On Tue, 2009-05-26 at 14:34 -0400, Alan Stern wrote:
> 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. :-)
Details?... In theory the sd driver can be attached at any time and
nothing should be relying on it being there when the inquiry scan
finishes, so if there's a bug it would be exposed by async scanning, not
really caused by it.
> > > 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();
No, scsi_complete_async_scans() here. There should be an
async_synchronize_full() in sd_remove.
> +
> restart:
> 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
> registered.
None of this really got reviewed through the SCSI list, so I'll let
Arjan answer.
> 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?
This one looks to be a mismerge between the async tree and the SCSI
tree.
James
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