scsi: iscsi: Add support for asynchronous iSCSI session destruction

iSCSI session destruction can be arbitrarily slow, since it might require
network operations and serialization inside the SCSI layer. This patch
adds a new user event to trigger the destruction work asynchronously,
releasing the rx_queue_mutex as soon as the operation is queued and before
it is performed. This change allows other operations to run in other
sessions in the meantime, removing one of the major iSCSI bottlenecks for
us.

To prevent the session from being used after the destruction request, we
remove it immediately from the sesslist. This simplifies the locking
required during the asynchronous removal.

Link: https://lore.kernel.org/r/[e-mail]
Co-developed-by: Gabriel Krisman Bertazi <[e-mail]>
Co-developed-by: Khazhismel Kumykov <[e-mail]>
Reviewed-by: Lee Duncan <[e-mail]>
Signed-off-by: Gabriel Krisman Bertazi <[e-mail]>
Signed-off-by: Frank Mayhar <[e-mail]>
Signed-off-by: Khazhismel Kumykov <[e-mail]>
Signed-off-by: Martin K. Petersen <[e-mail]>

drivers/scsi/scsi_transport_iscsi.c

···
 static atomic_t iscsi_session_nr; /* sysfs session id for next new session */
 static struct workqueue_struct *iscsi_eh_timer_workq;
 
+static struct workqueue_struct *iscsi_destroy_workq;
+
 static DEFINE_IDA(iscsi_sess_ida);
 /*
  * list of registered transports and lock that must
···
 static DEFINE_MUTEX(rx_queue_mutex);
 
 static LIST_HEAD(sesslist);
+static LIST_HEAD(sessdestroylist);
 static DEFINE_SPINLOCK(sesslock);
 static LIST_HEAD(connlist);
 static LIST_HEAD(connlist_err);
···
 	ISCSI_DBG_TRANS_SESSION(session, "Completed target removal\n");
 }
 
+static void __iscsi_destroy_session(struct work_struct *work)
+{
+	struct iscsi_cls_session *session =
+		container_of(work, struct iscsi_cls_session, destroy_work);
+
+	session->transport->destroy_session(session);
+}
+
 struct iscsi_cls_session *
 iscsi_alloc_session(struct Scsi_Host *shost, struct iscsi_transport *transport,
 		    int dd_size)
···
 	INIT_WORK(&session->block_work, __iscsi_block_session);
 	INIT_WORK(&session->unbind_work, __iscsi_unbind_session);
 	INIT_WORK(&session->scan_work, iscsi_scan_session);
+	INIT_WORK(&session->destroy_work, __iscsi_destroy_session);
 	spin_lock_init(&session->lock);
 
 	/* this is released in the dev's release function */
···
 		else
 			transport->destroy_session(session);
 		break;
+	case ISCSI_UEVENT_DESTROY_SESSION_ASYNC:
+		session = iscsi_session_lookup(ev->u.d_session.sid);
+		if (!session)
+			err = -EINVAL;
+		else if (iscsi_session_has_conns(ev->u.d_session.sid))
+			err = -EBUSY;
+		else {
+			unsigned long flags;
+
+			/* Prevent this session from being found again */
+			spin_lock_irqsave(&sesslock, flags);
+			list_move(&session->sess_list, &sessdestroylist);
+			spin_unlock_irqrestore(&sesslock, flags);
+
+			queue_work(iscsi_destroy_workq, &session->destroy_work);
+		}
+		break;
 	case ISCSI_UEVENT_UNBIND_SESSION:
 		session = iscsi_session_lookup(ev->u.d_session.sid);
 		if (session)
···
 		goto release_nls;
 	}
 
+	iscsi_destroy_workq = create_singlethread_workqueue("iscsi_destroy");
+	if (!iscsi_destroy_workq) {
+		err = -ENOMEM;
+		goto destroy_wq;
+	}
+
 	return 0;
 
+destroy_wq:
+	destroy_workqueue(iscsi_eh_timer_workq);
 release_nls:
 	netlink_kernel_release(nls);
 unregister_flashnode_bus:
···
 
 static void __exit iscsi_transport_exit(void)
 {
+	destroy_workqueue(iscsi_destroy_workq);
 	destroy_workqueue(iscsi_eh_timer_workq);
 	netlink_kernel_release(nls);
 	bus_unregister(&iscsi_flashnode_bus);

include/scsi/iscsi_if.h

···
 	ISCSI_UEVENT_LOGOUT_FLASHNODE_SID	= UEVENT_BASE + 30,
 	ISCSI_UEVENT_SET_CHAP		= UEVENT_BASE + 31,
 	ISCSI_UEVENT_GET_HOST_STATS	= UEVENT_BASE + 32,
+	ISCSI_UEVENT_DESTROY_SESSION_ASYNC	= UEVENT_BASE + 33,
 
 	/* up events */
 	ISCSI_KEVENT_RECV_PDU		= KEVENT_BASE + 1,

include/scsi/scsi_transport_iscsi.h

···
 	struct work_struct unblock_work;
 	struct work_struct scan_work;
 	struct work_struct unbind_work;
+	struct work_struct destroy_work;
 
 	/* recovery fields */
 	int recovery_tmo;