4046 dsl_dataset_t ds_dir->dd_lock is highly contended
Reviewed by: Eric Schrock <eric.schrock@delphix.com>
Reviewed by: George Wilson <george.wilson@delphix.com>

*** 839,862 ****
  void
  dsl_dir_diduse_space(dsl_dir_t *dd, dd_used_t type,
      int64_t used, int64_t compressed, int64_t uncompressed, dmu_tx_t *tx)
  {
          int64_t accounted_delta;
          boolean_t needlock = !MUTEX_HELD(&dd->dd_lock);
  
          ASSERT(dmu_tx_is_syncing(tx));
          ASSERT(type < DD_USED_NUM);
  
          if (needlock)
                  mutex_enter(&dd->dd_lock);
          accounted_delta = parent_delta(dd, dd->dd_phys->dd_used_bytes, used);
          ASSERT(used >= 0 || dd->dd_phys->dd_used_bytes >= -used);
          ASSERT(compressed >= 0 ||
              dd->dd_phys->dd_compressed_bytes >= -compressed);
          ASSERT(uncompressed >= 0 ||
              dd->dd_phys->dd_uncompressed_bytes >= -uncompressed);
-         dmu_buf_will_dirty(dd->dd_dbuf, tx);
          dd->dd_phys->dd_used_bytes += used;
          dd->dd_phys->dd_uncompressed_bytes += uncompressed;
          dd->dd_phys->dd_compressed_bytes += compressed;
  
          if (dd->dd_phys->dd_flags & DD_FLAG_USED_BREAKDOWN) {
--- 839,871 ----
  void
  dsl_dir_diduse_space(dsl_dir_t *dd, dd_used_t type,
      int64_t used, int64_t compressed, int64_t uncompressed, dmu_tx_t *tx)
  {
          int64_t accounted_delta;
+ 
+         /*
+          * dsl_dataset_set_refreservation_sync_impl() calls this with
+          * dd_lock held, so that it can atomically update
+          * ds->ds_reserved and the dsl_dir accounting, so that
+          * dsl_dataset_check_quota() can see dataset and dir accounting
+          * consistently.
+          */
          boolean_t needlock = !MUTEX_HELD(&dd->dd_lock);
  
          ASSERT(dmu_tx_is_syncing(tx));
          ASSERT(type < DD_USED_NUM);
  
+         dmu_buf_will_dirty(dd->dd_dbuf, tx);
+ 
          if (needlock)
                  mutex_enter(&dd->dd_lock);
          accounted_delta = parent_delta(dd, dd->dd_phys->dd_used_bytes, used);
          ASSERT(used >= 0 || dd->dd_phys->dd_used_bytes >= -used);
          ASSERT(compressed >= 0 ||
              dd->dd_phys->dd_compressed_bytes >= -compressed);
          ASSERT(uncompressed >= 0 ||
              dd->dd_phys->dd_uncompressed_bytes >= -uncompressed);
          dd->dd_phys->dd_used_bytes += used;
          dd->dd_phys->dd_uncompressed_bytes += uncompressed;
          dd->dd_phys->dd_compressed_bytes += compressed;
  
          if (dd->dd_phys->dd_flags & DD_FLAG_USED_BREAKDOWN) {
*** 885,913 ****
  
  void
  dsl_dir_transfer_space(dsl_dir_t *dd, int64_t delta,
      dd_used_t oldtype, dd_used_t newtype, dmu_tx_t *tx)
  {
-         boolean_t needlock = !MUTEX_HELD(&dd->dd_lock);
- 
          ASSERT(dmu_tx_is_syncing(tx));
          ASSERT(oldtype < DD_USED_NUM);
          ASSERT(newtype < DD_USED_NUM);
  
          if (delta == 0 || !(dd->dd_phys->dd_flags & DD_FLAG_USED_BREAKDOWN))
                  return;
  
!         if (needlock)
                  mutex_enter(&dd->dd_lock);
          ASSERT(delta > 0 ?
              dd->dd_phys->dd_used_breakdown[oldtype] >= delta :
              dd->dd_phys->dd_used_breakdown[newtype] >= -delta);
          ASSERT(dd->dd_phys->dd_used_bytes >= ABS(delta));
-         dmu_buf_will_dirty(dd->dd_dbuf, tx);
          dd->dd_phys->dd_used_breakdown[oldtype] -= delta;
          dd->dd_phys->dd_used_breakdown[newtype] += delta;
-         if (needlock)
                  mutex_exit(&dd->dd_lock);
  }
  
  typedef struct dsl_dir_set_qr_arg {
          const char *ddsqra_name;
--- 894,918 ----
  
  void
  dsl_dir_transfer_space(dsl_dir_t *dd, int64_t delta,
      dd_used_t oldtype, dd_used_t newtype, dmu_tx_t *tx)
  {
          ASSERT(dmu_tx_is_syncing(tx));
          ASSERT(oldtype < DD_USED_NUM);
          ASSERT(newtype < DD_USED_NUM);
  
          if (delta == 0 || !(dd->dd_phys->dd_flags & DD_FLAG_USED_BREAKDOWN))
                  return;
  
!         dmu_buf_will_dirty(dd->dd_dbuf, tx);
          mutex_enter(&dd->dd_lock);
          ASSERT(delta > 0 ?
              dd->dd_phys->dd_used_breakdown[oldtype] >= delta :
              dd->dd_phys->dd_used_breakdown[newtype] >= -delta);
          ASSERT(dd->dd_phys->dd_used_bytes >= ABS(delta));
          dd->dd_phys->dd_used_breakdown[oldtype] -= delta;
          dd->dd_phys->dd_used_breakdown[newtype] += delta;
          mutex_exit(&dd->dd_lock);
  }
  
  typedef struct dsl_dir_set_qr_arg {
          const char *ddsqra_name;