Illumos #3741

3741 zfs needs better comments
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Eric Schrock <eric.schrock@delphix.com>
Approved by: Christopher Siden <christopher.siden@delphix.com>

References:
  https://www.illumos.org/issues/3741
  illumos/illumos-gate@3e30c24aee

Ported-by: Richard Yao <ryao@gentoo.org>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #1775

module/zfs/txg.c

@@ -354,6 +354,12 @@ txg_rele_to_sync(txg_handle_t *th)
 	th->th_cpu = NULL;	/* defensive */
 }
 
+/*
+ * Blocks until all transactions in the group are committed.
+ *
+ * On return, the transaction group has reached a stable state in which it can
+ * then be passed off to the syncing context.
+ */
 static void
 txg_quiesce(dsl_pool_t *dp, uint64_t txg)
 {
@@ -409,6 +415,9 @@ txg_do_callbacks(list_t *cb_list)
 
 /*
  * Dispatch the commit callbacks registered on this txg to worker threads.
+ *
+ * If no callbacks are registered for a given TXG, nothing happens.
+ * This function creates a taskq for the associated pool, if needed.
  */
 static void
 txg_dispatch_callbacks(dsl_pool_t *dp, uint64_t txg)
@@ -419,7 +428,10 @@ txg_dispatch_callbacks(dsl_pool_t *dp, uint64_t txg)
 
 	for (c = 0; c < max_ncpus; c++) {
 		tx_cpu_t *tc = &tx->tx_cpu[c];
-		/* No need to lock tx_cpu_t at this point */
+		/*
+		 * No need to lock tx_cpu_t at this point, since this can
+		 * only be called once a txg has been synced.
+		 */
 
 		int g = txg & TXG_MASK;