[heap] Only run MarkTransitiveClosureLinear() with serial marking

This method was intended to run single-threaded, so with parallel
marking disabled. While correctness isn't an issue here (simply because
we will later run this method single-threaded as well), some CHECKs do
not hold when parallel marking is active.

Bug: chromium:1325628
Change-Id: Iedebcf6241835011982217b1452271e38a36ce9d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3700074
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81073}

src/heap/mark-compact.cc

@@ -2196,6 +2196,8 @@ bool MarkCompactCollector::ProcessEphemerons() {
 void MarkCompactCollector::MarkTransitiveClosureLinear() {
   TRACE_GC(heap()->tracer(),
            GCTracer::Scope::MC_MARK_WEAK_CLOSURE_EPHEMERON_LINEAR);
+  // This phase doesn't support parallel marking.
+  DCHECK(heap()->concurrent_marking()->IsStopped());
   std::unordered_multimap<HeapObject, HeapObject, Object::Hasher> key_to_values;
   Ephemeron ephemeron;
 
@@ -2275,6 +2277,8 @@ void MarkCompactCollector::MarkTransitiveClosureLinear() {
   ResetNewlyDiscovered();
   ephemeron_marking_.newly_discovered.shrink_to_fit();
 
+  CHECK(local_marking_worklists()->IsEmpty());
+
   CHECK(weak_objects_.current_ephemerons.IsEmpty());
   CHECK(weak_objects_.discovered_ephemerons.IsEmpty());
 
@@ -2406,8 +2410,9 @@ void MarkCompactCollector::MarkTransitiveClosure() {
 
   if (!MarkTransitiveClosureUntilFixpoint()) {
     // Fixpoint iteration needed too many iterations and was cancelled. Use the
-    // guaranteed linear algorithm.
-    MarkTransitiveClosureLinear();
+    // guaranteed linear algorithm. But only in the final single-thread marking
+    // phase.
+    if (!parallel_marking_) MarkTransitiveClosureLinear();
   }
 }