diff --git a/lci/runtime/completion/sync_flag.c b/lci/runtime/completion/sync_flag.c
index 4421c5ee..27fcfa11 100644
--- a/lci/runtime/completion/sync_flag.c
+++ b/lci/runtime/completion/sync_flag.c
@@ -1,8 +1,11 @@
 #include "runtime/lcii.h"
 
 struct LCII_sync_t {
-  atomic_int count;
-  atomic_int confirm;
+  atomic_uint_fast64_t top;
+  atomic_uint_fast64_t top2;
+  LCIU_CACHE_PADDING(2 * sizeof(atomic_uint_fast64_t));
+  atomic_uint_fast64_t tail;
+  LCIU_CACHE_PADDING(sizeof(atomic_uint_fast64_t));
   int threshold;
   LCII_context_t** ctx;
 };
@@ -15,8 +18,9 @@ LCI_error_t LCI_sync_create(LCI_device_t device, int threshold,
   LCI_DBG_Assert(threshold > 0, "threshold (%d) <= 0!\n", threshold);
   LCII_sync_t* sync = LCIU_malloc(sizeof(LCII_sync_t));
   sync->threshold = threshold;
-  atomic_init(&sync->count, 0);
-  atomic_init(&sync->confirm, 0);
+  atomic_init(&sync->top, 0);
+  atomic_init(&sync->top2, 0);
+  atomic_init(&sync->tail, 0);
   sync->ctx = LCIU_malloc(sizeof(LCII_context_t*) * sync->threshold);
   *completion = sync;
   atomic_thread_fence(LCIU_memory_order_seq_cst);
@@ -34,19 +38,23 @@ LCI_error_t LCI_sync_free(LCI_comp_t* completion)
   return LCI_OK;
 }
 
+// Assumption: no LCI_sync_wait/test will succeed when
+// LCI_sync_signal is called. Otherwise, this sync can receive more signals than
+// its threshold.
 LCI_error_t LCII_sync_signal(LCI_comp_t completion, LCII_context_t* ctx)
 {
   LCII_sync_t* sync = completion;
   LCI_DBG_Assert(sync != NULL, "synchronizer is a NULL pointer!\n");
-  int pos = 0;
-  if (sync->threshold > 1)
-    pos = atomic_fetch_add_explicit(&sync->count, 1, LCIU_memory_order_relaxed);
-  LCI_DBG_Assert(pos < sync->threshold, "Receive more signals than expected\n");
-  sync->ctx[pos] = ctx;
-  if (sync->threshold > 1)
-    atomic_fetch_add_explicit(&sync->confirm, 1, LCIU_memory_order_release);
-  else
-    atomic_store_explicit(&sync->confirm, 1, LCIU_memory_order_release);
+  uint_fast64_t tail = 0;
+  uint_fast64_t pos = 0;
+  if (sync->threshold > 1) {
+    pos = atomic_fetch_add_explicit(&sync->top, 1, LCIU_memory_order_relaxed);
+    tail = atomic_load_explicit(&sync->tail, LCIU_memory_order_acquire);
+  }
+  LCI_DBG_Assert(pos < tail + sync->threshold,
+                 "Receive more signals than expected\n");
+  sync->ctx[pos - tail] = ctx;
+  atomic_fetch_add_explicit(&sync->top2, 1, LCIU_memory_order_release);
   LCII_PCOUNTER_ADD(comp_produce, 1);
   return LCI_OK;
 }
@@ -63,55 +71,45 @@ LCI_error_t LCI_sync_signal(LCI_comp_t completion, LCI_request_t request)
   return LCI_OK;
 }
 
-// LCI_sync_wait is thread-safe against LCI(I)_sync_signal
-// but not thread-safe against other LCI_sync_wait/test
+// LCI_sync_wait is thread-safe against LCI(I)_sync_signal and other
+// LCI_sync_wait/test
 LCI_error_t LCI_sync_wait(LCI_comp_t completion, LCI_request_t request[])
 {
-  LCII_sync_t* sync = completion;
-  LCI_DBG_Assert(sync != NULL, "synchronizer is a NULL pointer!\n");
-  while (atomic_load_explicit(&sync->confirm, LCIU_memory_order_acquire) <
-         sync->threshold)
-    continue;
-  if (request)
-    for (int i = 0; i < sync->threshold; ++i) {
-      request[i] = LCII_ctx2req(sync->ctx[i]);
-    }
-  else
-    for (int i = 0; i < sync->threshold; ++i) {
-      LCIU_free(sync->ctx[i]);
-    }
-  atomic_store_explicit(&sync->confirm, 0, LCIU_memory_order_relaxed);
-  if (sync->threshold > 1)
-    atomic_store_explicit(&sync->count, 0, LCIU_memory_order_relaxed);
-  LCII_PCOUNTER_ADD(comp_consume, sync->threshold);
-  return LCI_OK;
+  LCI_error_t ret;
+  do {
+    ret = LCI_sync_test(completion, request);
+  } while (ret == LCI_ERR_RETRY);
+  return ret;
 }
 
-// LCI_sync_test is thread-safe against LCI(I)_sync_signal
-// but not thread-safe against other LCI_sync_wait/test
+// LCI_sync_test is thread-safe against LCI(I)_sync_signal and other
+// LCI_sync_wait/test
 LCI_error_t LCI_sync_test(LCI_comp_t completion, LCI_request_t request[])
 {
   LCII_sync_t* sync = completion;
   LCI_DBG_Assert(sync != NULL, "synchronizer is a NULL pointer!\n");
-  if (atomic_load_explicit(&sync->confirm, LCIU_memory_order_acquire) <
-      sync->threshold) {
+  int top2 = atomic_load_explicit(&sync->top2, LCIU_memory_order_acquire);
+  int tail = atomic_load_explicit(&sync->tail, LCIU_memory_order_acquire);
+  if (top2 != tail + sync->threshold) {
     return LCI_ERR_RETRY;
   } else {
-    LCI_DBG_Assert(sync->confirm == sync->threshold,
-                   "Receive more signals (%d) than expected (%d)\n",
-                   sync->confirm, sync->threshold);
-    if (request)
-      for (int i = 0; i < sync->threshold; ++i) {
-        request[i] = LCII_ctx2req(sync->ctx[i]);
-      }
-    else
-      for (int i = 0; i < sync->threshold; ++i) {
-        LCIU_free(sync->ctx[i]);
-      }
-    atomic_store_explicit(&sync->confirm, 0, LCIU_memory_order_relaxed);
-    if (sync->threshold > 1)
-      atomic_store_explicit(&sync->count, 0, LCIU_memory_order_relaxed);
-    LCII_PCOUNTER_ADD(comp_consume, sync->threshold);
-    return LCI_OK;
+    uint_fast64_t expected = tail;
+    bool succeed = atomic_compare_exchange_weak_explicit(
+        &sync->tail, &expected, top2, LCIU_memory_order_release,
+        LCIU_memory_order_relaxed);
+    if (succeed) {
+      if (request)
+        for (int i = 0; i < sync->threshold; ++i) {
+          request[i] = LCII_ctx2req(sync->ctx[i]);
+        }
+      else
+        for (int i = 0; i < sync->threshold; ++i) {
+          LCIU_free(sync->ctx[i]);
+        }
+      LCII_PCOUNTER_ADD(comp_consume, sync->threshold);
+      return LCI_OK;
+    } else {
+      return LCI_ERR_RETRY;
+    }
   }
 }