linux-kernel - [PATCH 3.12 018/123] scsi: fix scsi_error_handler vs. scsi_host_dev

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Message-Id: <ee2101730dae3748e94a3032091c8ecb0077e49e.1446021555.git.jslaby@suse.cz>
Date:	Wed, 28 Oct 2015 14:50:35 +0100
From:	Jiri Slaby <jslaby@...e.cz>
To:	stable@...r.kernel.org
Cc:	linux-kernel@...r.kernel.org, Michal Hocko <mhocko@...e.com>,
	James Bottomley <JBottomley@...n.com>,
	Jiri Slaby <jslaby@...e.cz>
Subject: [PATCH 3.12 018/123] scsi: fix scsi_error_handler vs. scsi_host_dev_release race

From: Michal Hocko <mhocko@...e.com>

3.12-stable review patch.  If anyone has any objections, please let me know.

===============

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@...adata.com>
Signed-off-by: Michal Hocko <mhocko@...e.com>
Reviewed-by: Dan Williams <dan.j.williams@...el.com>
Reviewed-by: Hannes Reinecke <hare@...e.de>
Signed-off-by: James Bottomley <JBottomley@...n.com>
Signed-off-by: Jiri Slaby <jslaby@...e.cz>
---
 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 066e3198838d..ff2689d01209 100644
--- a/drivers/scsi/scsi_error.c
+++ b/drivers/scsi/scsi_error.c
@@ -1906,8 +1906,17 @@ 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 != shost->host_busy) {
 			SCSI_LOG_ERROR_RECOVERY(1,
-- 
2.6.2

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