Fix race in dnode_check_slots_free()

Currently, dnode_check_slots_free() works by checking dn->dn_type
in the dnode to determine if the dnode is reclaimable. However,
there is a small window of time between dnode_free_sync() in the
first call to dsl_dataset_sync() and when the useraccounting code
is run when the type is set DMU_OT_NONE, but the dnode is not yet
evictable, leading to crashes. This patch adds the ability for
dnodes to track which txg they were last dirtied in and adds a
check for this before performing the reclaim.

This patch also corrects several instances when dn_dirty_link was
treated as a list_node_t when it is technically a multilist_node_t.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tom Caputi <tcaputi@datto.com>
Closes #7147 
Closes #7388

include/sys/dmu_impl.h

@@ -161,6 +161,7 @@ extern "C" {
  * 	dn_allocated_txg
  * 	dn_free_txg
  * 	dn_assigned_txg
+ * 	dn_dirty_txg
  * 	dd_assigned_tx
  * 	dn_notxholds
  * 	dn_dirtyctx

include/sys/dnode.h

@@ -329,6 +329,7 @@ struct dnode {
 	uint64_t dn_allocated_txg;
 	uint64_t dn_free_txg;
 	uint64_t dn_assigned_txg;
+	uint64_t dn_dirty_txg;			/* txg dnode was last dirtied */
 	kcondvar_t dn_notxholds;
 	enum dnode_dirtycontext dn_dirtyctx;
 	uint8_t *dn_dirtyctx_firstset;		/* dbg: contents meaningless */
@@ -432,6 +433,9 @@ void dnode_evict_dbufs(dnode_t *dn);
 void dnode_evict_bonus(dnode_t *dn);
 void dnode_free_interior_slots(dnode_t *dn);
 
+#define	DNODE_IS_DIRTY(_dn)						\
+	((_dn)->dn_dirty_txg >= spa_syncing_txg((_dn)->dn_objset->os_spa))
+
 #define	DNODE_IS_CACHEABLE(_dn)						\
 	((_dn)->dn_objset->os_primary_cache == ZFS_CACHE_ALL ||		\
 	(DMU_OT_IS_METADATA((_dn)->dn_type) &&				\

module/zfs/dbuf.c

@@ -1818,6 +1818,9 @@ dbuf_dirty(dmu_buf_impl_t *db, dmu_tx_t *tx)
 			    FTAG);
 		}
 	}
+
+	if (tx->tx_txg > dn->dn_dirty_txg)
+		dn->dn_dirty_txg = tx->tx_txg;
 	mutex_exit(&dn->dn_mtx);
 
 	if (db->db_blkid == DMU_SPILL_BLKID)

module/zfs/dmu.c

@@ -2284,7 +2284,7 @@ dmu_offset_next(objset_t *os, uint64_t object, boolean_t hole, uint64_t *off)
 	 * Check if dnode is dirty
 	 */
 	for (i = 0; i < TXG_SIZE; i++) {
-		if (list_link_active(&dn->dn_dirty_link[i])) {
+		if (multilist_link_active(&dn->dn_dirty_link[i])) {
 			clean = B_FALSE;
 			break;
 		}

module/zfs/dmu_objset.c

@@ -1416,10 +1416,23 @@ dmu_objset_sync_dnodes(multilist_sublist_t *list, dmu_tx_t *tx)
 		ASSERT3U(dn->dn_nlevels, <=, DN_MAX_LEVELS);
 		multilist_sublist_remove(list, dn);
 
+		/*
+		 * If we are not doing useraccounting (os_synced_dnodes == NULL)
+		 * we are done with this dnode for this txg. Unset dn_dirty_txg
+		 * if later txgs aren't dirtying it so that future holders do
+		 * not get a stale value. Otherwise, we will do this in
+		 * userquota_updates_task() when processing has completely
+		 * finished for this txg.
+		 */
 		multilist_t *newlist = dn->dn_objset->os_synced_dnodes;
 		if (newlist != NULL) {
 			(void) dnode_add_ref(dn, newlist);
 			multilist_insert(newlist, dn);
+		} else {
+			mutex_enter(&dn->dn_mtx);
+			if (dn->dn_dirty_txg == tx->tx_txg)
+				dn->dn_dirty_txg = 0;
+			mutex_exit(&dn->dn_mtx);
 		}
 
 		dnode_sync(dn, tx);
@@ -1889,6 +1902,8 @@ userquota_updates_task(void *arg)
 				dn->dn_id_flags |= DN_ID_CHKED_BONUS;
 		}
 		dn->dn_id_flags &= ~(DN_ID_NEW_EXIST);
+		if (dn->dn_dirty_txg == spa_syncing_txg(os->os_spa))
+			dn->dn_dirty_txg = 0;
 		mutex_exit(&dn->dn_mtx);
 
 		multilist_sublist_remove(list, dn);

module/zfs/dnode.c

@@ -139,7 +139,7 @@ dnode_cons(void *arg, void *unused, int kmflag)
 	bzero(&dn->dn_next_maxblkid[0], sizeof (dn->dn_next_maxblkid));
 
 	for (i = 0; i < TXG_SIZE; i++) {
-		list_link_init(&dn->dn_dirty_link[i]);
+		multilist_link_init(&dn->dn_dirty_link[i]);
 		dn->dn_free_ranges[i] = NULL;
 		list_create(&dn->dn_dirty_records[i],
 		    sizeof (dbuf_dirty_record_t),
@@ -149,6 +149,7 @@ dnode_cons(void *arg, void *unused, int kmflag)
 	dn->dn_allocated_txg = 0;
 	dn->dn_free_txg = 0;
 	dn->dn_assigned_txg = 0;
+	dn->dn_dirty_txg = 0;
 	dn->dn_dirtyctx = 0;
 	dn->dn_dirtyctx_firstset = NULL;
 	dn->dn_bonus = NULL;
@@ -188,7 +189,7 @@ dnode_dest(void *arg, void *unused)
 	ASSERT(!list_link_active(&dn->dn_link));
 
