Issue was that if a BGC thread handles a mark stack overflow, but runs into yet another mark stack overflow on another heap, we set a flag on the region, and the containing heap. However, the BGC handling the other heap may have already decided to move on, and may thus not see the flag. (#74814)

Fix is to set the flag on the heap doing the scan rather than the heap containing the object causing the mark stack stack overflow. The thread handling that heap will indeed recheck the flag and rescan if necessary. This necessitates another change because in the concurrent case, we need each BGC thread to enter mark stack overflow scanning if there was a mark stack overflow on its heap. So we need to propagate the per-heap flag to all the heaps.

Fixed another issue for regions where the small_object_segments local variable in background_process_mark_overflow_internal would be set incorrectly in the non-concurrent case. It would be set to FALSE as soon as all the regions for gen 0 are processed.

Co-authored-by: Peter Sollich <petersol@microsoft.com>

Author: github-actions[bot]

src/coreclr/gc/gc.cpp

@@ -24659,12 +24659,7 @@ void gc_heap::set_background_overflow_p (uint8_t* oo)
     heap_segment* overflow_region = get_region_info_for_address (oo);
     overflow_region->flags |= heap_segment_flags_overflow;
     dprintf (3,("setting overflow flag for region %p", heap_segment_mem (overflow_region)));
-#ifdef MULTIPLE_HEAPS
-    gc_heap* overflow_heap = heap_segment_heap (overflow_region);
-#else
-    gc_heap* overflow_heap = nullptr;
-#endif
-    overflow_heap->background_overflow_p = TRUE;
+    background_overflow_p = TRUE;
 }
 #endif //USE_REGIONS
 
@@ -25311,13 +25306,13 @@ void gc_heap::background_process_mark_overflow_internal (uint8_t* min_add, uint8
 
             dprintf (2, ("h%d: SOH: ov-mo: %Id", heap_number, total_marked_objects));
             fire_overflow_event (min_add, max_add, total_marked_objects, i);
-            if (small_object_segments)
+            if (i >= soh_gen2)
             {
                 concurrent_print_time_delta (concurrent_p ? "Cov SOH" : "Nov SOH");
+                small_object_segments = FALSE;
             }
 
             total_marked_objects = 0;
-            small_object_segments = FALSE;
         }
     }
 }
@@ -33980,23 +33975,35 @@ void gc_heap::background_scan_dependent_handles (ScanContext *sc)
 
             if (!s_fScanRequired)
             {
-#ifndef USE_REGIONS
+#ifdef USE_REGIONS
+                BOOL all_heaps_background_overflow_p = FALSE;
+#else //USE_REGIONS
                 uint8_t* all_heaps_max = 0;
                 uint8_t* all_heaps_min = MAX_PTR;
+#endif //USE_REGIONS
                 int i;
                 for (i = 0; i < n_heaps; i++)
                 {
+#ifdef USE_REGIONS
+                    // in the regions case, compute the OR of all the per-heap flags
+                    if (g_heaps[i]->background_overflow_p)
+                        all_heaps_background_overflow_p = TRUE;
+#else //USE_REGIONS
                     if (all_heaps_max < g_heaps[i]->background_max_overflow_address)
                         all_heaps_max = g_heaps[i]->background_max_overflow_address;
                     if (all_heaps_min > g_heaps[i]->background_min_overflow_address)
                         all_heaps_min = g_heaps[i]->background_min_overflow_address;
+#endif //USE_REGIONS
                 }
                 for (i = 0; i < n_heaps; i++)
                 {
+#ifdef USE_REGIONS
+                    g_heaps[i]->background_overflow_p = all_heaps_background_overflow_p;
+#else //USE_REGIONS
                     g_heaps[i]->background_max_overflow_address = all_heaps_max;
                     g_heaps[i]->background_min_overflow_address = all_heaps_min;
+#endif //USE_REGIONS
                 }
-#endif //!USE_REGIONS
             }
 
             dprintf(2, ("Starting all gc thread mark stack overflow processing"));
@@ -34740,15 +34747,24 @@ void gc_heap::background_mark_phase ()
 
     enable_preemptive ();
 
-#if defined(MULTIPLE_HEAPS) && !defined(USE_REGIONS)
+#if defined(MULTIPLE_HEAPS)
     bgc_t_join.join(this, gc_join_concurrent_overflow);
     if (bgc_t_join.joined())
     {
+#ifdef USE_REGIONS
+        BOOL all_heaps_background_overflow_p = FALSE;
+#else //USE_REGIONS
         uint8_t* all_heaps_max = 0;
         uint8_t* all_heaps_min = MAX_PTR;
+#endif //USE_REGIONS
         int i;
         for (i = 0; i < n_heaps; i++)
         {
+#ifdef USE_REGIONS
+            // in the regions case, compute the OR of all the per-heap flags
+            if (g_heaps[i]->background_overflow_p)
+                all_heaps_background_overflow_p = TRUE;
+#else //USE_REGIONS
             dprintf (3, ("heap %d overflow max is %Ix, min is %Ix",
                 i,
                 g_heaps[i]->background_max_overflow_address,
@@ -34757,17 +34773,21 @@ void gc_heap::background_mark_phase ()
                 all_heaps_max = g_heaps[i]->background_max_overflow_address;
             if (all_heaps_min > g_heaps[i]->background_min_overflow_address)
                 all_heaps_min = g_heaps[i]->background_min_overflow_address;
-
+#endif //USE_REGIONS
         }
         for (i = 0; i < n_heaps; i++)
         {
+#ifdef USE_REGIONS
+            g_heaps[i]->background_overflow_p = all_heaps_background_overflow_p;
+#else //USE_REGIONS
             g_heaps[i]->background_max_overflow_address = all_heaps_max;
             g_heaps[i]->background_min_overflow_address = all_heaps_min;
+#endif //USE_REGIONS
         }
         dprintf(3, ("Starting all bgc threads after updating the overflow info"));
         bgc_t_join.restart();
     }
-#endif //MULTIPLE_HEAPS && !USE_REGIONS
+#endif //MULTIPLE_HEAPS
 
     disable_preemptive (true);