[BUG] bfq-iosched: possible null-pointer dereference in __bfq_insert_request()

[Li Tuo]

Hello,

Our static analysis tool finds a possible null-pointer dereference in 
bfq-iosched.c in Linux 5.14.0-rc3:

The variable bfqq is checked in:
5920:    waiting = bfqq && bfq_bfqq_wait_request(bfqq);

This indicates that bfqq can be NULL.
But the address of its field fifo, which is an illegal pointer, is 
dereferenced in the function list_add_tail():
5925:    list_add_tail(&rq->queuelist, &bfqq->fifo);

The pointer bfqq is also dereferenced in the function bfq_rq_enqueued() 
which is called at line 5927.
Some dereference operations are:
5826:    bfqq->meta_pending++;
5828:    bfqq->last_request_pos = blk_rq_pos(rq) + blk_rq_sectors(rq);

I am not quite sure whether this possible null-pointer dereference is 
real and how to fix it if it is real.
Any feedback would be appreciated, thanks!

Reported-by: TOTE Robot <oslab@xxxxxxxxxxxxxxx>

Best wishes,
Tuo Li