Long hold the dataset during upgrade

If the receive or rollback is performed while filesystem is upgrading
the objset may be evicted in `dsl_dataset_clone_swap_sync_impl`. This
will lead to NULL pointer dereference when upgrade tries to access
evicted objset.

This commit adds long hold of dataset during whole upgrade process.
The receive and rollback will return an EBUSY error until the
upgrade is not finished.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Arkadiusz Bubała <arkadiusz.bubala@open-e.com>
Closes #5295 
Closes #6837

module/zfs/dmu_objset.c

@@ -28,6 +28,7 @@
  * Copyright (c) 2015, STRATO AG, Inc. All rights reserved.
  * Copyright (c) 2016 Actifio, Inc. All rights reserved.
  * Copyright 2017 Nexenta Systems, Inc.
+ * Copyright (c) 2017 Open-E, Inc. All Rights Reserved.
  */
 
 /* Portions Copyright 2010 Robert Milkowski */
@@ -79,6 +80,8 @@ int dmu_find_threads = 0;
  */
 int dmu_rescan_dnode_threshold = 1 << DN_MAX_INDBLKSHIFT;
 
+static char *upgrade_tag = "upgrade_tag";
+
 static void dmu_objset_find_dp_cb(void *arg);
 
 static void dmu_objset_upgrade(objset_t *os, dmu_objset_upgrade_cb_t cb);
@@ -704,13 +707,12 @@ dmu_objset_own(const char *name, dmu_objset_type_t type,
 		return (err);
 	}
 
-	dsl_pool_rele(dp, FTAG);
-
 	/* user accounting requires the dataset to be decrypted */
 	if (dmu_objset_userobjspace_upgradable(*osp) &&
 	    (ds->ds_dir->dd_crypto_obj == 0 || decrypt))
 		dmu_objset_userobjspace_upgrade(*osp);
 
+	dsl_pool_rele(dp, FTAG);
 	return (0);
 }
 
@@ -1311,6 +1313,7 @@ dmu_objset_upgrade_task_cb(void *data)
 	os->os_upgrade_exit = B_TRUE;
 	os->os_upgrade_id = 0;
 	mutex_exit(&os->os_upgrade_lock);
+	dsl_dataset_long_rele(dmu_objset_ds(os), upgrade_tag);
 }
 
 static void
@@ -1319,15 +1322,20 @@ dmu_objset_upgrade(objset_t *os, dmu_objset_upgrade_cb_t cb)
 	if (os->os_upgrade_id != 0)
 		return;
 
+	ASSERT(dsl_pool_config_held(dmu_objset_pool(os)));
+	dsl_dataset_long_hold(dmu_objset_ds(os), upgrade_tag);
+
 	mutex_enter(&os->os_upgrade_lock);
 	if (os->os_upgrade_id == 0 && os->os_upgrade_status == 0) {
 		os->os_upgrade_exit = B_FALSE;
 		os->os_upgrade_cb = cb;
 		os->os_upgrade_id = taskq_dispatch(
 		    os->os_spa->spa_upgrade_taskq,
 		    dmu_objset_upgrade_task_cb, os, TQ_SLEEP);
-		if (os->os_upgrade_id == TASKQID_INVALID)
+		if (os->os_upgrade_id == TASKQID_INVALID) {
+			dsl_dataset_long_rele(dmu_objset_ds(os), upgrade_tag);
 			os->os_upgrade_status = ENOMEM;
+		}
 	}
 	mutex_exit(&os->os_upgrade_lock);
 }
@@ -1343,7 +1351,9 @@ dmu_objset_upgrade_stop(objset_t *os)
 		os->os_upgrade_id = 0;
 		mutex_exit(&os->os_upgrade_lock);
 
-		taskq_cancel_id(os->os_spa->spa_upgrade_taskq, id);
+		if ((taskq_cancel_id(os->os_spa->spa_upgrade_taskq, id)) == 0) {
+			dsl_dataset_long_rele(dmu_objset_ds(os), upgrade_tag);
+		}
 		txg_wait_synced(os->os_spa->spa_dsl_pool, 0);
 	} else {
 		mutex_exit(&os->os_upgrade_lock);

module/zfs/zfs_ioctl.c

@@ -5275,9 +5275,6 @@ zfs_ioc_userobjspace_upgrade(zfs_cmd_t *zc)
 	if (error != 0)
 		return (error);
 
-	dsl_dataset_long_hold(dmu_objset_ds(os), FTAG);
-	dsl_pool_rele(dmu_objset_pool(os), FTAG);
-
 	if (dmu_objset_userobjspace_upgradable(os)) {
 		mutex_enter(&os->os_upgrade_lock);
 		if (os->os_upgrade_id == 0) {
@@ -5290,11 +5287,14 @@ zfs_ioc_userobjspace_upgrade(zfs_cmd_t *zc)
 			mutex_exit(&os->os_upgrade_lock);
 		}
 
+		dsl_pool_rele(dmu_objset_pool(os), FTAG);
+
 		taskq_wait_id(os->os_spa->spa_upgrade_taskq, os->os_upgrade_id);
 		error = os->os_upgrade_status;
+	} else {
+		dsl_pool_rele(dmu_objset_pool(os), FTAG);
 	}
 
-	dsl_dataset_long_rele(dmu_objset_ds(os), FTAG);
 	dsl_dataset_rele_flags(dmu_objset_ds(os), DS_HOLD_FLAG_DECRYPT, FTAG);
 
 	return (error);

module/zfs/zfs_vfsops.c

@@ -834,8 +834,13 @@ zfs_fuid_overobjquota(zfsvfs_t *zfsvfs, boolean_t isgroup, uint64_t fuid)
 	int err;
 
 	if (!dmu_objset_userobjspace_present(zfsvfs->z_os)) {
-		if (dmu_objset_userobjspace_upgradable(zfsvfs->z_os))
+		if (dmu_objset_userobjspace_upgradable(zfsvfs->z_os)) {
+			dsl_pool_config_enter(
+			    dmu_objset_pool(zfsvfs->z_os), FTAG);
 			dmu_objset_userobjspace_upgrade(zfsvfs->z_os);
+			dsl_pool_config_exit(
+			    dmu_objset_pool(zfsvfs->z_os), FTAG);
+		}
 		return (B_FALSE);
 	}