runtime: set userForced while holding gcsema

Fixes #52515

Change-Id: If10068a97733daf61aba72da5ed3935db8eed8b9
Reviewed-on: https://go-review.googlesource.com/c/go/+/401776
Reviewed-by: hopehook <[email protected]>
Run-TryBot: hopehook <[email protected]>
Reviewed-by: Michael Knyszek <[email protected]>
Auto-Submit: Jenny Rakoczy <[email protected]>
Run-TryBot: Michael Knyszek <[email protected]>
Reviewed-by: Jenny Rakoczy <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>

Author: WangLeonard

src/runtime/mgc.go

@@ -612,9 +612,6 @@ func gcStart(trigger gcTrigger) {
 		return
 	}
 
-	// For stats, check if this GC was forced by the user.
-	work.userForced = trigger.kind == gcTriggerCycle
-
 	// In gcstoptheworld debug mode, upgrade the mode accordingly.
 	// We do this after re-checking the transition condition so
 	// that multiple goroutines that detect the heap trigger don't
@@ -630,6 +627,10 @@ func gcStart(trigger gcTrigger) {
 	semacquire(&gcsema)
 	semacquire(&worldsema)
 
+	// For stats, check if this GC was forced by the user.
+	// Update it under gcsema to avoid gctrace getting wrong values.
+	work.userForced = trigger.kind == gcTriggerCycle
+
 	if trace.enabled {
 		traceGCStart()
 	}