[3.19.y-ckt stable] Patch "scsi: fix scsi_error_handler vs. scsi_host_dev_release race" has been added to staging queue

[Kamal Mostafa]

This is a note to let you know that I have just added a patch titled

    scsi: fix scsi_error_handler vs. scsi_host_dev_release race

to the linux-3.19.y-queue branch of the 3.19.y-ckt extended stable tree 
which can be found at:

    http://kernel.ubuntu.com/git/ubuntu/linux.git/log/?h=linux-3.19.y-queue

This patch is scheduled to be released in version 3.19.8-ckt8.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.19.y-ckt tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Kamal

------

>From 27a09678cde0a340172e8fded0515b36f76494c6 Mon Sep 17 00:00:00 2001
From: Michal Hocko <mhocko at suse.com>
Date: Thu, 27 Aug 2015 20:16:37 +0200
Subject: scsi: fix scsi_error_handler vs. scsi_host_dev_release race

commit 537b604c8b3aa8b96fe35f87dd085816552e294c upstream.

b9d5c6b7ef57 ("[SCSI] cleanup setting task state in
scsi_error_handler()") has introduced a race between scsi_error_handler
and scsi_host_dev_release resulting in the hang when the device goes
away because scsi_error_handler might miss a wake up:

CPU0					CPU1
scsi_error_handler			scsi_host_dev_release
  					  kthread_stop()
  kthread_should_stop()
    test_bit(KTHREAD_SHOULD_STOP)
					    set_bit(KTHREAD_SHOULD_STOP)
					    wake_up_process()
					    wait_for_completion()

  set_current_state(TASK_INTERRUPTIBLE)
  schedule()

The most straightforward solution seems to be to invert the ordering of
the set_current_state and kthread_should_stop.

The issue has been noticed during reboot test on a 3.0 based kernel but
the current code seems to be affected in the same way.

[jejb: additional comment added]
Reported-and-debugged-by: Mike Mayer <Mike.Meyer at teradata.com>
Signed-off-by: Michal Hocko <mhocko at suse.com>
Reviewed-by: Dan Williams <dan.j.williams at intel.com>
Reviewed-by: Hannes Reinecke <hare at suse.de>
Signed-off-by: James Bottomley <JBottomley at Odin.com>
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
---
 drivers/scsi/scsi_error.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/drivers/scsi/scsi_error.c b/drivers/scsi/scsi_error.c
index 9e3d16f..7f55278 100644
--- a/drivers/scsi/scsi_error.c
+++ b/drivers/scsi/scsi_error.c
@@ -2177,8 +2177,17 @@ int scsi_error_handler(void *data)
 	 * We never actually get interrupted because kthread_run
 	 * disables signal delivery for the created thread.
 	 */
-	while (!kthread_should_stop()) {
+	while (true) {
+		/*
+		 * The sequence in kthread_stop() sets the stop flag first
+		 * then wakes the process.  To avoid missed wakeups, the task
+		 * should always be in a non running state before the stop
+		 * flag is checked
+		 */
 		set_current_state(TASK_INTERRUPTIBLE);
+		if (kthread_should_stop())
+			break;
+
 		if ((shost->host_failed == 0 && shost->host_eh_scheduled == 0) ||
 		    shost->host_failed != atomic_read(&shost->host_busy)) {
 			SCSI_LOG_ERROR_RECOVERY(1,
--
1.9.1