iscsi-target: Fix race with thread_pre_handler flush_signals + ISCSI_THREAD_SET_DIE

This patch addresses an long standing race in iscsi_[rx,tx]_thread_pre_handler()
use of flush_signals(), and between iscsi_deallocate_extra_thread_sets() setting
ISCSI_THREAD_SET_DIE before calling kthread_stop().

It addresses the issue by both holding ts_state_lock before calling send_sig()
in iscsi_deallocate_extra_thread_sets(), as well as only calling flush_signals()
when ts->status != ISCSI_THREAD_SET_DIE within iscsi_[rx,tx]_thread_pre_handler()
code.

v2 changes:
  - Add explicit complete(&ts->[rx,tx]_start_comp); before kthread_stop() in
    iscsi_deallocate_extra_thread_sets()
  - Drop left-over send_sig() calls in iscsi_deallocate_extra_thread_sets()
  - Add kthread_should_stop() check in iscsi_signal_thread_pre_handler()
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>

drivers/target/iscsi/iscsi_target_tq.c

@@ -189,16 +189,20 @@ static void iscsi_deallocate_extra_thread_sets(void)
 
 		spin_lock_bh(&ts->ts_state_lock);
 		ts->status = ISCSI_THREAD_SET_DIE;
-		spin_unlock_bh(&ts->ts_state_lock);
 
 		if (ts->rx_thread) {
-			send_sig(SIGINT, ts->rx_thread, 1);
+			complete(&ts->rx_start_comp);
+			spin_unlock_bh(&ts->ts_state_lock);
 			kthread_stop(ts->rx_thread);
+			spin_lock_bh(&ts->ts_state_lock);
 		}
 		if (ts->tx_thread) {
-			send_sig(SIGINT, ts->tx_thread, 1);
+			complete(&ts->tx_start_comp);
+			spin_unlock_bh(&ts->ts_state_lock);
 			kthread_stop(ts->tx_thread);
+			spin_lock_bh(&ts->ts_state_lock);
 		}
+		spin_unlock_bh(&ts->ts_state_lock);
 		/*
 		 * Release this thread_id in the thread_set_bitmap
 		 */
@@ -400,7 +404,8 @@ static void iscsi_check_to_add_additional_sets(void)
 static int iscsi_signal_thread_pre_handler(struct iscsi_thread_set *ts)
 {
 	spin_lock_bh(&ts->ts_state_lock);
-	if ((ts->status == ISCSI_THREAD_SET_DIE) || signal_pending(current)) {
+	if (ts->status == ISCSI_THREAD_SET_DIE || kthread_should_stop() ||
+	    signal_pending(current)) {
 		spin_unlock_bh(&ts->ts_state_lock);
 		return -1;
 	}
@@ -419,6 +424,7 @@ struct iscsi_conn *iscsi_rx_thread_pre_handler(struct iscsi_thread_set *ts)
 		goto sleep;
 	}
 
+	if (ts->status != ISCSI_THREAD_SET_DIE)
 		flush_signals(current);
 
 	if (ts->delay_inactive && (--ts->thread_count == 0)) {
@@ -472,6 +478,7 @@ struct iscsi_conn *iscsi_tx_thread_pre_handler(struct iscsi_thread_set *ts)
 		goto sleep;
 	}
 
+	if (ts->status != ISCSI_THREAD_SET_DIE)
 		flush_signals(current);
 
 	if (ts->delay_inactive && (--ts->thread_count == 0)) {