Re: Bug in SCSI async probing

From: Alan Stern
Date: Tue May 26 2009 - 14:35:02 EST

On Tue, 26 May 2009, James Bottomley wrote:

> On Tue, 2009-05-26 at 11:22 -0400, Alan Stern wrote:
> > James & Arjan:
> >
> > Am I missing something here? It looks like
> >
> > fastboot: make scsi probes asynchronous
> >
> > has introduced a bug in the sd probing code. AFAICT, there is now
> > nothing to prevent do_scan_async() from returning before
> > sd_probe_async() has run.
> True, but this isn't really a problem.

Why not? I'd say an oops is a problem. :-)

> > Doesn't this mean that there's nothing to prevent sd_remove() from
> > being called and trying to unregister the disk _before_
> > sd_probe_async() has managed to register it?
> Yes, we've been discussing this ... most of the removal functions now
> need async_synchronize calls to mitigate this type of race.

Such as this?

Index: usb-2.6/drivers/scsi/scsi_scan.c
--- usb-2.6.orig/drivers/scsi/scsi_scan.c
+++ usb-2.6/drivers/scsi/scsi_scan.c
@@ -1866,6 +1866,12 @@ void scsi_forget_host(struct Scsi_Host *
struct scsi_device *sdev;
unsigned long flags;

+ /*
+ * Don't try to get rid of this host's devices until all the async
+ * probing is finished.
+ */
+ async_synchronize_full();
spin_lock_irqsave(shost->host_lock, flags);
list_for_each_entry(sdev, &shost->__devices, siblings) {

(Which reminds me... Are the calls in wait_scan_init() really enough?
wait_for_device_probe() does async_synchronize_full() and then
scsi_complete_async_scans() finishes the SCSI scanning. But if this
scanning involves calling sd_probe(), then more async work will be
queued. Maybe a second call to wait_for_device_probe() is needed.)

There's still more; the patch above isn't sufficient. What happens if
the "device_add(&sdkp->dev)" call in sd_probe_async() fails? Then in
sd_remove(), sdkp will be NULL and &sdkp->dev will be meaningless. The
device_del() call will crash and the actual scsi_disk structure will be
leaked. This could be fixed by moving the dev_set_drvdata() call from
the end of sd_probe_async() back into sd_probe(), but then we'd find
sd_remove trying to unregister a device which was never successfully

And why is it that the "out_free_index:" code in sd_probe() acquires
sd_index_lock but the corresponding code in sd_probe_async() doesn't?

Alan Stern

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