iscsi-target: Fix iscsi_np reset hung task during parallel delete

This patch fixes a bug associated with iscsit_reset_np_thread()
that can occur during parallel configfs rmdir of a single iscsi_np
used across multiple iscsi-target instances, that would result in
hung task(s) similar to below where configfs rmdir process context
was blocked indefinately waiting for iscsi_np->np_restart_comp
to finish:

[ 6726.112076] INFO: task dcp_proxy_node_:15550 blocked for more than 120 seconds.
[ 6726.119440]       Tainted: G        W  O     4.1.26-3321 #2
[ 6726.125045] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
[ 6726.132927] dcp_proxy_node_ D ffff8803f202bc88     0 15550      1 0x00000000
[ 6726.140058]  ffff8803f202bc88 ffff88085c64d960 ffff88083b3b1ad0 ffff88087fffeb08
[ 6726.147593]  ffff8803f202c000 7fffffffffffffff ffff88083f459c28 ffff88083b3b1ad0
[ 6726.155132]  ffff88035373c100 ffff8803f202bca8 ffffffff8168ced2 ffff8803f202bcb8
[ 6726.162667] Call Trace:
[ 6726.165150]  [<ffffffff8168ced2>] schedule+0x32/0x80
[ 6726.170156]  [<ffffffff8168f5b4>] schedule_timeout+0x214/0x290
[ 6726.176030]  [<ffffffff810caef2>] ? __send_signal+0x52/0x4a0
[ 6726.181728]  [<ffffffff8168d7d6>] wait_for_completion+0x96/0x100
[ 6726.187774]  [<ffffffff810e7c80>] ? wake_up_state+0x10/0x10
[ 6726.193395]  [<ffffffffa035d6e2>] iscsit_reset_np_thread+0x62/0xe0 [iscsi_target_mod]
[ 6726.201278]  [<ffffffffa0355d86>] iscsit_tpg_disable_portal_group+0x96/0x190 [iscsi_target_mod]
[ 6726.210033]  [<ffffffffa0363f7f>] lio_target_tpg_store_enable+0x4f/0xc0 [iscsi_target_mod]
[ 6726.218351]  [<ffffffff81260c5a>] configfs_write_file+0xaa/0x110
[ 6726.224392]  [<ffffffff811ea364>] vfs_write+0xa4/0x1b0
[ 6726.229576]  [<ffffffff811eb111>] SyS_write+0x41/0xb0
[ 6726.234659]  [<ffffffff8169042e>] system_call_fastpath+0x12/0x71

It would happen because each iscsit_reset_np_thread() sets state
to ISCSI_NP_THREAD_RESET, sends SIGINT, and then blocks waiting
for completion on iscsi_np->np_restart_comp.

However, if iscsi_np was active processing a login request and
more than a single iscsit_reset_np_thread() caller to the same
iscsi_np was blocked on iscsi_np->np_restart_comp, iscsi_np
kthread process context in __iscsi_target_login_thread() would
flush pending signals and only perform a single completion of
np->np_restart_comp before going back to sleep within transport
specific iscsit_transport->iscsi_accept_np code.

To address this bug, add a iscsi_np->np_reset_count and update
__iscsi_target_login_thread() to keep completing np->np_restart_comp
until ->np_reset_count has reached zero.
Reported-by: Gary Guo <ghg@datera.io>
Tested-by: Gary Guo <ghg@datera.io>
Cc: Mike Christie <mchristi@redhat.com>
Cc: Hannes Reinecke <hare@suse.de>
Cc: stable@vger.kernel.org # 3.10+
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>

drivers/target/iscsi/iscsi_target.c

@@ -418,6 +418,7 @@ int iscsit_reset_np_thread(
 		return 0;
 	}
 	np->np_thread_state = ISCSI_NP_THREAD_RESET;
+	atomic_inc(&np->np_reset_count);
 
 	if (np->np_thread) {
 		spin_unlock_bh(&np->np_thread_lock);

drivers/target/iscsi/iscsi_target_login.c

@@ -1243,9 +1243,11 @@ static int __iscsi_target_login_thread(struct iscsi_np *np)
 	flush_signals(current);
 
 	spin_lock_bh(&np->np_thread_lock);
-	if (np->np_thread_state == ISCSI_NP_THREAD_RESET) {
+	if (atomic_dec_if_positive(&np->np_reset_count) >= 0) {
 		np->np_thread_state = ISCSI_NP_THREAD_ACTIVE;
+		spin_unlock_bh(&np->np_thread_lock);
 		complete(&np->np_restart_comp);
+		return 1;
 	} else if (np->np_thread_state == ISCSI_NP_THREAD_SHUTDOWN) {
 		spin_unlock_bh(&np->np_thread_lock);
 		goto exit;
@@ -1278,7 +1280,8 @@ static int __iscsi_target_login_thread(struct iscsi_np *np)
 		goto exit;
 	} else if (rc < 0) {
 		spin_lock_bh(&np->np_thread_lock);
-		if (np->np_thread_state == ISCSI_NP_THREAD_RESET) {
+		if (atomic_dec_if_positive(&np->np_reset_count) >= 0) {
+			np->np_thread_state = ISCSI_NP_THREAD_ACTIVE;
 			spin_unlock_bh(&np->np_thread_lock);
 			complete(&np->np_restart_comp);
 			iscsit_put_transport(conn->conn_transport);

include/target/iscsi/iscsi_target_core.h

@@ -786,6 +786,7 @@ struct iscsi_np {
 	int			np_sock_type;
 	enum np_thread_state_table np_thread_state;
 	bool                    enabled;
+	atomic_t		np_reset_count;
 	enum iscsi_timer_flags_table np_login_timer_flags;
 	u32			np_exports;
 	enum np_flags_table	np_flags;