blkcg: fix "scheduling while atomic" in blk_queue_bypass_start

Since 749fefe677 in v3.7 ("block: lift the initial queue bypass mode
on blk_register_queue() instead of blk_init_allocated_queue()"),
the following warning appears when multipath is used with CONFIG_PREEMPT=y.

This patch moves blk_queue_bypass_start() before radix_tree_preload()
to avoid the sleeping call while preemption is disabled.

  BUG: scheduling while atomic: multipath/2460/0x00000002
  1 lock held by multipath/2460:
   #0:  (&md->type_lock){......}, at: [<ffffffffa019fb05>] dm_lock_md_type+0x17/0x19 [dm_mod]
  Modules linked in: ...
  Pid: 2460, comm: multipath Tainted: G        W    3.7.0-rc2 #1
  Call Trace:
   [<ffffffff810723ae>] __schedule_bug+0x6a/0x78
   [<ffffffff81428ba2>] __schedule+0xb4/0x5e0
   [<ffffffff814291e6>] schedule+0x64/0x66
   [<ffffffff8142773a>] schedule_timeout+0x39/0xf8
   [<ffffffff8108ad5f>] ? put_lock_stats+0xe/0x29
   [<ffffffff8108ae30>] ? lock_release_holdtime+0xb6/0xbb
   [<ffffffff814289e3>] wait_for_common+0x9d/0xee
   [<ffffffff8107526c>] ? try_to_wake_up+0x206/0x206
   [<ffffffff810c0eb8>] ? kfree_call_rcu+0x1c/0x1c
   [<ffffffff81428aec>] wait_for_completion+0x1d/0x1f
   [<ffffffff810611f9>] wait_rcu_gp+0x5d/0x7a
   [<ffffffff81061216>] ? wait_rcu_gp+0x7a/0x7a
   [<ffffffff8106fb18>] ? complete+0x21/0x53
   [<ffffffff810c0556>] synchronize_rcu+0x1e/0x20
   [<ffffffff811dd903>] blk_queue_bypass_start+0x5d/0x62
   [<ffffffff811ee109>] blkcg_activate_policy+0x73/0x270
   [<ffffffff81130521>] ? kmem_cache_alloc_node_trace+0xc7/0x108
   [<ffffffff811f04b3>] cfq_init_queue+0x80/0x28e
   [<ffffffffa01a1600>] ? dm_blk_ioctl+0xa7/0xa7 [dm_mod]
   [<ffffffff811d8c41>] elevator_init+0xe1/0x115
   [<ffffffff811e229f>] ? blk_queue_make_request+0x54/0x59
   [<ffffffff811dd743>] blk_init_allocated_queue+0x8c/0x9e
   [<ffffffffa019ffcd>] dm_setup_md_queue+0x36/0xaa [dm_mod]
   [<ffffffffa01a60e6>] table_load+0x1bd/0x2c8 [dm_mod]
   [<ffffffffa01a7026>] ctl_ioctl+0x1d6/0x236 [dm_mod]
   [<ffffffffa01a5f29>] ? table_clear+0xaa/0xaa [dm_mod]
   [<ffffffffa01a7099>] dm_ctl_ioctl+0x13/0x17 [dm_mod]
   [<ffffffff811479fc>] do_vfs_ioctl+0x3fb/0x441
   [<ffffffff811b643c>] ? file_has_perm+0x8a/0x99
   [<ffffffff81147aa0>] sys_ioctl+0x5e/0x82
   [<ffffffff812010be>] ? trace_hardirqs_on_thunk+0x3a/0x3f
   [<ffffffff814310d9>] system_call_fastpath+0x16/0x1b

Signed-off-by: Jun'ichi Nomura <j-nomura@ce.jp.nec.com>
Acked-by: Vivek Goyal <vgoyal@redhat.com>
Acked-by: Tejun Heo <tj@kernel.org>
Cc: Alasdair G Kergon <agk@redhat.com>
Cc: stable@kernel.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-cgroup.c

@@ -972,10 +972,10 @@ int blkcg_activate_policy(struct request_queue *q,
 	if (!new_blkg)
 		return -ENOMEM;
 
-	preloaded = !radix_tree_preload(GFP_KERNEL);
-
 	blk_queue_bypass_start(q);
 
+	preloaded = !radix_tree_preload(GFP_KERNEL);
+
 	/*
 	 * Make sure the root blkg exists and count the existing blkgs.  As
 	 * @q is bypassing at this point, blkg_lookup_create() can't be