Effect of write lock attempt on read lock attempts

rwmutex.go

package main

import (
	"fmt"
	"sync"
	"time"
)

type info struct {
	step, thread    uint64
	val, prev, next string
}

func main() {
	this := "A"
	lock := &sync.RWMutex{}

	start := time.Now()
	report := make(chan *info)
	go func() {
		for reported := range report {
			val := reported.val
			if val == "" {
				val = "?"
			}
			fmt.Printf("(%d-%d) %s @ %v %s|%s\n",
				reported.step,
				reported.thread,
				val,
				time.Since(start),
				reported.prev,
				reported.next)
		}
	}()

	for i := uint64(0); i < 4; i++ {
		go func(j uint64) {
			report <- &info{step: 1, thread: j, next: "Rlock"}
			lock.RLock()
			defer lock.RUnlock()
			report <- &info{step: 1, thread: j, val: this, prev: "Rlock", next: "Sleep 750ms"}
			time.Sleep(750 * time.Millisecond)
			report <- &info{step: 1, thread: j, val: this, prev: "Sleep 750ms", next: "defer RUnlock"}
		}(i)
	}

	go func() {
		report <- &info{step: 2, next: "Sleep 250ms"}
		time.Sleep(time.Millisecond * 250)
		report <- &info{step: 2, prev: "Sleep 250ms", next: "Lock"}
		lock.Lock()
		defer lock.Unlock()
		report <- &info{step: 2, val: this, prev: "Lock", next: "Set B"}
		this = "B"
		report <- &info{step: 2, val: this, prev: "Set B", next: "defer Unlock"}
	}()

	go func() {
		report <- &info{step: 3, next: "Sleep 500ms"}
		time.Sleep(time.Millisecond * 500)
		report <- &info{step: 3, prev: "Sleep 500ms", next: "RLock"}
		lock.RLock()
		defer lock.RUnlock()
		report <- &info{step: 3, val: this, prev: "RLock", next: "Sleep 100ms"}
		time.Sleep(time.Millisecond * 100)
		report <- &info{step: 3, val: this, prev: "Sleep 100ms", next: "defer RUnlock"}
	}()

	time.Sleep(time.Second)
}
// (1-1) ? @ 166.255µs |Rlock
// (1-2) ? @ 270.705µs |Rlock
// (1-3) ? @ 283.906µs |Rlock
// (2-0) ? @ 295.047µs |Sleep 250ms
// (3-0) ? @ 306.541µs |Sleep 500ms
// (1-0) ? @ 317.837µs |Rlock
// (1-2) A @ 328.542µs Rlock|Sleep 750ms
// (1-1) A @ 373.706µs Rlock|Sleep 750ms
// (1-3) A @ 509.196µs Rlock|Sleep 750ms
// (1-0) A @ 557.335µs Rlock|Sleep 750ms
// (2-0) ? @ 250.930793ms Sleep 250ms|Lock
// (3-0) ? @ 506.0245ms Sleep 500ms|RLock
// (1-2) A @ 750.904071ms Sleep 750ms|defer RUnlock
// (1-3) A @ 750.997367ms Sleep 750ms|defer RUnlock
// (1-1) A @ 751.055258ms Sleep 750ms|defer RUnlock
// (1-0) A @ 751.131485ms Sleep 750ms|defer RUnlock
// (2-0) A @ 751.260427ms Lock|Set B
// (2-0) B @ 751.335704ms Set B|defer Unlock
// (3-0) B @ 751.43604ms RLock|Sleep 100ms
// (3-0) B @ 852.007714ms Sleep 100ms|defer RUnlock