iocg_wake_fn() open-codes wait_queue_entry removal and wakeup because it
wants the wq_entry to be always removed whether it ended up waking the
task or not. finish_wait() tests whether wq_entry needs removal without
grabbing the wait_queue lock and expects the waker to use
list_del_init_careful() after all waking operations are complete, which
iocg_wake_fn() didn't do. The operation order was wrong and the regular
list_del_init() was used.

The result is that if a waiter wakes up racing the waker, it can free pop
the wq_entry off stack before the waker is still looking at it, which can
lead to a backtrace like the following.

  [7312084.588951] general protection fault, probably for non-canonical address 0x586bf4005b2b88: 0000 [#1] SMP
  ...
  [7312084.647079] RIP: 0010:queued_spin_lock_slowpath+0x171/0x1b0
  ...
  [7312084.858314] Call Trace:
  [7312084.863548]  _raw_spin_lock_irqsave+0x22/0x30
  [7312084.872605]  try_to_wake_up+0x4c/0x4f0
  [7312084.880444]  iocg_wake_fn+0x71/0x80
  [7312084.887763]  __wake_up_common+0x71/0x140
  [7312084.895951]  iocg_kick_waitq+0xe8/0x2b0
  [7312084.903964]  ioc_rqos_throttle+0x275/0x650
  [7312084.922423]  __rq_qos_throttle+0x20/0x30
  [7312084.930608]  blk_mq_make_request+0x120/0x650
  [7312084.939490]  generic_make_request+0xca/0x310
  [7312084.957600]  submit_bio+0x173/0x200
  [7312084.981806]  swap_readpage+0x15c/0x240
  [7312084.989646]  read_swap_cache_async+0x58/0x60
  [7312084.998527]  swap_cluster_readahead+0x201/0x320
  [7312085.023432]  swapin_readahead+0x2df/0x450
  [7312085.040672]  do_swap_page+0x52f/0x820
  [7312085.058259]  handle_mm_fault+0xa16/0x1420
  [7312085.066620]  do_page_fault+0x2c6/0x5c0
  [7312085.074459]  page_fault+0x2f/0x40

Fix it by switching to list_del_init_careful() and putting it at the end.
Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Rik van Riel <riel@surriel.com>
Fixes: 7caa4715 ("blkcg: implement blk-iocost")
Cc: stable@vger.kernel.org # v5.4+
Signed-off-by: Jens Axboe <axboe@kernel.dk>