scsi: target: Perform ALUA group changes in one step

When userspace changes the LUN's ALUA group, it will set the LUN's group to
NULL then to the new group. Before the new group is set,
target_alua_state_check() will return 0 and allow the I/O to execute. This
has us skip the NULL stage, and just swap in the new group.

Link: https://lore.kernel.org/r/20210930020422.92578-6-michael.christie@oracle.comSigned-off-by: Mike Christie <michael.christie@oracle.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

drivers/target/target_core_alua.c

@@ -1835,8 +1835,6 @@ static void __target_detach_tg_pt_gp(struct se_lun *lun,
 	list_del_init(&lun->lun_tg_pt_gp_link);
 	tg_pt_gp->tg_pt_gp_members--;
 	spin_unlock(&tg_pt_gp->tg_pt_gp_lock);
-
-	rcu_assign_pointer(lun->lun_tg_pt_gp, NULL);
 }
 
 void target_detach_tg_pt_gp(struct se_lun *lun)
@@ -1846,12 +1844,25 @@ void target_detach_tg_pt_gp(struct se_lun *lun)
 	spin_lock(&lun->lun_tg_pt_gp_lock);
 	tg_pt_gp = rcu_dereference_check(lun->lun_tg_pt_gp,
 				lockdep_is_held(&lun->lun_tg_pt_gp_lock));
-	if (tg_pt_gp)
+	if (tg_pt_gp) {
 		__target_detach_tg_pt_gp(lun, tg_pt_gp);
+		rcu_assign_pointer(lun->lun_tg_pt_gp, NULL);
+	}
 	spin_unlock(&lun->lun_tg_pt_gp_lock);
 	synchronize_rcu();
 }
 
+static void target_swap_tg_pt_gp(struct se_lun *lun,
+				 struct t10_alua_tg_pt_gp *old_tg_pt_gp,
+				 struct t10_alua_tg_pt_gp *new_tg_pt_gp)
+{
+	assert_spin_locked(&lun->lun_tg_pt_gp_lock);
+
+	if (old_tg_pt_gp)
+		__target_detach_tg_pt_gp(lun, old_tg_pt_gp);
+	__target_attach_tg_pt_gp(lun, new_tg_pt_gp);
+}
+
 ssize_t core_alua_show_tg_pt_gp_info(struct se_lun *lun, char *page)
 {
 	struct config_item *tg_pt_ci;
@@ -1941,18 +1952,16 @@ ssize_t core_alua_store_tg_pt_gp_info(
 					&tg_pt_gp->tg_pt_gp_group.cg_item),
 				tg_pt_gp->tg_pt_gp_id);
 
-			__target_detach_tg_pt_gp(lun, tg_pt_gp);
-			__target_attach_tg_pt_gp(lun,
+			target_swap_tg_pt_gp(lun, tg_pt_gp,
 					dev->t10_alua.default_tg_pt_gp);
 			spin_unlock(&lun->lun_tg_pt_gp_lock);
 
 			goto sync_rcu;
 		}
-		__target_detach_tg_pt_gp(lun, tg_pt_gp);
 		move = 1;
 	}
 
-	__target_attach_tg_pt_gp(lun, tg_pt_gp_new);
+	target_swap_tg_pt_gp(lun, tg_pt_gp, tg_pt_gp_new);
 	spin_unlock(&lun->lun_tg_pt_gp_lock);
 	pr_debug("Target_Core_ConfigFS: %s %s/tpgt_%hu/%s to ALUA"
 		" Target Port Group: alua/%s, ID: %hu\n", (move) ?