block: lift the initial queue bypass mode on blk_register_queue() instead of blk_init_allocated_queue()

b82d4b197c ("blkcg: make request_queue bypassing on allocation") made
request_queues bypassed on allocation to avoid switching on and off
bypass mode on a queue being initialized.  Some drivers allocate and
then destroy a lot of queues without fully initializing them and
incurring bypass latency overhead on each of them could add upto
significant overhead.

Unfortunately, blk_init_allocated_queue() is never used by queues of
bio-based drivers, which means that all bio-based driver queues are in
bypass mode even after initialization and registration complete
successfully.

Due to the limited way request_queues are used by bio drivers, this
problem is hidden pretty well but it shows up when blk-throttle is
used in combination with a bio-based driver.  Trying to configure
(echoing to cgroupfs file) blk-throttle for a bio-based driver hangs
indefinitely in blkg_conf_prep() waiting for bypass mode to end.

This patch moves the initial blk_queue_bypass_end() call from
blk_init_allocated_queue() to blk_register_queue() which is called for
any userland-visible queues regardless of its type.

I believe this is correct because I don't think there is any block
driver which needs or wants working elevator and blk-cgroup on a queue
which isn't visible to userland.  If there are such users, we need a
different solution.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Joseph Glanville <joseph.glanville@orionvm.com.au>
Cc: stable@vger.kernel.org
Acked-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-core.c

@@ -608,8 +608,8 @@ struct request_queue *blk_alloc_queue_node(gfp_t gfp_mask, int node_id)
 	/*
 	 * A queue starts its life with bypass turned on to avoid
 	 * unnecessary bypass on/off overhead and nasty surprises during
-	 * init.  The initial bypass will be finished at the end of
-	 * blk_init_allocated_queue().
+	 * init.  The initial bypass will be finished when the queue is
+	 * registered by blk_register_queue().
 	 */
 	q->bypass_depth = 1;
 	__set_bit(QUEUE_FLAG_BYPASS, &q->queue_flags);
@@ -712,9 +712,6 @@ blk_init_allocated_queue(struct request_queue *q, request_fn_proc *rfn,
 	/* init elevator */
 	if (elevator_init(q, NULL))
 		return NULL;
-
-	/* all done, end the initial bypass */
-	blk_queue_bypass_end(q);
 	return q;
 }
 EXPORT_SYMBOL(blk_init_allocated_queue);

block/blk-sysfs.c

@@ -561,6 +561,12 @@ int blk_register_queue(struct gendisk *disk)
 	if (WARN_ON(!q))
 		return -ENXIO;
 
+	/*
+	 * Initialization must be complete by now.  Finish the initial
+	 * bypass from queue allocation.
+	 */
+	blk_queue_bypass_end(q);
+
 	ret = blk_trace_init_sysfs(dev);
 	if (ret)
 		return ret;