From f6b6d2984157cc24ee69afb9b025b5d250a5a9ad Mon Sep 17 00:00:00 2001
From: Craig Tiller <ctiller@google.com>
Date: Fri, 2 Sep 2016 09:27:26 -0700
Subject: Address review comments

---
 src/core/lib/support/mpscq.c | 11 +++++++----
 src/core/lib/support/mpscq.h |  4 ++--
 2 files changed, 9 insertions(+), 6 deletions(-)

(limited to 'src/core/lib/support')

diff --git a/src/core/lib/support/mpscq.c b/src/core/lib/support/mpscq.c
index 25b055b172..cdd6335f82 100644
--- a/src/core/lib/support/mpscq.c
+++ b/src/core/lib/support/mpscq.c
@@ -38,7 +38,7 @@
 void gpr_mpscq_init(gpr_mpscq *q) {
   gpr_atm_no_barrier_store(&q->head, (gpr_atm)&q->stub);
   q->tail = &q->stub;
-  gpr_atm_no_barrier_store(&q->stub.next, 0);
+  gpr_atm_no_barrier_store(&q->stub.next, (gpr_atm)NULL);
 }
 
 void gpr_mpscq_destroy(gpr_mpscq *q) {
@@ -47,16 +47,17 @@ void gpr_mpscq_destroy(gpr_mpscq *q) {
 }
 
 void gpr_mpscq_push(gpr_mpscq *q, gpr_mpscq_node *n) {
-  gpr_atm_no_barrier_store(&n->next, 0);
+  gpr_atm_no_barrier_store(&n->next, (gpr_atm)NULL);
   gpr_mpscq_node *prev =
       (gpr_mpscq_node *)gpr_atm_full_xchg(&q->head, (gpr_atm)n);
-  gpr_atm_rel_store(&prev->next, (gpr_atm)n);
+  gpr_atm_no_barrier_store(&prev->next, (gpr_atm)n);
 }
 
 gpr_mpscq_node *gpr_mpscq_pop(gpr_mpscq *q) {
   gpr_mpscq_node *tail = q->tail;
   gpr_mpscq_node *next = (gpr_mpscq_node *)gpr_atm_acq_load(&tail->next);
   if (tail == &q->stub) {
+    // indicates the list is actually (ephemerally) empty
     if (next == NULL) return NULL;
     q->tail = next;
     tail = next;
@@ -68,7 +69,8 @@ gpr_mpscq_node *gpr_mpscq_pop(gpr_mpscq *q) {
   }
   gpr_mpscq_node *head = (gpr_mpscq_node *)gpr_atm_acq_load(&q->head);
   if (tail != head) {
-    return 0;
+    // indicates a retry is in order: we're still adding
+    return NULL;
   }
   gpr_mpscq_push(q, &q->stub);
   next = (gpr_mpscq_node *)gpr_atm_acq_load(&tail->next);
@@ -76,5 +78,6 @@ gpr_mpscq_node *gpr_mpscq_pop(gpr_mpscq *q) {
     q->tail = next;
     return tail;
   }
+  // indicates a retry is in order: we're still adding
   return NULL;
 }
diff --git a/src/core/lib/support/mpscq.h b/src/core/lib/support/mpscq.h
index 1201edceb1..977a117952 100644
--- a/src/core/lib/support/mpscq.h
+++ b/src/core/lib/support/mpscq.h
@@ -41,8 +41,8 @@
 // implementation from Dmitry Vyukov here:
 // http://www.1024cores.net/home/lock-free-algorithms/queues/intrusive-mpsc-node-based-queue
 
-// List node (include this in a data structure and dangle the rest of the
-// interesting bits off the end)
+// List node (include this in a data structure at the top, and add application
+// fields after it - to simulate inheritance)
 typedef struct gpr_mpscq_node { gpr_atm next; } gpr_mpscq_node;
 
 // Actual queue type
-- 
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