[vm] Fix TSAN data race when accessing Thread::top_

This is a possible fix for the following benign TSAN error:

WARNING: ThreadSanitizer: data race (pid=40436)
  Read of size 8 at 0x7b7c000bfa48 by thread T6 (mutexes: write M0):
    #0 dart::Page::object_end() const out/ReleaseTSANX64/../../runtime/vm/heap/page.h (dart+0x25ade83)
    #1 dart::Page::used() const out/ReleaseTSANX64/../../runtime/vm/heap/page.h:107:34 (dart+0x25ade83)
    #2 dart::SemiSpace::used_in_words() const out/ReleaseTSANX64/../../runtime/vm/heap/scavenger.h:46:18 (dart+0x25ade83)
    #3 dart::Scavenger::UsedInWords() const out/ReleaseTSANX64/../../runtime/vm/heap/scavenger.h:157:17 (dart+0x25ade83)
    #4 dart::GetProcessMemoryUsageHelper(dart::JSONStream*)::$_0::operator()(dart::IsolateGroup*) const out/ReleaseTSANX64/../../runtime/vm/service.cc:4729:50 (dart+0x25ade83)

  Previous write of size 8 at 0x7b7c000bfa48 by thread T9:
    #0 dart::Thread::set_top(unsigned long) out/ReleaseTSANX64/../../runtime/vm/thread.h:698:34 (dart+0x2622071)
    #1 dart::Scavenger::TryAllocateFromTLAB(dart::Thread*, long) out/ReleaseTSANX64/../../runtime/vm/heap/scavenger.h:256:13 (dart+0x2622071)
    #2 dart::Scavenger::TryAllocate(dart::Thread*, long) out/ReleaseTSANX64/../../runtime/vm/heap/scavenger.h:139:12 (dart+0x2622071)
    #3 dart::Heap::AllocateNew(dart::Thread*, long) out/ReleaseTSANX64/../../runtime/vm/heap/heap.cc:84:27 (dart+0x2622071)
    #4 dart::Heap::Allocate(dart::Thread*, long, dart::Heap::Space) out/ReleaseTSANX64/../../runtime/vm/heap/heap.h:87:16 (dart+0x2416cab)

TEST=ci

Change-Id: Id9077cff2c1143adf999b2b26c941701d63cd844
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/409180
Reviewed-by: Ryan Macnak <rmacnak@google.com>
Commit-Queue: Alexander Markov <alexmarkov@google.com>

Author: alexmarkov

runtime/vm/thread.cc

@@ -304,7 +304,7 @@ void Thread::AssertEmptyStackInvariants() {
 void Thread::AssertEmptyThreadInvariants() {
   AssertEmptyStackInvariants();
 
-  ASSERT(top_ == 0);
+  ASSERT(top() == 0);
   ASSERT(end_ == 0);
   ASSERT(true_end_ == 0);
   ASSERT(isolate_ == nullptr);

runtime/vm/thread.h

@@ -720,10 +720,10 @@ class Thread : public ThreadState {
   // TLAB and end() is the chosen sampling boundary for the thread.
   //
   // When the heap sampling profiler is disabled, true_end() == end().
-  uword top() const { return top_; }
+  uword top() const { return top_.load(std::memory_order_relaxed); }
   uword end() const { return end_; }
   uword true_end() const { return true_end_; }
-  void set_top(uword top) { top_ = top; }
+  void set_top(uword top) { top_.store(top, std::memory_order_relaxed); }
   void set_end(uword end) { end_ = end; }
   void set_true_end(uword true_end) { true_end_ = true_end; }
   static intptr_t top_offset() { return OFFSET_OF(Thread, top_); }
@@ -1267,7 +1267,7 @@ class Thread : public ThreadState {
 #if defined(DART_COMPRESSED_POINTERS)
   uword heap_base_ = 0;
 #endif
-  uword top_ = 0;
+  std::atomic<uword> top_ = 0;
   uword end_ = 0;
   const uword* dispatch_table_array_ = nullptr;
   ObjectPtr* field_table_values_ = nullptr;