[PATCH] mtd: use refcount to prevent corruption

[Winkler, Tomas]

> 
> ----- Ursprüngliche Mail -----
> >> > When underlying device is removed mtd core will crash in case user
> >> > space is still holding an open handle to a mtd device node.
> >> > A proper refcounting is needed so device is release only when a
> >> > partition has no active users. The current simple counter is not
> >> > sufficient.
> >>
> >> Can you please explain a little more what devices are involved?
> >> Does it implement _get_device() and _put_device()?
> > No this is not connected to those handlers of the underlying device
> > and those won't help.
> > I have a spi device provided by MFD framework so it can go away anytime.
> 
> Can it go away physically or just in software?
Software, but since this is mfd it's basically hotplug. The kernel is crashing when I simulate hardware failure.
> 
> Usually the pattern is that you make sure in the device driver that nobody can
> orphan the MTD while it is in use.
> e.g. drivers/mtd/ubi/gluebi.c does so. In _get_device() it grabs a reference on
> the underlying UBI volume to make sure it cannot go away while the MTD (on
> top of UBI) is in use.

I can try that if it helps, because we are simulating possible lower level crash. 
In an case I believe that the proper refcouting is much more robust solution, than the current one.
I'd appreciate if someone can review the actual implementation. 

Thanks
Tomas