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Re: [Xen-devel] [PATCH V2 2/4] Introduce xen-scsifront module

On 08/01/2014 02:08 PM, Christoph Hellwig wrote:
On Wed, Jul 30, 2014 at 06:53:59AM +0200, Juergen Gross wrote:
Hmm, I looked into scsi_add_device(). It seems as if the caller can't
distinguish between a new created and an already existing device. Am I
missing something?

That's right.  If you need that I still think it's better to add a
variant of scsi_add_device helping you with that.

I'm open to that solution.

Do you have preferences how to do it (IOW: can you give me a hint)?

The race is not existing: scsi_add_device() (and scsi_remove_device()
as well) for this scsi_host is called in scsifront_do_lun_hotplug()
only, and this function is always called in the same thread (xenbus
watch). A comment seems to be a good idea.

Do you disable scanning through procfs and sysfs as well?

No, I don't.

OTOH I don't think I see the problem. What could go wrong? Either
scsi_device_lookup() does find an existing device, then I refuse to add
it again. If I don't find it, it will be added. I can't see how any harm
would be done in either case when the device is added/removed between
the check and the action. What am I missing?


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