Re: Locking scheme of /proc/scsi/scsi

[Hannes Reinecke <hare@suse.de>]

On 11/21/2011 06:32 PM, Petr Tesarik wrote:
> Hi folks,
> 
> I've been working on a kernel crash dump of an ancient kernel recently, and I 
> have come to the conculsion that walking the scsi devices via 
> bus_find_device() is completely flawed. While looking for an upstream fix, I 
> didn't find any, so the same flaw is probably still there. However, let me ask 
> here to check how this is supposed to work.
> 
> First, this is how I understand the issue. The "/proc/scsi/scsi" file is 
> handled as a pretty standard seqfile, iterating over the devices with the 
> following function:
> 
> static inline struct device *next_scsi_device(struct device *start)
> {
> 	struct device *next = bus_find_device(&scsi_bus_type, start, NULL,
> 					      always_match);
> 	put_device(start);
> 	return next;
> }
> 
> The returned value is used for the next iteration. Now, bus_find_device() 
> assumes that the device is still attached to the knode_bus klist, because 
> that's how it initializes the klist iterator. When it finds the next device, 
> it increments the reference count on the device with get_device(), but it 
> doesn't do anything about the knode_bus field. So, when somebody calls 
> scsi_remove_device() on the current device between two calls to 
> next_scsi_device, then it does:
> 
> 	if (sdev->is_visible) {
> [...]
> 		device_del(dev);
> 
> which in turn calls:
> 
> 	bus_remove_device(dev);
> 
> which does:
> 
> 		if (klist_node_attached(&dev->p->knode_bus))
> 			klist_del(&dev->p->knode_bus);
> 
> So, even though the struct device has a non-zero refcount, the code in 
> next_scsi_device cannot continue, because it only has a stale pointer to an 
> already detached klist, right?
> 
> At least that's what I saw in 2.6.16, and I can still see the same thing 
> possible in 3.1.
> 
Hmm. Looks like we need to increase the refcount to knode_bus when
we iterate devices.
Let me check ...

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		      zSeries & Storage
hare@suse.de			      +49 911 74053 688
SUSE LINUX Products GmbH, Maxfeldstr. 5, 90409 Nürnberg
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