In 2.6.15.1 I encountered some IDE crashes when unplugging IDE cables to
emulate disk errors.  Below is a patch against 2.6.16 which I think still
applies.

1.  The first BUG_ON could trigger when a PREFLUSH IO fails (it would
   fail the original barrier request which hasn't been marked REQ_STARTED
   yet).

2. the rq could have been dequeued already (same as 1).

3.  HWGROUP(drive)->rq could be NULL because of the ide_error() several
   lines earlier.
Signed-off-by: Hua Zhong <hzhong@gmail.com>
Cc: Bartlomiej Zolnierkiewicz <B.Zolnierkiewicz@elka.pw.edu.pl>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>