blk-throttle: fix NULL pointer dereference in throtl_schedule_pending_timer

I have encountered a NULL pointer dereference in
throtl_schedule_pending_timer:
  [  413.735396] BUG: unable to handle kernel NULL pointer dereference at 0000000000000038
  [  413.735535] IP: [<ffffffff812ebbbf>] throtl_schedule_pending_timer+0x3f/0x210
  [  413.735643] PGD 22c8cf067 PUD 22cb34067 PMD 0
  [  413.735713] Oops: 0000 [#1] SMP
  ......

This is caused by the following case:
  blk_throtl_bio
    throtl_schedule_next_dispatch  <= sq is top level one without parent
      throtl_schedule_pending_timer
        sq_to_tg(sq)->td->throtl_slice  <= sq_to_tg(sq) returns NULL

Fix it by using sq_to_td instead of sq_to_tg(sq)->td, which will always
return a valid td.

Fixes: 297e3d854784 ("blk-throttle: make throtl_slice tunable")
Signed-off-by: Joseph Qi <qijiang.qj@alibaba-inc.com>
Reviewed-by: Shaohua Li <shli@fb.com>
Signed-off-by: Jens Axboe <axboe@fb.com>

diff --git a/block/blk-throttle.c b/block/blk-throttle.c
index fc13dd0c6e3956a84913d9e71132c0f321a67280..3b751f706c6180c490cfd38a44f49621a5780021 100644 (file)
--- a/block/blk-throttle.c
+++ b/block/blk-throttle.c
@@ -698,7 +698,7 @@ static void throtl_dequeue_tg(struct throtl_grp *tg)
 static void throtl_schedule_pending_timer(struct throtl_service_queue *sq,
                                          unsigned long expires)
 {
-       unsigned long max_expire = jiffies + 8 * sq_to_tg(sq)->td->throtl_slice;
+       unsigned long max_expire = jiffies + 8 * sq_to_td(sq)->throtl_slice;
 
        /*
         * Since we are adjusting the throttle limit dynamically, the sleep