在取消引用 NULL 'request->q' 指针时，'blk_mq_rq_timed_out()' 崩溃

Issue

• 在取消引用 NULL request->q 指针时，blk_mq_rq_timed_out() 崩溃。

BUG: unable to handle kernel NULL pointer dereference at 00000000000000d0
IP: [<ffffffff86d5f628>] blk_mq_rq_timed_out+0x18/0xa0
PGD 8000000035eae067 PUD 0 
Oops: 0000 [#1] SMP 
CPU: 2 PID: 1041 Comm: kworker/2:1H Kdump: loaded Tainted: P           OE  ------------   3.10.0-1127.19.1.el7.x86_64 #1
Hardware name: Red Hat KVM, BIOS 0.5.1 01/01/2011
Workqueue: kblockd blk_mq_timeout_work
task: ffff93dbd3fa8000 ti: ffff93dbd3f60000 task.ti: ffff93dbd3f60000
RIP: 0010:[<ffffffff86d5f628>]  [<ffffffff86d5f628>] blk_mq_rq_timed_out+0x18/0xa0
....
Call Trace:
 [<ffffffff86d5f70c>] blk_mq_check_expired+0x5c/0x60
 [<ffffffff86d62be4>] bt_iter+0x54/0x60
 [<ffffffff86d635eb>] blk_mq_queue_tag_busy_iter+0x13b/0x320
 [<ffffffff86d5f6b0>] ? blk_mq_rq_timed_out+0xa0/0xa0
 [<ffffffff86d5f6b0>] ? blk_mq_rq_timed_out+0xa0/0xa0
 [<ffffffff86d5e33b>] blk_mq_timeout_work+0x8b/0x180
 [<ffffffff86abe6bf>] process_one_work+0x17f/0x440
 [<ffffffff86abf7d6>] worker_thread+0x126/0x3c0
....
RIP  [<ffffffff86d5f628>] blk_mq_rq_timed_out+0x18/0xa0
....