linux-kernel - Re: [PATCH for v3.19, v2] Avoid that sd_shutdown() triggers a kernel warning

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Date:	Wed, 14 Jan 2015 01:33:02 -0800
From:	Christoph Hellwig <hch@...radead.org>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	Christoph Hellwig <hch@...radead.org>,
	Bart Van Assche <bvanassche@....org>,
	James Bottomley <jbottomley@...allels.com>,
	Hannes Reinecke <hare@...e.de>,
	"linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH for v3.19, v2] Avoid that sd_shutdown() triggers a kernel
 warning

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.
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