 	for (i = 0; i < TXG_SIZE; i++) {
-		ASSERT(!list_link_active(&dn->dn_dirty_link[i]));
+		ASSERT(!multilist_link_active(&dn->dn_dirty_link[i]));
 		ASSERT3P(dn->dn_free_ranges[i], ==, NULL);
 		list_destroy(&dn->dn_dirty_records[i]);
 		ASSERT0(dn->dn_next_nblkptr[i]);
@@ -204,6 +205,7 @@ dnode_dest(void *arg, void *unused)
 	ASSERT0(dn->dn_allocated_txg);
 	ASSERT0(dn->dn_free_txg);
 	ASSERT0(dn->dn_assigned_txg);
+	ASSERT0(dn->dn_dirty_txg);
 	ASSERT0(dn->dn_dirtyctx);
 	ASSERT3P(dn->dn_dirtyctx_firstset, ==, NULL);
 	ASSERT3P(dn->dn_bonus, ==, NULL);
@@ -530,6 +532,7 @@ dnode_destroy(dnode_t *dn)
 	dn->dn_allocated_txg = 0;
 	dn->dn_free_txg = 0;
 	dn->dn_assigned_txg = 0;
+	dn->dn_dirty_txg = 0;
 
 	dn->dn_dirtyctx = 0;
 	if (dn->dn_dirtyctx_firstset != NULL) {
@@ -601,6 +604,7 @@ dnode_allocate(dnode_t *dn, dmu_object_type_t ot, int blocksize, int ibs,
 	ASSERT0(dn->dn_maxblkid);
 	ASSERT0(dn->dn_allocated_txg);
 	ASSERT0(dn->dn_assigned_txg);
+	ASSERT0(dn->dn_dirty_txg);
 	ASSERT(refcount_is_zero(&dn->dn_tx_holds));
 	ASSERT3U(refcount_count(&dn->dn_holds), <=, 1);
 	ASSERT(avl_is_empty(&dn->dn_dbufs));
@@ -614,7 +618,7 @@ dnode_allocate(dnode_t *dn, dmu_object_type_t ot, int blocksize, int ibs,
 		ASSERT0(dn->dn_rm_spillblk[i]);
 		ASSERT0(dn->dn_next_blksz[i]);
 		ASSERT0(dn->dn_next_maxblkid[i]);
-		ASSERT(!list_link_active(&dn->dn_dirty_link[i]));
+		ASSERT(!multilist_link_active(&dn->dn_dirty_link[i]));
 		ASSERT3P(list_head(&dn->dn_dirty_records[i]), ==, NULL);
 		ASSERT3P(dn->dn_free_ranges[i], ==, NULL);
 	}
@@ -792,6 +796,7 @@ dnode_move_impl(dnode_t *odn, dnode_t *ndn)
 	ndn->dn_allocated_txg = odn->dn_allocated_txg;
 	ndn->dn_free_txg = odn->dn_free_txg;
 	ndn->dn_assigned_txg = odn->dn_assigned_txg;
+	ndn->dn_dirty_txg = odn->dn_dirty_txg;
 	ndn->dn_dirtyctx = odn->dn_dirtyctx;
 	ndn->dn_dirtyctx_firstset = odn->dn_dirtyctx_firstset;
 	ASSERT(refcount_count(&odn->dn_tx_holds) == 0);
@@ -860,6 +865,7 @@ dnode_move_impl(dnode_t *odn, dnode_t *ndn)
 	odn->dn_allocated_txg = 0;
 	odn->dn_free_txg = 0;
 	odn->dn_assigned_txg = 0;
+	odn->dn_dirty_txg = 0;
 	odn->dn_dirtyctx = 0;
 	odn->dn_dirtyctx_firstset = NULL;
 	odn->dn_have_spill = B_FALSE;
@@ -1086,6 +1092,10 @@ dnode_check_slots_free(dnode_children_t *children, int idx, int slots)
 {
 	ASSERT3S(idx + slots, <=, DNODES_PER_BLOCK);
 
+	/*
+	 * If all dnode slots are either already free or
+	 * evictable return B_TRUE.
+	 */
 	for (int i = idx; i < idx + slots; i++) {
 		dnode_handle_t *dnh = &children->dnc_children[i];
 		dnode_t *dn = dnh->dnh_dnode;
@@ -1094,18 +1104,17 @@ dnode_check_slots_free(dnode_children_t *children, int idx, int slots)
 			continue;
 		} else if (DN_SLOT_IS_PTR(dn)) {
 			mutex_enter(&dn->dn_mtx);
-			dmu_object_type_t type = dn->dn_type;
+			boolean_t can_free = (dn->dn_type == DMU_OT_NONE &&
+			    !DNODE_IS_DIRTY(dn));
 			mutex_exit(&dn->dn_mtx);
 
-			if (type != DMU_OT_NONE)
+			if (!can_free)
 				return (B_FALSE);
-
+			else
-			continue;
+				continue;
 		} else {
 			return (B_FALSE);
 		}
-
-		return (B_FALSE);
 	}
 
 	return (B_TRUE);
@@ -1633,7 +1642,7 @@ dnode_setdirty(dnode_t *dn, dmu_tx_t *tx)
 	/*
 	 * If we are already marked dirty, we're done.
 	 */
-	if (list_link_active(&dn->dn_dirty_link[txg & TXG_MASK])) {
+	if (multilist_link_active(&dn->dn_dirty_link[txg & TXG_MASK])) {
 		multilist_sublist_unlock(mls);
 		return;
 	}