3 files changed, 317 insertions, 0 deletions
diff --git a/pkg/gate/BUILD b/pkg/gate/BUILD
new file mode 100644
index 000000000..381474d9e
--- /dev/null
+++ b/pkg/gate/BUILD
@@ -0,0 +1,22 @@
+package(licenses = ["notice"])  # BSD
+
+load("@io_bazel_rules_go//go:def.bzl", "go_library", "go_test")
+
+go_library(
+    name = "gate",
+    srcs = [
+        "gate.go",
+    ],
+    importpath = "gvisor.googlesource.com/gvisor/pkg/gate",
+    visibility = ["//visibility:public"],
+)
+
+go_test(
+    name = "gate_test",
+    srcs = [
+        "gate_test.go",
+    ],
+    deps = [
+        ":gate",
+    ],
+)
diff --git a/pkg/gate/gate.go b/pkg/gate/gate.go
new file mode 100644
index 000000000..47651c761
--- /dev/null
+++ b/pkg/gate/gate.go
@@ -0,0 +1,120 @@
+// Package gate provides a usage Gate synchronization primitive.
+package gate
+
+import (
+	"sync/atomic"
+)
+
+const (
+	// gateClosed is the bit set in the gate's user count to indicate that
+	// it has been closed. It is the MSB of the 32-bit field; the other 31
+	// bits carry the actual count.
+	gateClosed = 0x80000000
+)
+
+// Gate is a synchronization primitive that allows concurrent goroutines to
+// "enter" it as long as it hasn't been closed yet. Once it's been closed,
+// goroutines cannot enter it anymore, but are allowed to leave, and the closer
+// will be informed when all goroutines have left.
+//
+// Many goroutines are allowed to enter the gate concurrently, but only one is
+// allowed to close it.
+//
+// This is similar to a r/w critical section, except that goroutines "entering"
+// never block: they either enter immediately or fail to enter. The closer will
+// block waiting for all goroutines currently inside the gate to leave.
+//
+// This function is implemented efficiently. On x86, only one interlocked
+// operation is performed on enter, and one on leave.
+//
+// This is useful, for example, in cases when a goroutine is trying to clean up
+// an object for which multiple goroutines have pointers. In such a case, users
+// would be required to enter and leave the gates, and the cleaner would wait
+// until all users are gone (and no new ones are allowed) before proceeding.
+//
+// Users:
+//
+//	if !g.Enter() {
+//		// Gate is closed, we can't use the object.
+//		return
+//	}
+//
+//	// Do something with object.
+//	[...]
+//
+//	g.Leave()
+//
+// Closer:
+//
+//	// Prevent new users from using the object, and wait for the existing
+//	// ones to complete.
+//	g.Close()
+//
+//	// Clean up the object.
+//	[...]
+//
+type Gate struct {
+	userCount uint32
+	done      chan struct{}
+}
+
+// Enter tries to enter the gate. It will succeed if it hasn't been closed yet,
+// in which case the caller must eventually call Leave().
+//
+// This function is thread-safe.
+func (g *Gate) Enter() bool {
+	if g == nil {
+		return false
+	}
+
+	for {
+		v := atomic.LoadUint32(&g.userCount)
+		if v&gateClosed != 0 {
+			return false
+		}
+
+		if atomic.CompareAndSwapUint32(&g.userCount, v, v+1) {
+			return true
+		}
+	}
+}
+
+// Leave leaves the gate. This must only be called after a successful call to
+// Enter(). If the gate has been closed and this is the last one inside the
+// gate, it will notify the closer that the gate is done.
+//
+// This function is thread-safe.
+func (g *Gate) Leave() {
+	for {
+		v := atomic.LoadUint32(&g.userCount)
+		if v&^gateClosed == 0 {
+			panic("leaving a gate with zero usage count")
+		}
+
+		if atomic.CompareAndSwapUint32(&g.userCount, v, v-1) {
+			if v == gateClosed+1 {
+				close(g.done)
+			}
+			return
+		}
+	}
+}
+
+// Close closes the gate for entering, and waits until all goroutines [that are
+// currently inside the gate] leave before returning.
+//
+// Only one goroutine can call this function.
