context: Revert "context: use CompareAndSwap in cancelCtx.Done"

This reverts commit 964f0c7a306998256f1c5a5fd78fc457a972f001.

Reason: cause increasing timeout in crypto/tls tests on race builders.

Change-Id: Id16d4fcd19c2ca2e89ad4d0c9d55ef1105b19c76
Reviewed-on: https://go-review.googlesource.com/c/go/+/422035
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>
Auto-Submit: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Michael Pratt <mpratt@google.com>

1 files changed, 8 insertions, 2 deletions

diff --git a/src/context/context.go b/src/context/context.go
index 04ac080402..1070111efa 100644
--- a/src/context/context.go
+++ b/src/context/context.go
@@ -360,8 +360,14 @@ func (c *cancelCtx) Done() <-chan struct{} {
 	if d != nil {
 		return d.(chan struct{})
 	}
-	c.done.CompareAndSwap(nil, make(chan struct{}))
-	return c.done.Load().(chan struct{})
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	d = c.done.Load()
+	if d == nil {
+		d = make(chan struct{})
+		c.done.Store(d)
+	}
+	return d.(chan struct{})
 }
 
 func (c *cancelCtx) Err() error {