Fix dnode_hold() freeing dnode behavior

Commit 4c5b89f59 refactored dnode_hold() and in the process
accidentally introduced a slight change in behavior which was
not intended.  The required behavior is that once the ZPL,
or other consumer, declares its intent to free a dnode then
dnode_hold() should immediately start failing.  This updated
code wouldn't return the failure until after it was freed.

When DNODE_MUST_BE_ALLOCATED is set it must return ENOENT, and
when DNODE_MUST_BE_FREE is set it must return EEXIST;

This issue was uncovered by ztest_remap() which attempted
to remap a freeing object which should have been skipped as
described by the comment in dmu_objset_remap_indirects_impl().

Reviewed-by: George Melikov <[email protected]>
Reviewed-by: Tom Caputi <[email protected]>
Reviewed-by: Olaf Faaland <[email protected]>
Signed-off-by: Brian Behlendorf <[email protected]>
Closes #8172 

1 files changed, 4 insertions, 2 deletions

diff --git a/module/zfs/dnode.c b/module/zfs/dnode.c
index 6b9f94053..0e9a4dabe 100644
--- a/module/zfs/dnode.c
+++ b/module/zfs/dnode.c
@@ -1253,8 +1253,10 @@ dnode_buf_evict_async(void *dbu)
  * EINVAL - Invalid object number or flags.
  * ENOSPC - Hole too small to fulfill "slots" request (DNODE_MUST_BE_FREE)
  * EEXIST - Refers to an allocated dnode (DNODE_MUST_BE_FREE)
+ *        - Refers to a freeing dnode (DNODE_MUST_BE_FREE)
  *        - Refers to an interior dnode slot (DNODE_MUST_BE_ALLOCATED)
  * ENOENT - The requested dnode is not allocated (DNODE_MUST_BE_ALLOCATED)
+ *        - The requested dnode is being freed (DNODE_MUST_BE_ALLOCATED)
  * EIO    - I/O error when reading the meta dnode dbuf.
  *
  * succeeds even for free dnodes.
@@ -1443,7 +1445,7 @@ dnode_hold_impl(objset_t *os, uint64_t object, int flag, int slots,
 		}
 
 		mutex_enter(&dn->dn_mtx);
-		if (dn->dn_type == DMU_OT_NONE) {
+		if (dn->dn_type == DMU_OT_NONE || dn->dn_free_txg != 0) {
 			DNODE_STAT_BUMP(dnode_hold_alloc_type_none);
 			mutex_exit(&dn->dn_mtx);
 			dnode_slots_rele(dnc, idx, slots);
@@ -1502,7 +1504,7 @@ dnode_hold_impl(objset_t *os, uint64_t object, int flag, int slots,
 		}
 
 		mutex_enter(&dn->dn_mtx);
-		if (!zfs_refcount_is_zero(&dn->dn_holds)) {
+		if (!zfs_refcount_is_zero(&dn->dn_holds) || dn->dn_free_txg) {
 			DNODE_STAT_BUMP(dnode_hold_free_refcount);
 			mutex_exit(&dn->dn_mtx);
 			dnode_slots_rele(dnc, idx, slots);