System unresponsive during FS activity

[Dominic Fandrey]

I'm running stable/10 r260037:
FreeBSD mobileKamikaze.norad 10.0-PRERELEASE FreeBSD 10.0-PRERELEASE #6 r260037: Sun Dec 29 14:58:09 CET 2013     root at mobileKamikaze.norad:/usr/obj/HP6510b-10/amd64/usr/src/sys/HP6510b-10  amd64

While moving a large file from a nullfs mount to a UFS+SU mount
(the nullfs mount is on the same UFS+SU mount) the system becomes fully
unresponsive.

I.e.:
- X freezes and doesn't react:
  - Screen frozen
  - Mouse cursor does not move
  - No keyboard interactions
- CTLR-ALT-Fn into the console does not work
- Skype video calls freeze and break down

When the mv is completed the system is fully responsive again.

Regards

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