3741 zfs needs better comments
Submitted by:   Will Andrews <willa@spectralogic.com>
Submitted by:   Justin Gibbs <justing@spectralogic.com>
Submitted by:   Alan Somers <alans@spectralogic.com>
Reviewed by:    Matthew Ahrens <mahrens@delphix.com>
Reviewed by:    Eric Schrock <eric.schrock@delphix.com>

*** 335,344 ****
--- 335,350 ----
          mutex_exit(&tc->tc_lock);
  
          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)
  {
          tx_state_t *tx = &dp->dp_tx;
          int g = txg & TXG_MASK;
*** 385,405 ****
          kmem_free(cb_list, sizeof (list_t));
  }
  
  /*
   * Dispatch the commit callbacks registered on this txg to worker threads.
   */
  static void
  txg_dispatch_callbacks(dsl_pool_t *dp, uint64_t txg)
  {
          int c;
          tx_state_t *tx = &dp->dp_tx;
          list_t *cb_list;
  
          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 */
  
                  int g = txg & TXG_MASK;
  
                  if (list_is_empty(&tc->tc_callbacks[g]))
                          continue;
--- 391,417 ----
          kmem_free(cb_list, sizeof (list_t));
  }
  
  /*
   * 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)
  {
          int c;
          tx_state_t *tx = &dp->dp_tx;
          list_t *cb_list;
  
          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, since this can
!                  * only be called once a txg has been synced.
!                  */
  
                  int g = txg & TXG_MASK;
  
                  if (list_is_empty(&tc->tc_callbacks[g]))
                          continue;