Re: [PATCH update] firewire: fix "kobject_add failed for fw* with -EEXIST"

[Stefan Richter <stefanr@s5r6.in-berlin.de>]

I wrote:
> But the interaction with userspace processes opening /dev/fwX while the
> respective node is being shut down gave me headaches.  I am still not
> entirely sure if I got it right in the patch update, i.e. if it is free
> from deadlocks.  fw_device_shutdown() and fw_device_op_open() can be
> entered at the same time.  Would device_unregister() have to acquire a
> driver core lock which open() already took?  If yes, device_unregister()
> would be blocked on this lock while fw_device_op_open() is blocked on
> idr_rwsem.
> 
> So why did I move device_unregister() into the idr_rwsem protected
> section in the first place?  That's because I wanted to guarantee that
> fw_device_op_open() wouldn't look up a fw_device which is just in the
> process of being unregistered.  But maybe we don't even need this guarantee.

Did some further thinking:  Yes, I have to guarantee that the pointer
which the lookup in the idr tree returned is valid.

But I don't have to shove device_unregister() into the idr_rwsem locked
section.  Instead, I can (and actually should) increment the device's
refcount when I stick the device pointer into the idr tree.  (And of
course I decrease the refcount again when I remove the pointer from the
idr tree.)

Will probably post another update tomorrow.
-- 
Stefan Richter
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http://arcgraph.de/sr/