suspect UBIFS async operations causing issues during reboot

[Brian Norris]

On Sun, Nov 09, 2014 at 09:10:03PM -0800, Scott Branden wrote:
> On 14-11-09 02:20 AM, Richard Weinberger wrote:
> >Well, I agree with David that anything we do in software will only hide the real problem
> >or trim down the window.
> Hi Richard,
> 
> Currently the NAND does not shut down in a clean manner for a reboot
> operation.  This is due to the asynchronous ubi_thread make flash
> erase calls.  unmount is done properly in ubi already and cleanly
> shuts down.  reboot is not done in a clean manner as there is no
> reboot_notifier to handle the situation.
> 
> This is not hiding a real problem.  It is just shutting down ubi
> properly rather than pulling the power from it in the middle of
> operations.
> 
> In addition to this - a reboot_notifier needs to be added at the mtd
> level to shut it down properly as well.
> 
> This is not trimming down a window.  It is having the drivers shut
> down properly so they do not look like a power failure to the NAND
> device.
> 
> There is no solution to the power failure - it will corrupt pages in
> the middle of erasure.  And you do handle this in UBI/UBIFS.  But
> why corrupt other erase pages unnecessarily when all that needs to
> be done is shut down the drivers properly.  I don't know what you
> are agreeing with David with?  It is not making a window smaller.
> It is changing the functionality so that the UBI and MTD drivers are
> shut down cleanly in reboot situations.  Right now, they are not
> shut down at all in these situations.

I agree with Scott's statements. While it's fine to talk about how all
layers (from bootloader to UBIFS) should be able to handle a power cut
in the midst of an erase, that does *not* mean that we should
intentionally deny the chance to shut down cleanly.

AFAICT, Scott's not trying to work around any unsound reset behaviors
(in UBIFS or in his bootloader); he's just trying to shut things down
gracefully, just as we would try to terminate processes, sync file
systems, etc., rather than just cutting power on reboot.

Brian