It still has no reply but I really hope someone will backport your fix soon.
Jens Axboe has determined that the proper fix is quite different: http://git.kernel.dk/cgit/linux-block/commit/?h=for-linus&id=85bd6e61f34dffa8ec2dc75ff3c02ee7b2f1cbceThis patch is already on its way to stable branches. I have tested it and confirmed that it resolves the problem.
(The line I originally found seems to actually be a mistake introduced in the master branch, but it somehow also prevents the deadlock).
Dragan