5056 ZFS deadlock on db_mtx and dn_holds
Reviewed by: Will Andrews <willa@spectralogic.com>
Reviewed by: Matt Ahrens <mahrens@delphix.com>
Reviewed by: George Wilson <george.wilson@delphix.com>
Approved by: Dan McDonald <danmcd@omniti.com>

*** 373,383 ****
  
  static uint64_t
  dsl_scan_ds_maxtxg(dsl_dataset_t *ds)
  {
          uint64_t smt = ds->ds_dir->dd_pool->dp_scan->scn_phys.scn_max_txg;
!         if (dsl_dataset_is_snapshot(ds))
                  return (MIN(smt, dsl_dataset_phys(ds)->ds_creation_txg));
          return (smt);
  }
  
  static void
--- 373,383 ----
  
  static uint64_t
  dsl_scan_ds_maxtxg(dsl_dataset_t *ds)
  {
          uint64_t smt = ds->ds_dir->dd_pool->dp_scan->scn_phys.scn_max_txg;
!         if (ds->ds_is_snapshot)
                  return (MIN(smt, dsl_dataset_phys(ds)->ds_creation_txg));
          return (smt);
  }
  
  static void
*** 809,819 ****
  
          if (scn->scn_phys.scn_state != DSS_SCANNING)
                  return;
  
          if (scn->scn_phys.scn_bookmark.zb_objset == ds->ds_object) {
!                 if (dsl_dataset_is_snapshot(ds)) {
                          /* Note, scn_cur_{min,max}_txg stays the same. */
                          scn->scn_phys.scn_bookmark.zb_objset =
                              dsl_dataset_phys(ds)->ds_next_snap_obj;
                          zfs_dbgmsg("destroying ds %llu; currently traversing; "
                              "reset zb_objset to %llu",
--- 809,819 ----
  
          if (scn->scn_phys.scn_state != DSS_SCANNING)
                  return;
  
          if (scn->scn_phys.scn_bookmark.zb_objset == ds->ds_object) {
!                 if (ds->ds_is_snapshot) {
                          /* Note, scn_cur_{min,max}_txg stays the same. */
                          scn->scn_phys.scn_bookmark.zb_objset =
                              dsl_dataset_phys(ds)->ds_next_snap_obj;
                          zfs_dbgmsg("destroying ds %llu; currently traversing; "
                              "reset zb_objset to %llu",
*** 831,841 ****
          } else if (zap_lookup_int_key(dp->dp_meta_objset,
              scn->scn_phys.scn_queue_obj, ds->ds_object, &mintxg) == 0) {
                  ASSERT3U(dsl_dataset_phys(ds)->ds_num_children, <=, 1);
                  VERIFY3U(0, ==, zap_remove_int(dp->dp_meta_objset,
                      scn->scn_phys.scn_queue_obj, ds->ds_object, tx));
!                 if (dsl_dataset_is_snapshot(ds)) {
                          /*
                           * We keep the same mintxg; it could be >
                           * ds_creation_txg if the previous snapshot was
                           * deleted too.
                           */
--- 831,841 ----
          } else if (zap_lookup_int_key(dp->dp_meta_objset,
              scn->scn_phys.scn_queue_obj, ds->ds_object, &mintxg) == 0) {
                  ASSERT3U(dsl_dataset_phys(ds)->ds_num_children, <=, 1);
                  VERIFY3U(0, ==, zap_remove_int(dp->dp_meta_objset,
                      scn->scn_phys.scn_queue_obj, ds->ds_object, tx));
!                 if (ds->ds_is_snapshot) {
                          /*
                           * We keep the same mintxg; it could be >
                           * ds_creation_txg if the previous snapshot was
                           * deleted too.
                           */
*** 1015,1025 ****
           * snapshots can have ZIL block pointers (which may be the same
           * BP as in the head), they must be ignored.  So we traverse the
           * ZIL here, rather than in scan_recurse(), because the regular
           * snapshot block-sharing rules don't apply to it.
           */
!         if (DSL_SCAN_IS_SCRUB_RESILVER(scn) && !dsl_dataset_is_snapshot(ds))
                  dsl_scan_zil(dp, &os->os_zil_header);
  
          /*
           * Iterate over the bps in this ds.
           */
--- 1015,1025 ----
           * snapshots can have ZIL block pointers (which may be the same
           * BP as in the head), they must be ignored.  So we traverse the
           * ZIL here, rather than in scan_recurse(), because the regular
           * snapshot block-sharing rules don't apply to it.
           */
!         if (DSL_SCAN_IS_SCRUB_RESILVER(scn) && !ds->ds_is_snapshot)
                  dsl_scan_zil(dp, &os->os_zil_header);
  
          /*
           * Iterate over the bps in this ds.
           */