Skip to content

Instantly share code, notes, and snippets.

@zviadm zviadm/cond2.go Secret
Created Jan 12, 2018

Embed
What would you like to do?
package main
import (
"log"
"runtime"
"sync"
"sync/atomic"
"time"
"unsafe"
)
// Conditional variable implementation that uses channels for notifications.
// Only supports .Broadcast() method, however supports timeout based Wait() calls
// unlike regular sync.Cond.
type Cond struct {
L sync.Locker
n unsafe.Pointer
}
func NewCond(l sync.Locker) *Cond {
c := &Cond{L: l}
n := make(chan struct{})
c.n = unsafe.Pointer(&n)
return c
}
// Waits for Broadcast calls. Similar to regular sync.Cond, this unlocks the underlying
// locker first, waits on changes and re-locks it before returning.
func (c *Cond) Wait() {
n := c.NotifyChan()
c.L.Unlock()
<-n
c.L.Lock()
}
// Same as Wait() call, but will only wait up to a given timeout.
func (c *Cond) WaitWithTimeout(t time.Duration) {
n := c.NotifyChan()
c.L.Unlock()
select {
case <-n:
case <-time.After(t):
}
c.L.Lock()
}
// Returns a channel that can be used to wait for next Broadcast() call.
func (c *Cond) NotifyChan() <-chan struct{} {
ptr := atomic.LoadPointer(&c.n)
return *((*chan struct{})(ptr))
}
// Broadcast call notifies everyone that something has changed.
func (c *Cond) Broadcast() {
n := make(chan struct{})
ptrOld := atomic.SwapPointer(&c.n, unsafe.Pointer(&n))
close(*(*chan struct{})(ptrOld))
}
func TestRace() {
x := 0
c := NewCond(&sync.Mutex{})
done := make(chan bool)
go func() {
c.L.Lock()
x = 1
c.Wait()
if x != 2 {
log.Fatal("want 2")
}
x = 3
c.Broadcast()
c.L.Unlock()
done <- true
}()
go func() {
c.L.Lock()
for {
if x == 1 {
x = 2
c.Broadcast()
break
}
c.L.Unlock()
runtime.Gosched()
c.L.Lock()
}
c.L.Unlock()
done <- true
}()
go func() {
c.L.Lock()
for {
if x == 2 {
c.Wait()
if x != 3 {
log.Fatal("want 3")
}
break
}
if x == 3 {
break
}
c.L.Unlock()
runtime.Gosched()
c.L.Lock()
}
c.L.Unlock()
done <- true
}()
<-done
<-done
<-done
}
func main() {
log.Println("testing...")
TestRace()
}
Sign up for free to join this conversation on GitHub. Already have an account? Sign in to comment
You can’t perform that action at this time.