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>

*** 98,110 ****
          if (ds == NULL) {
                  dsl_pool_mos_diduse_space(tx->tx_pool,
                      used, compressed, uncompressed);
                  return;
          }
-         dmu_buf_will_dirty(ds->ds_dbuf, tx);
  
!         mutex_enter(&ds->ds_dir->dd_lock);
          mutex_enter(&ds->ds_lock);
          delta = parent_delta(ds, used);
          ds->ds_phys->ds_referenced_bytes += used;
          ds->ds_phys->ds_compressed_bytes += compressed;
          ds->ds_phys->ds_uncompressed_bytes += uncompressed;
--- 98,109 ----
          if (ds == NULL) {
                  dsl_pool_mos_diduse_space(tx->tx_pool,
                      used, compressed, uncompressed);
                  return;
          }
  
!         dmu_buf_will_dirty(ds->ds_dbuf, tx);
          mutex_enter(&ds->ds_lock);
          delta = parent_delta(ds, used);
          ds->ds_phys->ds_referenced_bytes += used;
          ds->ds_phys->ds_compressed_bytes += compressed;
          ds->ds_phys->ds_uncompressed_bytes += uncompressed;
*** 112,122 ****
          mutex_exit(&ds->ds_lock);
          dsl_dir_diduse_space(ds->ds_dir, DD_USED_HEAD, delta,
              compressed, uncompressed, tx);
          dsl_dir_transfer_space(ds->ds_dir, used - delta,
              DD_USED_REFRSRV, DD_USED_HEAD, tx);
-         mutex_exit(&ds->ds_dir->dd_lock);
  }
  
  int
  dsl_dataset_block_kill(dsl_dataset_t *ds, const blkptr_t *bp, dmu_tx_t *tx,
      boolean_t async)
--- 111,120 ----
*** 147,157 ****
                  int64_t delta;
  
                  dprintf_bp(bp, "freeing ds=%llu", ds->ds_object);
                  dsl_free(tx->tx_pool, tx->tx_txg, bp);
  
-                 mutex_enter(&ds->ds_dir->dd_lock);
                  mutex_enter(&ds->ds_lock);
                  ASSERT(ds->ds_phys->ds_unique_bytes >= used ||
                      !DS_UNIQUE_IS_ACCURATE(ds));
                  delta = parent_delta(ds, -used);
                  ds->ds_phys->ds_unique_bytes -= used;
--- 145,154 ----
*** 158,168 ****
                  mutex_exit(&ds->ds_lock);
                  dsl_dir_diduse_space(ds->ds_dir, DD_USED_HEAD,
                      delta, -compressed, -uncompressed, tx);
                  dsl_dir_transfer_space(ds->ds_dir, -used - delta,
                      DD_USED_REFRSRV, DD_USED_HEAD, tx);
-                 mutex_exit(&ds->ds_dir->dd_lock);
          } else {
                  dprintf_bp(bp, "putting on dead list: %s", "");
                  if (async) {
                          /*
                           * We are here as part of zio's write done callback,
--- 155,164 ----
*** 589,623 ****
                          }
                  }
          }
  }
  
- static int
- dsl_dataset_namelen(dsl_dataset_t *ds)
- {
-         int result;
- 
-         if (ds == NULL) {
-                 result = 3;     /* "mos" */
-         } else {
-                 result = dsl_dir_namelen(ds->ds_dir);
-                 VERIFY0(dsl_dataset_get_snapname(ds));
-                 if (ds->ds_snapname[0]) {
-                         ++result;       /* adding one for the @-sign */
-                         if (!MUTEX_HELD(&ds->ds_lock)) {
-                                 mutex_enter(&ds->ds_lock);
-                                 result += strlen(ds->ds_snapname);
-                                 mutex_exit(&ds->ds_lock);
-                         } else {
-                                 result += strlen(ds->ds_snapname);
-                         }
-                 }
-         }
- 
-         return (result);
- }
- 
  void
  dsl_dataset_rele(dsl_dataset_t *ds, void *tag)
  {
          dmu_buf_rele(ds->ds_dbuf, tag);
  }
--- 585,594 ----