[heap] Relax DCHECK in TRACE_GC

We might run TRACE_GC with ThreadKind::kMain not only on each isolate's
main thread but also on the shared isolate's thread during a shared GC.
The DCHECK is too restrictive for the latter case. This is safe because
the shared GC will stop all main threads before starting its work.

Bug: v8:11708
Change-Id: I1f40140d6502b1ec797dfa783fb693ed213efb3b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3380522Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/main@{#78588}

src/heap/gc-tracer.cc

@@ -72,7 +72,9 @@ GCTracer::Scope::Scope(GCTracer* tracer, ScopeId scope, ThreadKind thread_kind)
 #ifdef V8_RUNTIME_CALL_STATS
   if (V8_LIKELY(!TracingFlags::is_runtime_stats_enabled())) return;
   if (thread_kind_ == ThreadKind::kMain) {
-    DCHECK_EQ(tracer_->heap_->isolate()->thread_id(), ThreadId::Current());
+#if DEBUG
+    AssertMainThread();
+#endif  // DEBUG
     runtime_stats_ =
         tracer_->heap_->isolate()->counters()->runtime_call_stats();
     runtime_stats_->Enter(&timer_, GCTracer::RCSCounterFromScope(scope));
@@ -89,7 +91,10 @@ GCTracer::Scope::~Scope() {
   double duration_ms = tracer_->MonotonicallyIncreasingTimeInMs() - start_time_;
 
   if (thread_kind_ == ThreadKind::kMain) {
-    DCHECK_EQ(tracer_->heap_->isolate()->thread_id(), ThreadId::Current());
+#if DEBUG
+    AssertMainThread();
+#endif  // DEBUG
+
     tracer_->AddScopeSample(scope_, duration_ms);
     if (scope_ == ScopeId::MC_INCREMENTAL ||
         scope_ == ScopeId::MC_INCREMENTAL_START ||
@@ -110,6 +115,19 @@ GCTracer::Scope::~Scope() {
 #endif  // defined(V8_RUNTIME_CALL_STATS)
 }
 
+#if DEBUG
+void GCTracer::Scope::AssertMainThread() {
+  Isolate* isolate = tracer_->heap_->isolate();
+  Isolate* shared_isolate = isolate->shared_isolate();
+  ThreadId thread_id = ThreadId::Current();
+
+  // Either run on isolate's main thread or on the current main thread of the
+  // shared isolate during shared GCs.
+  DCHECK(isolate->thread_id() == thread_id ||
+         (shared_isolate && shared_isolate->thread_id() == thread_id));
+}
+#endif  // DEBUG
+
 const char* GCTracer::Scope::Name(ScopeId id) {
 #define CASE(scope)  \
   case Scope::scope: \

src/heap/gc-tracer.h

@@ -112,6 +112,10 @@ class V8_EXPORT_PRIVATE GCTracer {
     static bool NeedsYoungEpoch(ScopeId id);
 
    private:
+#if DEBUG
+    void AssertMainThread();
+#endif  // DEBUG
+
     GCTracer* tracer_;
     ScopeId scope_;
     ThreadKind thread_kind_;