bfq: fix waker_bfqq inconsistency crash

This fixes crashes in bfq_add_bfqq_busy due to waker_bfqq being NULL,
but woken_list_node still being hashed. This would happen when
bfq_init_rq() expects a brand new allocated queue to be returned from
bfq_get_bfqq_handle_split() and unconditionally updates waker_bfqq
without resetting woken_list_node. Since we can always return oom_bfqq
when attempting to allocate, we cannot assume waker_bfqq starts as NULL.

Avoid setting woken_bfqq for oom_bfqq entirely, as it's not useful.

Crashes would have a stacktrace like:
[160595.656560]  bfq_add_bfqq_busy+0x110/0x1ec
[160595.661142]  bfq_add_request+0x6bc/0x980
[160595.666602]  bfq_insert_request+0x8ec/0x1240
[160595.671762]  bfq_insert_requests+0x58/0x9c
[160595.676420]  blk_mq_sched_insert_request+0x11c/0x198
[160595.682107]  blk_mq_submit_bio+0x270/0x62c
[160595.686759]  __submit_bio_noacct_mq+0xec/0x178
[160595.691926]  submit_bio+0x120/0x184
[160595.695990]  ext4_mpage_readpages+0x77c/0x7c8
[160595.701026]  ext4_readpage+0x60/0xb0
[160595.705158]  filemap_read_page+0x54/0x114
[160595.711961]  filemap_fault+0x228/0x5f4
[160595.716272]  do_read_fault+0xe0/0x1f0
[160595.720487]  do_fault+0x40/0x1c8

Tested by injecting random failures into bfq_get_queue, crashes go away
completely.

Fixes: 8ef3fc3a04 ("block, bfq: make shared queues inherit wakers")
Signed-off-by: Khazhismel Kumykov <khazhy@google.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20221108181030.1611703-1-khazhy@google.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/bfq-iosched.c

@@ -6717,6 +6717,12 @@ static struct bfq_queue *bfq_init_rq(struct request *rq)
 				bfqq = bfq_get_bfqq_handle_split(bfqd, bic, bio,
 								 true, is_sync,
 								 NULL);
+				if (unlikely(bfqq == &bfqd->oom_bfqq))
+					bfqq_already_existing = true;
+			} else
+				bfqq_already_existing = true;
+
+			if (!bfqq_already_existing) {
 				bfqq->waker_bfqq = old_bfqq->waker_bfqq;
 				bfqq->tentative_waker_bfqq = NULL;
 
@@ -6730,8 +6736,7 @@ static struct bfq_queue *bfq_init_rq(struct request *rq)
 				if (bfqq->waker_bfqq)
 					hlist_add_head(&bfqq->woken_list_node,
 						       &bfqq->waker_bfqq->woken_list);
-			} else
-				bfqq_already_existing = true;
+			}
 		}
 	}