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

[Douglas Gilbert]

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


--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/