3 files changed, 40 insertions, 4 deletions
diff --git a/src/cmd/trace/trace_test.go b/src/cmd/trace/trace_test.go
index 9e90f50d4b..ef2d06c961 100644
--- a/src/cmd/trace/trace_test.go
+++ b/src/cmd/trace/trace_test.go
@@ -12,7 +12,9 @@ import (
 	"io/ioutil"
 	rtrace "runtime/trace"
 	"strings"
+	"sync"
 	"testing"
+	"time"
 )
 
 // stacks is a fake stack map populated for test.
@@ -233,3 +235,34 @@ func TestFoo(t *testing.T) {
 	}
 
 }
+
+func TestDirectSemaphoreHandoff(t *testing.T) {
+	prog0 := func() {
+		var mu sync.Mutex
+		var wg sync.WaitGroup
+		mu.Lock()
+		// This is modeled after src/sync/mutex_test.go to trigger Mutex
+		// starvation mode, in which the goroutine that calls Unlock hands off
+		// both the semaphore and its remaining time slice. See issue 36186.
+		for i := 0; i < 2; i++ {
+			wg.Add(1)
+			go func() {
+				defer wg.Done()
+				for i := 0; i < 100; i++ {
+					mu.Lock()
+					time.Sleep(100 * time.Microsecond)
+					mu.Unlock()
+				}
+			}()
+		}
+		mu.Unlock()
+		wg.Wait()
+	}
+	if err := traceProgram(t, prog0, "TestDirectSemaphoreHandoff"); err != nil {
+		t.Fatalf("failed to trace the program: %v", err)
+	}
+	_, err := parseTrace()
+	if err != nil {
+		t.Fatalf("failed to parse the trace: %v", err)
+	}
+}
diff --git a/src/runtime/proc.go b/src/runtime/proc.go
index d264e1d120..26af68e3c1 100644
--- a/src/runtime/proc.go
+++ b/src/runtime/proc.go
@@ -2767,7 +2767,7 @@ func preemptPark(gp *g) {
 }
 
 // goyield is like Gosched, but it:
-// - does not emit a GoSched trace event
+// - emits a GoPreempt trace event instead of a GoSched trace event
 // - puts the current G on the runq of the current P instead of the globrunq
 func goyield() {
 	checkTimeouts()
@@ -2775,6 +2775,9 @@ func goyield() {
 }
 
 func goyield_m(gp *g) {
+	if trace.enabled {
+		traceGoPreempt()
+	}
 	pp := gp.m.p.ptr()
 	casgstatus(gp, _Grunning, _Grunnable)
 	dropg()
diff --git a/src/runtime/sema.go b/src/runtime/sema.go
index 7bbf871caa..9bfd4f96d5 100644
--- a/src/runtime/sema.go
+++ b/src/runtime/sema.go
@@ -199,9 +199,9 @@ func semrelease1(addr *uint32, handoff bool, skipframes int) {
 			// the waiter G immediately.
 			// Note that waiter inherits our time slice: this is desirable
 			// to avoid having a highly contended semaphore hog the P
-			// indefinitely. goyield is like Gosched, but it does not emit a
-			// GoSched trace event and, more importantly, puts the current G
-			// on the local runq instead of the global one.
+			// indefinitely. goyield is like Gosched, but it emits a
+			// "preempted" trace event instead and, more importantly, puts
+			// the current G on the local runq instead of the global one.
 			// We only do this in the starving regime (handoff=true), as in
 			// the non-starving case it is possible for a different waiter
 			// to acquire the semaphore while we are yielding/scheduling,