src/hotspot/share/gc/shenandoah/shenandoahHeapRegion.hpp

rev 58411 : 8241081: Shenandoah: Do not modify update-watermark concurrently

*** 259,269 ****
    uint64_t _seqnum_last_alloc_gc;
  
    volatile size_t _live_data;
    volatile size_t _critical_pins;
  
!   HeapWord* volatile _update_watermark;
  
    // Claim some space at the end to protect next region
    DEFINE_PAD_MINUS_SIZE(0, DEFAULT_CACHE_LINE_SIZE, 0);
  
  public:
--- 259,269 ----
    uint64_t _seqnum_last_alloc_gc;
  
    volatile size_t _live_data;
    volatile size_t _critical_pins;
  
!   HeapWord* _update_watermark;
  
    // Claim some space at the end to protect next region
    DEFINE_PAD_MINUS_SIZE(0, DEFAULT_CACHE_LINE_SIZE, 0);
  
  public:
*** 429,445 ****
    uint64_t seqnum_last_alloc_gc()  const {
      return _seqnum_last_alloc_gc;
    }
  
    HeapWord* get_update_watermark() const {
      assert(bottom() <= _update_watermark && _update_watermark <= top(), "within bounds");
!     return Atomic::load_acquire(&_update_watermark);
    }
  
    void set_update_watermark(HeapWord* w) {
      assert(bottom() <= w && w <= top(), "within bounds");
!     Atomic::release_store(&_update_watermark, w);
    }
  
  private:
    void do_commit();
    void do_uncommit();
--- 429,451 ----
    uint64_t seqnum_last_alloc_gc()  const {
      return _seqnum_last_alloc_gc;
    }
  
    HeapWord* get_update_watermark() const {
+     // Updates to the update-watermark only happen at safepoints or, when pushing
+     // back the watermark for evacuation regions, under the Shenandoah heap-lock.
+     // Consequently, we should access the field under the same lock. However, since
+     // those updates are only monotonically increasing, possibly reading a stale value
+     // is only conservative - we would not miss to update any fields.
      assert(bottom() <= _update_watermark && _update_watermark <= top(), "within bounds");
!     return _update_watermark;
    }
  
    void set_update_watermark(HeapWord* w) {
+     _heap->assert_heaplock_or_safepoint();
      assert(bottom() <= w && w <= top(), "within bounds");
!     _update_watermark = w;
    }
  
  private:
    void do_commit();
    void do_uncommit();