Patch "target: Fix NULL dereference during LUN lookup + active I/O shutdown" has been added to the 4.10-stable tree

@@ -78,12 +78,16 @@ transport_lookup_cmd_lun(struct se_cmd *
 					&deve->read_bytes);
 
 		se_lun = rcu_dereference(deve->se_lun);
+
+		if (!percpu_ref_tryget_live(&se_lun->lun_ref)) {
+			se_lun = NULL;
+			goto out_unlock;
+		}
+
 		se_cmd->se_lun = rcu_dereference(deve->se_lun);
 		se_cmd->pr_res_key = deve->pr_res_key;
 		se_cmd->orig_fe_lun = unpacked_lun;
 		se_cmd->se_cmd_flags |= SCF_SE_LUN_CMD;
-
-		percpu_ref_get(&se_lun->lun_ref);
 		se_cmd->lun_ref_active = true;
 
 		if ((se_cmd->data_direction == DMA_TO_DEVICE) &&

@@ -97,6 +101,7 @@ transport_lookup_cmd_lun(struct se_cmd *
 			goto ref_dev;
 		}
 	}
+out_unlock:
 	rcu_read_unlock();
 
 	if (!se_lun) {

@@ -816,6 +821,7 @@ struct se_device *target_alloc_device(st
 	xcopy_lun = &dev->xcopy_lun;
 	rcu_assign_pointer(xcopy_lun->lun_se_dev, dev);
 	init_completion(&xcopy_lun->lun_ref_comp);
+	init_completion(&xcopy_lun->lun_shutdown_comp);
 	INIT_LIST_HEAD(&xcopy_lun->lun_deve_list);
 	INIT_LIST_HEAD(&xcopy_lun->lun_dev_link);
 	mutex_init(&xcopy_lun->lun_tg_pt_md_mutex);

@@ -445,7 +445,7 @@ static void core_tpg_lun_ref_release(str
 {
 	struct se_lun *lun = container_of(ref, struct se_lun, lun_ref);
 
-	complete(&lun->lun_ref_comp);
+	complete(&lun->lun_shutdown_comp);
 }
 
 int core_tpg_register(

@@ -571,6 +571,7 @@ struct se_lun *core_tpg_alloc_lun(
 	lun->lun_link_magic = SE_LUN_LINK_MAGIC;
 	atomic_set(&lun->lun_acl_count, 0);
 	init_completion(&lun->lun_ref_comp);
+	init_completion(&lun->lun_shutdown_comp);
 	INIT_LIST_HEAD(&lun->lun_deve_list);
 	INIT_LIST_HEAD(&lun->lun_dev_link);
 	atomic_set(&lun->lun_tg_pt_secondary_offline, 0);

@@ -2706,10 +2706,39 @@ void target_wait_for_sess_cmds(struct se
 }
 EXPORT_SYMBOL(target_wait_for_sess_cmds);
 
+static void target_lun_confirm(struct percpu_ref *ref)
+{
+	struct se_lun *lun = container_of(ref, struct se_lun, lun_ref);
+
+	complete(&lun->lun_ref_comp);
+}
+
 void transport_clear_lun_ref(struct se_lun *lun)
 {
-	percpu_ref_kill(&lun->lun_ref);
+	/*
+	 * Mark the percpu-ref as DEAD, switch to atomic_t mode, drop
+	 * the initial reference and schedule confirm kill to be
+	 * executed after one full RCU grace period has completed.
+	 */
+	percpu_ref_kill_and_confirm(&lun->lun_ref, target_lun_confirm);
+	/*
+	 * The first completion waits for percpu_ref_switch_to_atomic_rcu()
+	 * to call target_lun_confirm after lun->lun_ref has been marked
+	 * as __PERCPU_REF_DEAD on all CPUs, and switches to atomic_t
+	 * mode so that percpu_ref_tryget_live() lookup of lun->lun_ref
+	 * fails for all new incoming I/O.
+	 */
 	wait_for_completion(&lun->lun_ref_comp);
+	/*
+	 * The second completion waits for percpu_ref_put_many() to
+	 * invoke ->release() after lun->lun_ref has switched to
+	 * atomic_t mode, and lun->lun_ref.count has reached zero.
+	 *
+	 * At this point all target-core lun->lun_ref references have
+	 * been dropped via transport_lun_remove_cmd(), and it's safe
+	 * to proceed with the remaining LUN shutdown.
+	 */
+	wait_for_completion(&lun->lun_shutdown_comp);
 }
 
 static bool

@@ -732,6 +732,7 @@ struct se_lun {
 	struct config_group	lun_group;
 	struct se_port_stat_grps port_stat_grps;
 	struct completion	lun_ref_comp;
+	struct completion	lun_shutdown_comp;
 	struct percpu_ref	lun_ref;
 	struct list_head	lun_dev_link;
 	struct hlist_node	link;