ch)
+	wp.lock.Unlock()
 	return true
 }
 
@@ -230,5 +245,7 @@ func (wp *workerPool) workerFunc(ch *workerChan) {
 		}
 	}
 
-	atomic.AddInt32(&wp.workersCount, -1)
+	wp.lock.Lock()
+	wp.workersCount--
+	wp.lock.Unlock()
 }