linux-kernel - [PATCH 2/2] target: Fix target_wait_for_sess

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Date:   Wed, 10 Oct 2018 03:23:10 +0000
From:   "Nicholas A. Bellinger" <nab@...ux-iscsi.org>
To:     target-devel <target-devel@...r.kernel.org>
Cc:     linux-scsi <linux-scsi@...r.kernel.org>,
        lkml <linux-kernel@...r.kernel.org>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        Mike Christie <mchristi@...hat.com>,
        Hannes Reinecke <hare@...e.com>,
        Christoph Hellwig <hch@....de>,
        Sagi Grimberg <sagi@...mberg.me>,
        "Bryant G. Ly" <bryantly@...ux.vnet.ibm.com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Nicholas Bellinger <nab@...ux-iscsi.org>,
        Bart Van Assche <bvanassche@....org>
Subject: [PATCH 2/2] target: Fix target_wait_for_sess_cmds breakage with active signals

From: Nicholas Bellinger <nab@...ux-iscsi.org>

With the addition of commit 00d909a107 in v4.19-rc, it incorrectly assumes no
signals will be pending for task_struct executing the normal session shutdown
and I/O quiesce code-path.

For example, iscsi-target and iser-target issue SIGINT to all kthreads as
part of session shutdown.  This has been the behaviour since day one.

As-is when signals are pending with se_cmds active in se_sess->sess_cmd_list,
wait_event_interruptible_lock_irq_timeout() returns a negative number and
immediately kills the machine because of the do while (ret <= 0) loop that
was added in commit 00d909a107 to spin while backend I/O is taking any
amount of extended time (say 30 seconds) to complete.

Here's what it looks like in action with debug plus delayed backend I/O
completion:

[ 4951.909951] se_sess: 000000003e7e08fa before target_wait_for_sess_cmds
[ 4951.914600] target_wait_for_sess_cmds: signal_pending: 1
[ 4951.918015] wait_event_interruptible_lock_irq_timeout ret: -512 signal_pending: 1 loop count: 0
[ 4951.921639] wait_event_interruptible_lock_irq_timeout ret: -512 signal_pending: 1 loop count: 1
[ 4951.921944] wait_event_interruptible_lock_irq_timeout ret: -512 signal_pending: 1 loop count: 2
[ 4951.921944] wait_event_interruptible_lock_irq_timeout ret: -512 signal_pending: 1 loop count: 3
[ 4951.921944] wait_event_interruptible_lock_irq_timeout ret: -512 signal_pending: 1 loop count: 4
[ 4951.921944] wait_event_interruptible_lock_irq_timeout ret: -512 signal_pending: 1 loop count: 5
[ 4951.921944] wait_event_interruptible_lock_irq_timeout ret: -512 signal_pending: 1 loop count: 6
[ 4951.921944] wait_event_interruptible_lock_irq_timeout ret: -512 signal_pending: 1 loop count: 7
[ 4951.921944] wait_event_interruptible_lock_irq_timeout ret: -512 signal_pending: 1 loop count: 8
[ 4951.921944] wait_event_interruptible_lock_irq_timeout ret: -512 signal_pending: 1 loop count: 9

... followed by the usual RCU CPU stalls and deadlock.

There was never a case pre commit 00d909a107 where wait_for_complete(&se_cmd->cmd_wait_comp)
was able to be interruptted, so to address this for v4.19+ moving forward go ahead and
use wait_event_lock_irq_timeout() instead so new code works with all fabric drivers.

Also for commit 00d909a107, fix a minor regression in target_release_cmd_kref()
to only wake_up the new se_sess->cmd_list_wq only when shutdown has actually
been triggered via se_sess->sess_tearing_down.

Fixes: 00d909a107 ("scsi: target: Make the session shutdown code also wait for commands that are being aborted")
Cc: Bart Van Assche <bvanassche@....org>
Cc: Mike Christie <mchristi@...hat.com>
Cc: Hannes Reinecke <hare@...e.com>
Cc: Christoph Hellwig <hch@....de>
Cc: Sagi Grimberg <sagi@...mberg.me>
Cc: Bryant G. Ly <bryantly@...ux.vnet.ibm.com>
Tested-by: Nicholas Bellinger <nab@...ux-iscsi.org>
Signed-off-by: Nicholas Bellinger <nab@...ux-iscsi.org>
---
 drivers/target/target_core_transport.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/target/target_core_transport.c b/drivers/target/target_core_transport.c
index 86c0156..fc3093d2 100644
--- a/drivers/target/target_core_transport.c
+++ b/drivers/target/target_core_transport.c
@@ -2754,7 +2754,7 @@ static void target_release_cmd_kref(struct kref *kref)
 	if (se_sess) {
 		spin_lock_irqsave(&se_sess->sess_cmd_lock, flags);
 		list_del_init(&se_cmd->se_cmd_list);
-		if (list_empty(&se_sess->sess_cmd_list))
+		if (se_sess->sess_tearing_down && list_empty(&se_sess->sess_cmd_list))
 			wake_up(&se_sess->cmd_list_wq);
 		spin_unlock_irqrestore(&se_sess->sess_cmd_lock, flags);
 	}
@@ -2907,7 +2907,7 @@ void target_wait_for_sess_cmds(struct se_session *se_sess)
 
 	spin_lock_irq(&se_sess->sess_cmd_lock);
 	do {
-		ret = wait_event_interruptible_lock_irq_timeout(
+		ret = wait_event_lock_irq_timeout(
 				se_sess->cmd_list_wq,
 				list_empty(&se_sess->sess_cmd_list),
 				se_sess->sess_cmd_lock, 180 * HZ);
-- 
1.9.1