On Sun, 2013-12-15 at 16:32 -0500, Alan Stern wrote:
> On Sat, 14 Dec 2013, James Bottomley wrote:
>
> > > The refcount test and state change race with scsi_alloc_target().
> > > Maybe the race won't occur in practice, but to be safe you should hold
> > > shost->host_lock throughout that time interval, as the original code
> > > here does.
> >
> > You mean the fact that using a state model to indicate whether we should
> > destroy a target without bothering to refcount isn't robust against two
> > threads of execution running through a scan on the same target?
>
> I meant that the patch you posted suffers from a race when one thread
> is adding a device to a target and another thread is removing an
> existing device below that target at the same time. Suppose the
> target's reap_ref count is initially equal to 1:
>
> Thread 0 Thread 1
> -------- --------
> In scsi_alloc_target(): In scsi_target_reap():
> lock host_lock scsi_target_reap_ref_put():
> find existing starget starget->reap_ref drops to 0
> incr starget->reap_ref In scsi_target_reap_ref_release():
> unlock host_lock device_del(&starget->dev);
> starget->state == STARGET_DEL?
> No => okay to use starget set starget->state = STARGET_DEL
>
> Result: We end up using starget _and_ removing it. The only way to
> avoid this race would be to guarantee that we never add and remove
> devices below the same target at the same time. In theory this is
> feasible, but I don't know if you want to do it.
>
> This doesn't seem to be what you are talking about above. In any case,
> it is a bug.
No, I was thinking of the two thread scan bug (i.e. two scan threads)
not one scan and one remove, which is a bug in the old code. This is a
race between put and get when the kref is incremented from zero (an
illegal operation which triggers a warn on).
The way to mediate this is to check for the kref already being zero
condition, like below.
James
---
diff --git a/drivers/scsi/scsi_scan.c b/drivers/scsi/scsi_scan.c
index 327c0e92..303d471 100644
--- a/drivers/scsi/scsi_scan.c
+++ b/drivers/scsi/scsi_scan.c
@@ -473,7 +473,13 @@ static struct scsi_target *scsi_alloc_target(struct device *parent,
return starget;
found:
- kref_get(&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);
if (found_target->state != STARGET_DEL) {
put_device(dev);
--
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