context: reduce contention in cancelCtx.Done

Use an atomic.Value to hold the done channel.
Conveniently, we have a mutex handy to coordinate writes to it.

name                 old time/op  new time/op  delta
ContextCancelDone-8  67.5ns ±10%   2.2ns ±11%  -96.74%  (p=0.000 n=30+28)

Fixes #42564

Change-Id: I5d72e0e87fb221d4e230209e5fb4698bea4053c6
Reviewed-on: https://go-review.googlesource.com/c/go/+/288193
Trust: Josh Bleecher Snyder <josharian@gmail.com>
Trust: Sameer Ajmani <sameer@golang.org>
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

src/context/benchmark_test.go

@@ -5,6 +5,7 @@
 package context_test
 
 import (
+	"context"
 	. "context"
 	"fmt"
 	"runtime"
@@ -138,3 +139,17 @@ func BenchmarkCheckCanceled(b *testing.B) {
 		}
 	})
 }
+
+func BenchmarkContextCancelDone(b *testing.B) {
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+
+	b.RunParallel(func(pb *testing.PB) {
+		for pb.Next() {
+			select {
+			case <-ctx.Done():
+			default:
+			}
+		}
+	})
+}

src/context/context.go

@@ -303,10 +303,8 @@ func parentCancelCtx(parent Context) (*cancelCtx, bool) {
 	if !ok {
 		return nil, false
 	}
-	p.mu.Lock()
-	ok = p.done == done
-	p.mu.Unlock()
-	if !ok {
+	pdone, _ := p.done.Load().(chan struct{})
+	if pdone != done {
 		return nil, false
 	}
 	return p, true
@@ -345,7 +343,7 @@ type cancelCtx struct {
 	Context
 
 	mu       sync.Mutex            // protects following fields
-	done     chan struct{}         // created lazily, closed by first cancel call
+	done     atomic.Value          // of chan struct{}, created lazily, closed by first cancel call
 	children map[canceler]struct{} // set to nil by the first cancel call
 	err      error                 // set to non-nil by the first cancel call
 }
@@ -358,13 +356,18 @@ func (c *cancelCtx) Value(key interface{}) interface{} {
 }
 
 func (c *cancelCtx) Done() <-chan struct{} {
+	d := c.done.Load()
+	if d != nil {
+		return d.(chan struct{})
+	}
 	c.mu.Lock()
-	if c.done == nil {
-		c.done = make(chan struct{})
+	defer c.mu.Unlock()
+	d = c.done.Load()
+	if d == nil {
+		d = make(chan struct{})
+		c.done.Store(d)
 	}
-	d := c.done
-	c.mu.Unlock()
-	return d
+	return d.(chan struct{})
 }
 
 func (c *cancelCtx) Err() error {
@@ -401,10 +404,11 @@ func (c *cancelCtx) cancel(removeFromParent bool, err error) {
 		return // already canceled
 	}
 	c.err = err
-	if c.done == nil {
-		c.done = closedchan
+	d, _ := c.done.Load().(chan struct{})
+	if d == nil {
+		c.done.Store(closedchan)
 	} else {
-		close(c.done)
+		close(d)
 	}
 	for child := range c.children {
 		// NOTE: acquiring the child's lock while holding parent's lock.