Index: FreeBSD/sys/contrib/ck/src/ck_epoch.c
===================================================================
--- FreeBSD/sys/contrib/ck/src/ck_epoch.c
+++ FreeBSD/sys/contrib/ck/src/ck_epoch.c
@@ -137,6 +137,14 @@
 CK_STACK_CONTAINER(struct ck_epoch_entry, stack_entry,
     ck_epoch_entry_container)
 
+/*
+ * CK_EPOCH_LENGTH must be a power-of-2 (because (CK_EPOCH_LENGTH - 1) is used
+ * as a mask, and it must be at least 3 (see comments above).
+ */
+#if (CK_EPOCH_LENGTH < 3 || (CK_EPOCH_LENGTH & (CK_EPOCH_LENGTH - 1)) != 0)
+#error "CK_EPOCH_LENGTH must be a power of 2 and >= 3"
+#endif
+
 #define CK_EPOCH_SENSE_MASK	(CK_EPOCH_SENSE - 1)
 
 bool
@@ -552,22 +560,36 @@
  * There are cases where it will fail to reclaim as early as it could. If this
  * becomes a problem, we could actually use a heap for epoch buckets but that
  * is far from ideal too.
+ *
+ * Note that this code assumes that nothing will modify the record while the
+ * code is running.
  */
 bool
 ck_epoch_poll_deferred(struct ck_epoch_record *record, ck_stack_t *deferred)
 {
 	bool active;
-	unsigned int epoch;
+	unsigned int epoch, startpend;
 	struct ck_epoch_record *cr = NULL;
 	struct ck_epoch *global = record->global;
 
+	startpend = record->n_pending;
 	epoch = ck_pr_load_uint(&global->epoch);
 
 	/* Serialize epoch snapshots with respect to global epoch. */
 	ck_pr_fence_memory();
+
+	/*
+	 * At this point, epoch is the current global epoch value.
+	 * There may or may not be active threads which observed epoch - 1.
+	 * (ck_epoch_scan() will tell us that.) However, there should be
+	 * no active threads which observed epoch - 2. (Note that checking
+	 * epoch - 2 is necessary, as race conditions can allow another
+	 * thread to increment the global epoch before this thread runs.)
+	 */
+	ck_epoch_dispatch(record, epoch - 2, deferred);
 	cr = ck_epoch_scan(global, cr, epoch, &active);
 	if (cr != NULL)
-		return false;
+		goto done;
 
 	/* We are at a grace period if all threads are inactive. */
 	if (active == false) {
@@ -575,14 +597,21 @@
 		for (epoch = 0; epoch < CK_EPOCH_LENGTH; epoch++)
 			ck_epoch_dispatch(record, epoch, deferred);
 
-		return true;
+		goto done;
 	}
 
-	/* If an active thread exists, rely on epoch observation. */
+	/*
+	 * If an active thread exists, rely on epoch observation.
+	 *
+	 * All the active threads entered the epoch section during
+	 * the current epoch. Therefore, we can now run the handlers
+	 * for the immediately preceding epoch and advance the epoch.
+	 */
 	(void)ck_pr_cas_uint(&global->epoch, epoch, epoch + 1);
+	ck_epoch_dispatch(record, epoch - 1, deferred);
 
-	ck_epoch_dispatch(record, epoch + 1, deferred);
-	return true;
+done:
+	return (record->n_pending != startpend);
 }
 
 bool