runtime: decrement netpollWaiters in netpollunblock

We used to decrement it in netpollgoready, but that missed
the common case of a descriptor becoming ready due to I/O.
All calls to netpollgoready go through netpollunblock,
so this shouldn't miss any decrements we missed before.

Fixes #60782

Change-Id: Ideefefa1ac96ca38e09fe2dd5d595c5dd7883237
Reviewed-on: https://go-review.googlesource.com/c/go/+/503923
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>

diff --git a/src/runtime/crash_test.go b/src/runtime/crash_test.go
index 8f11333b462535f4e94e2646c177d57dc1c834eb..5eccf86e1a751914e1c48ace1125cd0265d1e2ff 100644 (file)
--- a/src/runtime/crash_test.go
+++ b/src/runtime/crash_test.go
@@ -869,3 +869,12 @@ func TestPanicOnUnsafeSlice(t *testing.T) {
                t.Errorf("output does not contain %q:\n%s", want, output)
        }
 }
+
+func TestNetpollWaiters(t *testing.T) {
+       t.Parallel()
+       output := runTestProg(t, "testprognet", "NetpollWaiters")
+       want := "OK\n"
+       if output != want {
+               t.Fatalf("output is not %q\n%s", want, output)
+       }
+}

diff --git a/src/runtime/netpoll.go b/src/runtime/netpoll.go
index 9b54e8e21f2dc30b7a73ee8c02eab45f12e99269..6877b2c350f0456102b9065ee9724d4af1fd96ce 100644 (file)
--- a/src/runtime/netpoll.go
+++ b/src/runtime/netpoll.go
@@ -526,7 +526,6 @@ func netpollblockcommit(gp *g, gpp unsafe.Pointer) bool {
 }
 
 func netpollgoready(gp *g, traceskip int) {
-       netpollWaiters.Add(-1)
        goready(gp, traceskip+1)
 }
 
@@ -587,13 +586,15 @@ func netpollunblock(pd *pollDesc, mode int32, ioready bool) *g {
                        // will check for timeout/cancel before waiting.
                        return nil
                }
-               var new uintptr
+               new := pdNil
                if ioready {
                        new = pdReady
                }
                if gpp.CompareAndSwap(old, new) {
                        if old == pdWait {
                                old = pdNil
+                       } else if old != pdNil {
+                               netpollWaiters.Add(-1)
                        }
                        return (*g)(unsafe.Pointer(old))
                }

diff --git a/src/runtime/testdata/testprognet/waiters.go b/src/runtime/testdata/testprognet/waiters.go
new file mode 100644 (file)
index 0000000..6c8db1f
--- /dev/null
+++ b/src/runtime/testdata/testprognet/waiters.go
@@ -0,0 +1,68 @@
+// Copyright 2023 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+import (
+       "fmt"
+       "io"
+       "log"
+       "net"
+       "runtime/internal/atomic"
+       "sync"
+       "time"
+       _ "unsafe" // for go:linkname
+)
+
+// The bug is that netpollWaiters increases monotonically.
+// This doesn't cause a problem until it overflows.
+// Use linkname to see the value.
+//go:linkname netpollWaiters runtime.netpollWaiters
+var netpollWaiters atomic.Uint32
+
+func init() {
+       register("NetpollWaiters", NetpollWaiters)
+}
+
+func NetpollWaiters() {
+       listener, err := net.Listen("tcp", "127.0.0.1:0")
+       if err != nil {
+               log.Fatal(err)
+       }
+
+       var wg sync.WaitGroup
+       wg.Add(1)
+       go func() {
+               defer wg.Done()
+               conn, err := listener.Accept()
+               if err != nil {
+                       log.Fatal(err)
+               }
+               defer conn.Close()
+               if _, err := io.Copy(io.Discard, conn); err != nil {
+                       log.Fatal(err)
+               }
+       }()
+
+       wg.Add(1)
+       go func() {
+               defer wg.Done()
+               conn, err := net.Dial("tcp", listener.Addr().String())
+               if err != nil {
+                       log.Fatal(err)
+               }
+               defer conn.Close()
+               for i := 0; i < 10; i++ {
+                       fmt.Fprintf(conn, "%d\n", i)
+                       time.Sleep(time.Millisecond)
+               }
+       }()
+
+       wg.Wait()
+       if v := netpollWaiters.Load(); v != 0 {
+               log.Fatalf("current waiters %v", v)
+       }
+
+       fmt.Println("OK")
+}