Re: [PATCH] scsi_debug: deadlock between completions and surprise module removal

From: Douglas Gilbert
Date: Mon Sep 01 2014 - 15:52:25 EST

On 14-09-01 11:36 AM, Christoph Hellwig wrote:
--- a/drivers/scsi/scsi_debug.c 2014-08-26 13:24:51.646948507 -0400
+++ b/drivers/scsi/scsi_debug.c 2014-08-30 18:04:54.589226679 -0400
@@ -2743,6 +2743,13 @@ static int stop_queued_cmnd(struct scsi_
if (test_bit(k, queued_in_use_bm)) {
sqcp = &queued_arr[k];
if (cmnd == sqcp->a_cmnd) {
+ devip = (struct sdebug_dev_info *)
+ cmnd->device->hostdata;
+ if (devip)
+ atomic_dec(&devip->num_in_q);
+ sqcp->a_cmnd = NULL;

Why would the hostdata every be NULL here? We should never
call ->slave_destroy on a device that has outstanding commands.

To your first question, perhaps it could not be. In
the scsi_debug driver almost all uses of 'devip'
check for NULL, so it may not always have been true.

'rmmod scsi_debug' would lead to scsi_debug_exit()
being called and that called stop_all_queued() which
deadlocked with a command completion, or worse command
commencement. scsi_debug_exit() looks a bit racy: the
first thing it does is stop_all_queued() but has
anything been done to stop new commands being issued?
Later scsi_debug_exit() brings down the hosts.

This patch is primarily a bug fix for the lk 3.17 series and
the code you highlight was simply moved from being under a
lock to outside that lock. I didn't want to be too creative,
it's too easy to slip up and upset the management.

Enhancements could be sent to your drivers-for-3.18 tree. Rob
Elliott already has a few in mind, to improve performance.
Removing all 'devip' NULL checks would also improve performance,
and readability.

Doug Gilbert

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