runtime: netpollWaiters typically not decremented

What is the semantic of runtime.netpollWaiters? If it should track number of goroutines waiting for poll result, then it is implemented incorrectly (at least on Linux with epoll).

I see that runtime.netpollWaiters is incremented every time new goroutine getting blocked on polling:

https://github.com/golang/go/blob/f686a2890b34996455c7d7aba9a0efba74b613f5/src/runtime/netpoll.go#L354-L363

and decremented only in func netpollgoready(gp *g, traceskip int):

https://github.com/golang/go/blob/f686a2890b34996455c7d7aba9a0efba74b613f5/src/runtime/netpoll.go#L365-L368

Looks like netpollgoready() is called only from internal/poll.runtime_pollSetDeadline(), i.e. in some codepaths related to setting polling deadlines:

https://github.com/golang/go/blob/f686a2890b34996455c7d7aba9a0efba74b613f5/src/runtime/netpoll.go#L204-L205

And most frequently parked goroutine waiting for poll result is being awakened somewhere in runtime.findrunnable():

https://github.com/golang/go/blob/61bb56ad63992a3199acc55b2537c8355ef887b6/src/runtime/proc.go#L2210-L2221

or runtime.pollWork():

https://github.com/golang/go/blob/61bb56ad63992a3199acc55b2537c8355ef887b6/src/runtime/proc.go#L2395-L2409

Apparently atomic.Load(&netpollWaiters) > 0 condition in the referenced above runtime.findrunnable() and runtime.pollWork() functions is always true, as soon as as single goroutine will wait for poll result and get awakened from those functions.

I verified that runtime.netpollWaiters is increased with each wait of a goroutine on network in an example of handling TCP conection:

// tcp-server.go

package main

import (
        "bufio"
        "fmt"
        "log"
        "net"
        "strings"
)

func main() {

        fmt.Println("Launching server...")

        ln, _ := net.Listen("tcp", ":8081")

        conn, _ := ln.Accept()

        for {
                message, err := bufio.NewReader(conn).ReadString('\n')
                if err != nil {
                        log.Fatal(err)
                }
                fmt.Print("Message Received:", string(message))
                newmessage := strings.ToUpper(message)
                conn.Write([]byte(newmessage + "\n"))
        }
}

$ dlv debug tcp-socket.go
Type 'help' for list of commands.
(dlv) p runtime.netpollWaiters
0
(dlv) c
Launching server...
received SIGINT, stopping process (will not forward signal)
> runtime.epollwait() /usr/lib/golang/src/runtime/sys_linux_amd64.s:675 (PC: 0x4619e0)                                                                               
Warning: debugging optimized function
(dlv) p runtime.netpollWaiters
1
(dlv) c
Message Received:message 1 from netcat
Message Received:message 2 from netcat
received SIGINT, stopping process (will not forward signal)
> runtime.epollwait() /usr/lib/golang/src/runtime/sys_linux_amd64.s:675 (PC: 0x4619e0)
Warning: debugging optimized function
(dlv) p runtime.netpollWaiters
4
(dlv)

$ go version
go version go1.12.5 linux/amd64

Snippet from go env:

$ go env
GOARCH="amd64"
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GCCGO="gccgo"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD=""
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"

golang / go

runtime: netpollWaiters typically not decremented #33624