scsi: fix scsi_error_handler vs. scsi_host_dev_release race

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]
Cc: <stable@vger.kernel.org> # 3.6+
Reported-and-debugged-by: Mike Mayer <Mike.Meyer@teradata.com>
Signed-off-by: Michal Hocko <mhocko@suse.com>
Reviewed-by: Dan Williams <dan.j.williams@intel.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: James Bottomley <JBottomley@Odin.com>

1 files changed, 10 insertions, 1 deletions

diff --git a/drivers/scsi/scsi_error.c b/drivers/scsi/scsi_error.c
index 410911c31c67..b5bbc122c414 100644
--- a/drivers/scsi/scsi_error.c
+++ b/drivers/scsi/scsi_error.c
@@ -2179,8 +2179,17 @@ 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,