iscsi-target: Fix missed wakeup race in TX thread

The sleeping code in iscsi_target_tx_thread() is susceptible to the classic
missed wakeup race:

 - TX thread finishes handle_immediate_queue() and handle_response_queue(),
   thinks both queues are empty.
 - Another thread adds a queue entry and does wake_up_process(), which does
   nothing because the TX thread is still awake.
 - TX thread does schedule_timeout() and sleeps forever.

In practice this can kill an iSCSI connection if for example an initiator
does single-threaded writes and the target misses the wakeup window when
queueing an R2T; in this case the connection will be stuck until the
initiator loses patience and does some task management operation (or kills
the connection entirely).

Fix this by converting to wait_event_interruptible(), which does not
suffer from this sort of race.
Signed-off-by: Roland Dreier <roland@purestorage.com>
Cc: Andy Grover <agrover@redhat.com>
Cc: Hannes Reinecke <hare@suse.de>
Cc: Christoph Hellwig <hch@lst.de>
Cc: stable@vger.kernel.org
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>

drivers/target/iscsi/iscsi_target.c

@@ -3719,7 +3719,9 @@ int iscsi_target_tx_thread(void *arg)
 		 */
 		iscsit_thread_check_cpumask(conn, current, 1);
 
-		schedule_timeout_interruptible(MAX_SCHEDULE_TIMEOUT);
+		wait_event_interruptible(conn->queues_wq,
+					 !iscsit_conn_all_queues_empty(conn) ||
+					 ts->status == ISCSI_THREAD_SET_RESET);
 
 		if ((ts->status == ISCSI_THREAD_SET_RESET) ||
 		     signal_pending(current))

drivers/target/iscsi/iscsi_target_core.h

@@ -486,6 +486,7 @@ struct iscsi_tmr_req {
 };
 
 struct iscsi_conn {
+	wait_queue_head_t	queues_wq;
 	/* Authentication Successful for this connection */
 	u8			auth_complete;
 	/* State connection is currently in */

drivers/target/iscsi/iscsi_target_login.c

@@ -41,6 +41,7 @@
 
 static int iscsi_login_init_conn(struct iscsi_conn *conn)
 {
+	init_waitqueue_head(&conn->queues_wq);
 	INIT_LIST_HEAD(&conn->conn_list);
 	INIT_LIST_HEAD(&conn->conn_cmd_list);
 	INIT_LIST_HEAD(&conn->immed_queue_list);

drivers/target/iscsi/iscsi_target_util.c

@@ -488,7 +488,7 @@ void iscsit_add_cmd_to_immediate_queue(
 	atomic_set(&conn->check_immediate_queue, 1);
 	spin_unlock_bh(&conn->immed_queue_lock);
 
-	wake_up_process(conn->thread_set->tx_thread);
+	wake_up(&conn->queues_wq);
 }
 
 struct iscsi_queue_req *iscsit_get_cmd_from_immediate_queue(struct iscsi_conn *conn)
@@ -562,7 +562,7 @@ void iscsit_add_cmd_to_response_queue(
 	atomic_inc(&cmd->response_queue_count);
 	spin_unlock_bh(&conn->response_queue_lock);
 
-	wake_up_process(conn->thread_set->tx_thread);
+	wake_up(&conn->queues_wq);
 }
 
 struct iscsi_queue_req *iscsit_get_cmd_from_response_queue(struct iscsi_conn *conn)
@@ -616,6 +616,24 @@ static void iscsit_remove_cmd_from_response_queue(
 	}
 }
 
+bool iscsit_conn_all_queues_empty(struct iscsi_conn *conn)
+{
+	bool empty;
+
+	spin_lock_bh(&conn->immed_queue_lock);
+	empty = list_empty(&conn->immed_queue_list);
+	spin_unlock_bh(&conn->immed_queue_lock);
+
+	if (!empty)
+		return empty;
+
+	spin_lock_bh(&conn->response_queue_lock);
+	empty = list_empty(&conn->response_queue_list);
+	spin_unlock_bh(&conn->response_queue_lock);
+
+	return empty;
+}
+
 void iscsit_free_queue_reqs_for_conn(struct iscsi_conn *conn)
 {
 	struct iscsi_queue_req *qr, *qr_tmp;

drivers/target/iscsi/iscsi_target_util.h

@@ -25,6 +25,7 @@ extern struct iscsi_queue_req *iscsit_get_cmd_from_immediate_queue(struct iscsi_
 extern void iscsit_add_cmd_to_response_queue(struct iscsi_cmd *, struct iscsi_conn *, u8);
 extern struct iscsi_queue_req *iscsit_get_cmd_from_response_queue(struct iscsi_conn *);
 extern void iscsit_remove_cmd_from_tx_queues(struct iscsi_cmd *, struct iscsi_conn *);
+extern bool iscsit_conn_all_queues_empty(struct iscsi_conn *);
 extern void iscsit_free_queue_reqs_for_conn(struct iscsi_conn *);
 extern void iscsit_release_cmd(struct iscsi_cmd *);
 extern void iscsit_free_cmd(struct iscsi_cmd *);