Re: [PATCH 14/15] libnvdimm: support read-only btt backing devices

[Christoph Hellwig]

On Wed, Jun 17, 2015 at 07:56:02PM -0400, Dan Williams wrote:
> Upon detection of a read-only backing device arrange for the btt to
> device to be read only.  Implement a catch for the BLKROSET ioctl and
> only allow a btt-instance to become read-write when the backing-device
> becomes read-write.  Conversely, if a backing-device becomes read-only
> arrange for its parent btt to be marked read-only.  Synchronize these
> changes under the bus lock.

Eww.  I have to say the deeper I look into this code the more I hate
the stacking nature of btt.  It seems more and more we should never
attach pmem if we want to use a device with btt. 
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