On Fri, 2014-01-03 at 10:58 -0500, Alan Stern wrote:
> On Thu, 2 Jan 2014, James Bottomley wrote:
>
> > In the highly unusual case where two threads are running concurrently
> > through
> > the scanning code scanning the same target, we run into the situation where
> > one may allocate the target while the other is still using it. In this
> > case,
> > because the reap checks for STARGET_CREATED and kills the target without
> > reference counting, the second thread will do the wrong thing on reap.
> >
> > Fix this by reference counting even creates and doing the STARGET_CREATED
> > check in the final put.
>
> I'm still concerned about one thing. The previous patch does this in
> scsi_alloc_target():
>
> > found:
> > - found_target->reap_ref++;
> > + if (!kref_get_unless_zero(&found_target->reap_ref))
> > + /*
> > + * release routine already fired. Target is dead, but
> > + * STARGET_DEL may not yet be set (set in the release
> > + * routine), so set here as well, just in case
> > + */
> > + found_target->state = STARGET_DEL;
> > spin_unlock_irqrestore(shost->host_lock, flags);
>
> As a result, the two comments in this patch aren't right:
>
> > @@ -384,9 +385,15 @@ static void scsi_target_reap_ref_release(struct kref
> > *kref)
> > struct scsi_target *starget
> > = container_of(kref, struct scsi_target, reap_ref);
> >
> > - transport_remove_device(&starget->dev);
> > - device_del(&starget->dev);
> > - starget->state = STARGET_DEL;
> > + /*
> > + * if we get here and the target is still in the CREATED state that
> > + * means it was allocated but never made visible (because a scan
> > + * turned up no LUNs), so don't call device_del() on it.
> > + */
> > + if (starget->state == STARGET_RUNNING) {
> > + transport_remove_device(&starget->dev);
> > + device_del(&starget->dev);
> > + }
>
> Here the state could already be STARGET_DEL, even though the target is
> still visible.
Well, I agree with the theory. In practise, there are only a few
machine instructions between the kref going to zero and us reaching that
point, because kref_release will jump into this routine next, so the
condition would be very hard to see. However, I suppose it's easy to
close by checking for != STARGET_CREATED and there's no reason not to do
that, so I'll change it.
> Also, it's a little odd that the comment talks about CREATED but the
> code really checks for RUNNING. They should be consistent.
!= STARGET_CREATED should solve this.
> > @@ -506,11 +513,13 @@ static struct scsi_target *scsi_alloc_target(struct
> > device *parent,
> > */
> > void scsi_target_reap(struct scsi_target *starget)
> > {
> > + /*
> > + * serious problem if this triggers: STARGET_DEL is only set in the
> > + * kref release routine, so we're doing another final put on an
> > + * already released kref
> > + */
> > BUG_ON(starget->state == STARGET_DEL);
>
> Here the code is okay but the comment is wrong: STARGET_DEL is set in
> _two_ places (but neither of them runs until reap_ref has reached 0).
>
> Would it be better in scsi_alloc_target() to behave as though the state
> were STARGET_DEL without actually setting it?
Yes, I'll update the comment to it only goes to DEL after the kref goes
to zero.
How does the attached diff look?
James
---
diff --git a/drivers/scsi/scsi_scan.c b/drivers/scsi/scsi_scan.c
index 82cf902..2f7de33 100644
--- a/drivers/scsi/scsi_scan.c
+++ b/drivers/scsi/scsi_scan.c
@@ -390,7 +390,7 @@ static void scsi_target_reap_ref_release(struct kref *kref)
* means it was allocated but never made visible (because a scan
* turned up no LUNs), so don't call device_del() on it.
*/
- if (starget->state == STARGET_RUNNING) {
+ if (starget->state != STARGET_CREATED) {
transport_remove_device(&starget->dev);
device_del(&starget->dev);
}
@@ -514,9 +514,9 @@ static struct scsi_target *scsi_alloc_target(struct device
*parent,
void scsi_target_reap(struct scsi_target *starget)
{
/*
- * serious problem if this triggers: STARGET_DEL is only set in the
- * kref release routine, so we're doing another final put on an
- * already released kref
+ * serious problem if this triggers: STARGET_DEL is only set in the if
+ * the reap_ref drops to zero, so we're trying to do another final put
+ * on an already released kref
*/
BUG_ON(starget->state == STARGET_DEL);
scsi_target_reap_ref_put(starget);
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