[PATCH 5.10 25/67] blk-iocost: fix operation ordering in iocg_wake_fn()

Greg Kroah-Hartman
2021-08-02T14:03:07Z

From: Tejun Heo <tj@kernel.org>

commit 5ab189cf3abbc9994bae3be524c5b88589ed56e2 upstream.

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: 7caa47151ab2 ("blkcg: implement blk-iocost")
Cc: stable@vger.kernel.org # v5.4+
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
block/blk-iocost.c | 11 ++++++-----
1 file changed, 6 insertions(+), 5 deletions(-)

--- a/block/blk-iocost.c
+++ b/block/blk-iocost.c
@@ -1394,16 +1394,17 @@ static int iocg_wake_fn(struct wait_queu
return -1;

iocg_commit_bio(ctx->iocg, wait->bio, wait->abs_cost, cost);
+ wait->committed = true;

/*
* autoremove_wake_function() removes the wait entry only when it
- * actually changed the task state. We want the wait always
- * removed. Remove explicitly and use default_wake_function().
+ * actually changed the task state. We want the wait always removed.
+ * Remove explicitly and use default_wake_function(). Note that the
+ * order of operations is important as finish_wait() tests whether
+ * @wq_entry is removed without grabbing the lock.
*/
- list_del_init(&wq_entry->entry);
- wait->committed = true;
-
default_wake_function(wq_entry, mode, flags, key);
+ list_del_init_careful(&wq_entry->entry);
return 0;
}