+func (g *Gate) Close() {
+	for {
+		v := atomic.LoadUint32(&g.userCount)
+		if v&^gateClosed != 0 && g.done == nil {
+			g.done = make(chan struct{})
+		}
+		if atomic.CompareAndSwapUint32(&g.userCount, v, v|gateClosed) {
+			if v&^gateClosed != 0 {
+				<-g.done
+			}
+			return
+		}
+	}
+}
diff --git a/pkg/gate/gate_test.go b/pkg/gate/gate_test.go
new file mode 100644
index 000000000..b3b101a0c
--- /dev/null
+++ b/pkg/gate/gate_test.go
@@ -0,0 +1,175 @@
+package gate_test
+
+import (
+	"sync"
+	"testing"
+	"time"
+
+	"gvisor.googlesource.com/gvisor/pkg/gate"
+)
+
+func TestBasicEnter(t *testing.T) {
+	var g gate.Gate
+
+	if !g.Enter() {
+		t.Fatalf("Failed to enter when it should be allowed")
+	}
+
+	g.Leave()
+
+	g.Close()
+
+	if g.Enter() {
+		t.Fatalf("Allowed to enter when it should fail")
+	}
+}
+
+func enterFunc(t *testing.T, g *gate.Gate, enter, leave, reenter chan struct{}, done1, done2, done3 *sync.WaitGroup) {
+	// Wait until instructed to enter.
+	<-enter
+	if !g.Enter() {
+		t.Errorf("Failed to enter when it should be allowed")
+	}
+
+	done1.Done()
+
+	// Wait until instructed to leave.
+	<-leave
+	g.Leave()
+
+	done2.Done()
+
+	// Wait until instructed to reenter.
+	<-reenter
+	if g.Enter() {
+		t.Errorf("Allowed to enter when it should fail")
+	}
+	done3.Done()
+}
+
+func TestConcurrentEnter(t *testing.T) {
+	var g gate.Gate
+	var done1, done2, done3 sync.WaitGroup
+
+	// Create 1000 worker goroutines.
+	enter := make(chan struct{})
+	leave := make(chan struct{})
+	reenter := make(chan struct{})
+	done1.Add(1000)
+	done2.Add(1000)
+	done3.Add(1000)
+	for i := 0; i < 1000; i++ {
+		go enterFunc(t, &g, enter, leave, reenter, &done1, &done2, &done3)
+	}
+
+	// Tell them all to enter, then leave.
+	close(enter)
+	done1.Wait()
+
+	close(leave)
+	done2.Wait()
+
+	// Close the gate, then have the workers try to enter again.
+	g.Close()
+	close(reenter)
+	done3.Wait()
+}
+
+func closeFunc(g *gate.Gate, done chan struct{}) {
+	g.Close()
+	close(done)
+}
+
+func TestCloseWaits(t *testing.T) {
+	var g gate.Gate
+
+	// Enter 10 times.
+	for i := 0; i < 10; i++ {
+		if !g.Enter() {
+			t.Fatalf("Failed to enter when it should be allowed")
+		}
+	}
+
+	// Launch closer. Check that it doesn't complete.
+	done := make(chan struct{})
+	go closeFunc(&g, done)
+
+	for i := 0; i < 10; i++ {
+		select {
+		case <-done:
+			t.Fatalf("Close function completed too soon")
+		case <-time.After(100 * time.Millisecond):
+		}
+
+		g.Leave()
+	}
+
+	// Now the closer must complete.
+	<-done
+}
+
+func TestMultipleSerialCloses(t *testing.T) {
+	var g gate.Gate
+
+	// Enter 10 times.
+	for i := 0; i < 10; i++ {
+		if !g.Enter() {
+			t.Fatalf("Failed to enter when it should be allowed")
+		}
+	}
+
+	// Launch closer. Check that it doesn't complete.
+	done := make(chan struct{})
+	go closeFunc(&g, done)
+
+	for i := 0; i < 10; i++ {
+		select {
+		case <-done:
+			t.Fatalf("Close function completed too soon")
+		case <-time.After(100 * time.Millisecond):
+		}
+
+		g.Leave()
+	}
+
+	// Now the closer must complete.
+	<-done
+
+	// Close again should not block.
+	done = make(chan struct{})
+	go closeFunc(&g, done)
+
+	select {
+	case <-done:
+	case <-time.After(2 * time.Second):
+		t.Fatalf("Second Close is blocking")
+	}
+}
+
+func worker(g *gate.Gate, done *sync.WaitGroup) {
+	for {
+		if !g.Enter() {
+			break
+		}
+		g.Leave()
+	}
+	done.Done()
+}
+
+func TestConcurrentAll(t *testing.T) {
+	var g gate.Gate
+	var done sync.WaitGroup
+
+	// Launch 1000 goroutines to concurrently enter/leave.
+	done.Add(1000)
+	for i := 0; i < 1000; i++ {
+		go worker(&g, &done)
+	}
+
+	// Wait for the goroutines to do some work, then close the gate.
+	time.Sleep(2 * time.Second)
+	g.Close()
+
+	// Wait for all of them to complete.
+	done.Wait()
+}