Patch "scsi: fix scsi_error_handler vs. scsi_host_dev_release race" has been added to the 4.2-stable tree

This is a note to let you know that I've just added the patch titled

scsi: fix scsi_error_handler vs. scsi_host_dev_release race

to the 4.2-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
scsi-fix-scsi_error_handler-vs.-scsi_host_dev_release-race.patch
and it can be found in the queue-4.2 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Michal Hocko <[email protected]>
Date: Thu, 27 Aug 2015 20:16:37 +0200
Subject: scsi: fix scsi_error_handler vs. scsi_host_dev_release race

From: Michal Hocko <[email protected]>

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.

---
 drivers/scsi/scsi_error.c |   11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

@@ -2169,8 +2169,17 @@ 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,

Patches currently in stable-queue which might be from [email protected] are

queue-4.2/scsi-fix-scsi_error_handler-vs.-scsi_host_dev_release-race.patch