From 953a1bec48662c57fb40f50a2a4f56f033d842c6 Mon Sep 17 00:00:00 2001
From: Damien Neil <dneil@google.com>
Date: Tue, 3 Dec 2024 13:18:38 -0800
Subject: [PATCH] runtime: avoid defer on system stack in synctestidle_c

Fixes #70661

Change-Id: I58a465cfb1cd16709ffbb072eca0997569540074
Reviewed-on: https://go-review.googlesource.com/c/go/+/633281
Auto-Submit: Damien Neil <dneil@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
---
 src/runtime/synctest.go | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/src/runtime/synctest.go b/src/runtime/synctest.go
index 09748d5c1c..498c3b92dd 100644
--- a/src/runtime/synctest.go
+++ b/src/runtime/synctest.go
@@ -217,13 +217,15 @@ func synctestRun(f func()) {
 
 func synctestidle_c(gp *g, _ unsafe.Pointer) bool {
 	lock(&gp.syncGroup.mu)
-	defer unlock(&gp.syncGroup.mu)
+	canIdle := true
 	if gp.syncGroup.running == 0 && gp.syncGroup.active == 1 {
 		// All goroutines in the group have blocked or exited.
-		return false
+		canIdle = false
+	} else {
+		gp.syncGroup.active--
 	}
-	gp.syncGroup.active--
-	return true
+	unlock(&gp.syncGroup.mu)
+	return canIdle
 }
 
 //go:linkname synctestWait internal/synctest.Wait
-- 
2.48.1