split out _mi_page_try_use_delayed_free for contention reduction (issue microsoft#630)

Author: daanx

include/mimalloc-internal.h

@@ -117,7 +117,8 @@ void       _mi_heap_delayed_free_all(mi_heap_t* heap);
 bool       _mi_heap_delayed_free_partial(mi_heap_t* heap);
 void       _mi_heap_collect_retired(mi_heap_t* heap, bool force);
 
-bool       _mi_page_use_delayed_free(mi_page_t* page, mi_delayed_t delay, bool override_never);
+void       _mi_page_use_delayed_free(mi_page_t* page, mi_delayed_t delay, bool override_never);
+bool       _mi_page_try_use_delayed_free(mi_page_t* page, mi_delayed_t delay, bool override_never);
 size_t     _mi_page_queue_append(mi_heap_t* heap, mi_page_queue_t* pq, mi_page_queue_t* append);
 void       _mi_deferred_free(mi_heap_t* heap, bool force);
 

src/alloc.c

@@ -531,7 +531,7 @@ bool _mi_free_delayed_block(mi_block_t* block) {
   // some blocks may end up in the page `thread_free` list with no blocks in the
   // heap `thread_delayed_free` list which may cause the page to be never freed!
   // (it would only be freed if we happen to scan it in `mi_page_queue_find_free_ex`)
-  if (!_mi_page_use_delayed_free(page, MI_USE_DELAYED_FREE, false /* dont overwrite never delayed */)) {
+  if (!_mi_page_try_use_delayed_free(page, MI_USE_DELAYED_FREE, false /* dont overwrite never delayed */)) {
     return false;
   }
 

src/page.c

@@ -122,7 +122,13 @@ bool _mi_page_is_valid(mi_page_t* page) {
 }
 #endif
 
-bool _mi_page_use_delayed_free(mi_page_t* page, mi_delayed_t delay, bool override_never) {
+void _mi_page_use_delayed_free(mi_page_t* page, mi_delayed_t delay, bool override_never) {
+  while (!_mi_page_try_use_delayed_free(page, delay, override_never)) {
+    mi_atomic_yield();
+  }
+}
+
+bool _mi_page_try_use_delayed_free(mi_page_t* page, mi_delayed_t delay, bool override_never) {
   mi_thread_free_t tfreex;
   mi_delayed_t     old_delay;
   mi_thread_free_t tfree;