errors during umount make SysRq fail

[Carl-Daniel Hailfinger <c-d.hailfinger.kernel.2004@gmx.net>]

Hi,

having removed an USB disk while umount for it was still running (yes,
that was stupid) I noticed that umount for this device hangs forever in
D state. That would be ok (consequences for user error), however *all*
other umounts I attempt also hang in D state and SysRq-U also hangs,
resulting in a broken system on the next reboot.

I assume the locking against concurrent umount is there to protect
against non-trivial namespace problems and makes sense for normal
umounting, but IIRC SysRq-U is there to ensure consistent filesystems
on the next startup. Would it make sense to allow SysRq-U to break
these locks?

Similar problem exists with SysRq-S. If syncing of one device hangs,
it will never proceed to the next one in the list. I agree that one
is not trivial (stacked devices, loop et al), but can't we make a
best effort to sync at least the physical devices in the machine?
Please don't shoot me for talking about physical devices, I know
there are some really grey areas trying to define that.

Regards,
Carl-Daniel
-- 
http://www.hailfinger.org/