ck_hp_stack: Add trypush/trypop variants.

Regressions tests pending an update (taking a snaphot, A/C is down).

include/ck_hp_stack.h

@@ -44,7 +44,14 @@ ck_hp_stack_push_mpmc(struct ck_stack *target, struct ck_stack_entry *entry)
 	return;
 }
 
-CK_CC_INLINE static void *
+CK_CC_INLINE static bool
+ck_hp_stack_trypush_mpmc(struct ck_stack *target, struct ck_stack_entry *entry)
+{
+
+	return ck_stack_trypush_upmc(target, entry);
+}
+
+CK_CC_INLINE static struct ck_stack_entry *
 ck_hp_stack_pop_mpmc(ck_hp_record_t *record, struct ck_stack *target)
 {
 	struct ck_stack_entry *entry, *update;
@@ -78,4 +85,29 @@ ck_hp_stack_pop_mpmc(ck_hp_record_t *record, struct ck_stack *target)
 	return (entry);
 }
 
+CK_CC_INLINE static bool
+ck_hp_stack_trypop_mpmc(ck_hp_record_t *record, struct ck_stack *target, struct ck_stack_entry **r)
+{
+	struct ck_stack_entry *entry;
+
+	entry = ck_pr_load_ptr(&target->head);
+	if (entry == NULL)
+		return false;
+
+	ck_hp_set(record, 0, entry);
+	ck_pr_fence_memory();
+	if (entry != ck_pr_load_ptr(&target->head))
+		goto leave;
+
+	if (ck_pr_cas_ptr_value(&target->head, entry, entry->next, &entry) == false)
+		goto leave;
+
+	*r = entry;
+	return true;
+
+leave:
+	ck_hp_set(record, 0, NULL);
+	return false;
+}
+
 #endif /* _CK_HP_STACK_H */