On Tue, Apr 12, 2011 at 06:49:53PM +0200, Jens Axboe wrote:

I realize that, in fact it's already safe as long as you pass in 'true'
for __blk_run_queue(). Before I had rewritten it to move the running
out, so that makes the trick a little difficult. This afternoon I also
tested it and saw no noticable difference, but I'll probably just do it
anyway as it makes sense.

We still need the lock for __elv_add_request, so we'll need to keep the
logic anyway.  But splitting out the just queue to kblockd case from
__blk_run_queue and giving the latter a sane prototype still sounds
like a good idea to me.

Btw, now that we don't call the request_fn directly any more and thus
can't block, can the unplugging be moved into the preempt notifiers?