hazard pointers: Fix flaky test

Summary:
Fix flaky test. Without this fix it is possible for the background asynchronous reclamation thread to be too slow in pushing p1 into the cohorts safe list before the main thread checks for p1's reclamation.

This fix lets the main thread wait for p1 to be reclaimed before checking that p2 was not reclaimed. Since p1 is considered for reclamation before p2, it is possible that this check would pass even if p2 was about to be reclaimed in error. So the test then retires p3 and waits for it to be reclaimed before checking again that p2 was not reclaimed.

Reviewed By: yfeldblum

Differential Revision: D30248673

fbshipit-source-id: 268edb299681ab4b7256dac591d8cacbb960a7cc

folly/synchronization/test/HazptrTest.cpp

@@ -906,20 +906,30 @@ void cohort_safe_list_children_test() {
   p1->set_next(p2); // p2 is p1's child
   hazard_pointer<> h = make_hazard_pointer<>();
   h.reset_protection(p2);
-  p1->retire();
   /* When p1 is retired, it is inserted into cohort, then pushed into
      the domain's tagged list, then when p1 is found unprotected by
      hazard pointers it will be pushed into cohort's safe list. When
      p1 is reclaimed, p2 (p1's child) will be automatically retired to
      the domain's tagged list. */
-  /* Retire enough nodes to trigger asynchronous reclamation to
-     reclaim p1. */
-  for (int i = 0; i < 1100; ++i) {
+  p1->retire();
+  /* Retire enough nodes to invoke asynchronous reclamation until p1
+     and/or p2 are reclaimed. */
+  while (sum == 0) {
     NodeA* p = new NodeA(cohort, sum);
     p->retire();
   }
-  /* At this point p1 should be reclaimed but not p2 */
+  /* At this point p1 must be already reclaimed but not p2 */
   DCHECK_EQ(sum, 1000);
+  NodeA* p3 = new NodeA(cohort, sum, 3000);
+  p3->retire();
+  /* Retire more nodes to invoke asynchronous reclamation until p3
+     and/or p2 are reclaimed. */
+  while (sum == 1000) {
+    NodeA* p = new NodeA(cohort, sum);
+    p->retire();
+  }
+  /* At this point p3 must be already reclaimed but not p2 */
+  DCHECK_EQ(sum, 4000);
 }
 
 void fork_test() {