[RFC PATCH] bfq: fix waker_bfqq inconsistency crash

[Khazhismel Kumykov <khazhy@chromium.org>]

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.
We must either reset woken_list_node, or avoid setting woken_list at all
for oom_bfqq - opt to do the former.

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: 8ef3fc3a043c ("block, bfq: make shared queues inherit wakers")
Signed-off-by: Khazhismel Kumykov <khazhy@google.com>
---
RFC mainly because it's not clear to me the best policy here - but the
patch is tested and fixes a real crash we started seeing in 5.15

This is following up my ramble over at
https://lore.kernel.org/lkml/CACGdZYLMnfcqwbAXDx+x9vUOMn2cz55oc+8WySBS3J2Xd_q7Lg@mail.gmail.com/

 block/bfq-iosched.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
index 7ea427817f7f..5d2861119d20 100644
--- a/block/bfq-iosched.c
+++ b/block/bfq-iosched.c
@@ -6793,7 +6793,12 @@ static struct bfq_queue *bfq_init_rq(struct request *rq)
 				 * reset. So insert new_bfqq into the
 				 * woken_list of the waker. See
 				 * bfq_check_waker for details.
+				 *
+				 * Also, if we got oom_bfqq, we must check if
+				 * it's already in a woken_list
 				 */
+				if (unlikely(!hlist_unhashed(&bfqq->woken_list_node)))
+					hlist_del_init(&bfqq->woken_list_node);
 				if (bfqq->waker_bfqq)
 					hlist_add_head(&bfqq->woken_list_node,
 						       &bfqq->waker_bfqq->woken_list);
-- 
2.38.1.273.g43a17bfeac-goog