On Wed, 14 Jan 2015, Christoph Hellwig wrote:
> On Mon, Jan 12, 2015 at 11:29:15AM -0500, Alan Stern wrote:
> > This seems like a good idea and the obvious (once it has been pointed
> > out!) approach.
> >
> > Perhaps not directly related to the issue at hand is this question: In
> > scsi_rescan_device() we will now have:
> >
> > mutex_lock(&shost->scan_mutex);
> > if (dev->driver && try_module_get(dev->driver->owner)) {
> > struct scsi_driver *drv = to_scsi_driver(dev->driver);
> >
> > if (drv->rescan)
> > drv->rescan(dev);
> > module_put(dev->driver->owner);
> > }
> > mutex_unlock(&shost->scan_mutex);
> >
> > What prevents the device from being unbound from its driver while the
> > rescan runs? Evaluating the argument to the module_put() would then
> > dereference a NULL pointer.
> >
> > Unbind events that happen through the normal scsi_remove_host()
> > mechanism are fine, because scsi_remove_host() locks the scan_mutex.
> > But what about writes to the driver's sysfs "unbind" attribute?
>
> Looks like we should still get an unconditional reference to
> the device using get_device in scsi_rescan_device at least.
>
> But this seems like a more generic problem, and at least a quick glance at
> the pci_driver methods seems like others don't have a good
> synchroniation of ->remove against random driver methods.
This particular problem comes down to the fact that
scsi_rescan_device() accesses dev->driver without appropriate mutual
exclusion. SCSI's scan_mutex won't help because it doesn't protect
dev->driver. Rather, dev->driver is protected by dev->mutex, and so
scsi_rescan_device() needs to use device_lock/unlock.
This suggests that the scan_mutex may not be necessary at all.
Historically, it seems to be quite old, predating the device model.
Now that we have the device model, maybe scan_mutex simply isn't
needed.
Scanning for channels or targets beneath a host should be protected by
shost->gendev.mutex. Scanning for logical units beneath a target
should be protected by starget->dev.mutex. Scanning for partitions
beneath a SCSI drive should be protected by sdev->sdev_gendev.mutex.
James, here's a related question. Suppose userspace writes to the
rescan attribute file for a disk drive for sd_probe_async() has
started. What will happen? What _ought_ to happen? Do we need to
call
async_synchronize_full_domain(&scsi_sd_probe_domain);
somewhere in this pathway, or will it be okay?
Alan Stern
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