[PATCH] CFQ io scheduler: scheduler switch oops

If cfq is managing a queue and a new scheduler is later selected, it is
possible for the cfqd unplug_work work to be queued after the kblockd
work struct has been flushed.  The problem is the ordering of
cfq_shutdown_timer_wq() and blk_put_queue() in cfq_put_cfqd().  The
latter may rearm the work, leaving cfq_kick_queue() with dead data.
Signed-off-by: Jens Axboe <axboe@suse.de>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

drivers/block/cfq-iosched.c

@@ -2249,10 +2249,11 @@ static void cfq_put_cfqd(struct cfq_data *cfqd)
 	if (!atomic_dec_and_test(&cfqd->ref))
 		return;
 
-	cfq_shutdown_timer_wq(cfqd);
-
 	blk_put_queue(q);
 
+	cfq_shutdown_timer_wq(cfqd);
+	q->elevator->elevator_data = NULL;
+
 	mempool_destroy(cfqd->crq_pool);
 	kfree(cfqd->crq_hash);
 	kfree(cfqd->cfq_hash);